Proper ventilation is imperative for a greenhouse to function properly. The ventilation system of a greenhouse provides fresh air to the plants (CO2), helps to control temperature and humidity, and reduces the likelihood of disease. Greenhouses are generally enclosed structures that will not ventilate entirely on their own. In order to ensure that proper ventilation in the greenhouse is achieved, gardeners must install either a passive or powered ventilation system.
Passive Ventilation System
A passive ventilation system is a ventilation system without powered fans. Instead, the greenhouse is ventilated via convection (hot air becomes less dense and rises) through the ridge vents. In addition to the ridge vents, a passive ventilation system needs intake vents to allow cool air to enter the greenhouse and displace the hot air as it rises. These intake vents are generally placed on the lower portion of the side walls of a greenhouse. The largest advantage of passive ventilation systems is that they are virtually silent. This can be advantageous for a gardener whose greenhouse is attached to his or her home and the noise of a powered fan would be a disturbance. The biggest disadvantages of a passive ventilation system are cost, maintenance, and efficiency. Most people don’t know that a ridge vent system will cost 3-4 times as much as a PVS (powered ventilation system). Also many people do not understand that maintenance is required with a passive ventilation system. The ridge vent needs to be kept free of debris and cleaned periodically. Also, the pistons on the vent system will need to be lubricated. Last, but not least, because air is not being forced through the greenhouse, a passive ventilation system is not as efficient as a powered ventilation system. All in all, passive ventilation systems are a great fit for greenhouses where the noise of a powered fan would be an annoyance.
Powered Ventilation System (PVS)
A powered ventilation system for a greenhouse is a ventilation system with a powered fan and intake vents. The cubic feet of the greenhouse space will be the determining factor for sizing a fan system. The minimum goal is to get the cubic volume of air turned over in less than two minutes. In many cases, a fan will be sized to turn over the air in less than one minute. A quick turnover makes it much easier to control temperature and/or humidity, keeping the greenhouse about 10 degrees cooler than a passive cooling system. The fan should be installed on the top section of the wall opposite of the intake vents and screen door. In other words, the intake vents should be installed on the same wall as the door and the fan should be installed on the top of the opposite wall. This ensures that the fresh air entering the greenhouse travels across the greenhouse before being evacuated. Powered ventilation systems are usually set up with a thermostatic control which will turn on the fan when the set-point temperature is reached. The intake vents are synced with the fan so they will open at the same time the fan is activated. In order to maintain the highest level of performance, the thermostat sensor should be placed at plant height.
In addition to the cooling/exhaust fan of a PVS, a greenhouse should also be equipped with a circulating fan. Circulating fans provide continuous air movement within the greenhouse which helps to maintain uniform temperatures and humidity while also increases the structural integrity of the plants (much like the wind strengthens plants in nature). Greenhouses with circulating fans are the most effective and efficient for maintaining desirable atmospheric conditions.
Whether you choose a passive or powered ventilation system, making sure a greenhouse is properly ventilated is crucial to creating an ideal growing environment for plants. A greenhouse’s temperature, humidity, and ambient CO2 levels are all determined by the ventilation system. In other words, the ventilation system is one of the most important determining factors over the way plants will perform in a greenhouse and should be contemplated by every potential greenhouse gardener.
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Greenhouse Space-Saving Techniques
With a little garden space planning, it is possible to drastically increase the productivity and enjoyment of a greenhouse garden. Like with any great structure, one can start from the foundation and work up to the top to make sure each area complements the others and is an efficient use of the space.
If a greenhouse is on a deck or already has permanent flooring, it may not be possible to take advantage of some great underground space. If at all possible, there are many advantages to having a “flexible” greenhouse floor. To start with, some area of floor that is not covered can allow for cultivation of the soil or the addition of a perfectly blended soil mix to grow plants at the ground level. This will result in an automatic space saving because the roots of the plants are occupying space below the greenhouse floor level that otherwise would be wasted. All that is generally necessary for walking and working in the greenhouse is to leave a three to four foot corridor down the middle and the rest of the floor area can be planted. But even this center corridor can be a door to an underground gardening powerhouse.
The Underground Greenhouse Engine
One of the best ways to turn a greenhouse into a full scale growing machine is to convert the center corridor into an underground composting and/or vermiculture center, and it is quite easy to do. Simply dig a trench three to four feet wide and two to three feet deep in the center of the greenhouse from end to end. Stack a brick barrier on the sides and ends and cover with strong plywood sections. The plywood makes a nice greenhouse floor for walking through and tending plants. Sections of the plywood can be lifted to reveal the perfect cool damp environment for composting and/or vermiculture and a dry box section can be included for garden supply storage.
Hydroponics and the Underground
Any discussion of greenhouse space saving techniques would be incomplete without some mention of hydroponics. Hydroponics presents a level of control and efficiency that is geared toward productivity and getting the most out of available space. Underground is the perfect place to put nutrient reservoirs. Many hydroponics systems require the reservoir to be below the plant growing medium and below ground reservoirs allow the plant grow beds to be as low as ground level. They do not take up space in the greenhouse and keep the nutrient solution cooler in summer and warmer in winter.
Using Floor Space Wisely
With a few cautions in mind, the stacking plant growing space with tiered benches or shelves can add greatly to productivity. It is important to consider how the shelves and plants themselves will shade other plants from light. Just as tall plants are generally positioned in the back of the garden so they will not shade shorter plants, it is important to observe the path of the sun in relationship to the greenhouse placement and plant accordingly. Place tall shelving and plants where they will not block too much light from other areas of the greenhouse. Shelves and racks are best made of mesh or screen construction which allows light, air and moisture to pass more easily. Always try to buy adjustable shelving. It is much more versatile and allows for spacing shelves based on the requirements of each crop. Specially designed triangular shelves are also available for the corners of the greenhouse to take advantage of what is often dead space. One last note on shelving, and just about anything else that is placed in the greenhouse, light colors are best because they reflect light allowing more of it to be absorbed by the plants.
Using Lights to Increase Productivity
As mentioned above, it is best to position benches, shelving and plants to take the best advantage of the natural light that is available to the greenhouse. That said it is possible to lengthen the growing season, volume of plants that can be grown and productivity with the use of artificial lights. T5 Fluorescent lights have several advantages for greenhouse space saving. They do not require large bulky external ballasts, and are very low profile and can be attached to the bottom of a shelf to provide light to the plants on the next shelf down. Just be sure to have proper channeling for moisture from the plants above. Bulbs are available in warm, cool and full spectrum and they produce very little heat allowing them to be positioned quite close to growing plants. They are available in 48 inch long 2, 4, 6 and 8 tube models that are perfect for any width shelf.
Using Space Saving Hydroponics Towers
Hydroponics towers are a growing innovation that is hard to ignore when it comes to getting the most from each square foot. These systems stack growing containers in clever configurations so that plants still get light, but are vertically stacked to save ground space. Several different varieties are available to suit almost any greenhouse application.
Last, but not least, because of their clear ceilings, greenhouses offer even more space for hanging plants. Once the floor and wall spaces have been planted, consider where hanging plants may work best without robbing too much light from other plants. As with shelves and other vertical plantings, it is important to study the path of the sun and shade in the greenhouse to determine where best to place hanging plants so the shade they provide is an asset not a determent to the greenhouse as a whole. This may be quite different depending on the season. In the summer, sun loving hanging plants may provide welcome shade when placed properly in the greenhouse. In the winter, they may need to be avoided entirely depending on the overall light requirements of the particular greenhouse application. Upside down tomato growers have also become popular for growing tomatoes and other plants hanging upside down and may make welcome space saving additions to the greenhouse.
It may perhaps be a habit from traditional outdoor gardening to look at the garden space as one dimensional and plan accordingly. A better approach is to take into consideration each level and surface from floor to sealing and how it will interact with the greenhouse as a whole. By taking a three dimensional view and planning the greenhouse garden from the floor up a better, more productive garden is within reach.
Sizing Your Greenhouse for Optimum Utilization of Space
Prior to setting up a greenhouse it is important to consider how the space within the greenhouse will be utilized. Unfortunately, many growers do not think about how they are going to use the space before they set up or purchase a greenhouse. This can lead to frustration and an unsatisfying greenhouse experience. However, with a little forethought, you can get the appropriately sized greenhouse for your particular needs and desires. When thinking about the utilization of space, it is important to examine the available widths of the greenhouse and how it will affect the greenhouse’s layout.
Greenhouses come in all lengths and widths. When contemplating the layout or floor plan of a greenhouse, a gardener should be most concerned with the width of the greenhouse because, in most cases, that is what will determine the dimensions of the benches and aisles. Hobby greenhouses are most commonly 8 or 10 feet wide but can also range from 12 – 20 foot widths. Most commercial or production greenhouses are usually wider and range from 20 feet and up. As previously mentioned, the width is important because it will determine the size of the benches that can be used. Benches are extremely popular for greenhouse gardeners and are used by most hobbyists. By examining the width of the greenhouse, a grower can determine what kind of bench and aisle spacing he or she can have.
8 Foot Width
Many smaller greenhouses are available in 8 foot widths. Unfortunately, the 8 foot measurement is a representation of the outside dimensions of the greenhouse. In other words, this measurement does not take the frame into consideration and, therefore, the actual width on the inside will not be a full 8 feet. Greenhouses with 8 foot widths typically have 2 foot wide benches on either side of a 3 foot wide aisle. Although this may be big enough for some hobbyists, many horticulturists will quickly fill up the bench space and wish they had more room.
10 Foot Width
Greenhouses with a width of 10 feet are very popular among hobbyists because they instantly give 30% more bench space than an 8 foot wide greenhouse. A typical set up in a 10 foot wide greenhouse is to have 3 foot benches on either side of a 3 foot wide aisle. A 3 foot bench is the perfect size because it provides sufficient space for plants while not being so deep that the horticulturist can’t reach the plants. All in all, a 10 foot wide greenhouse is a nice, comfortable width for hobbyists.
12 – 20 Foot Widths
Greenhouses that range from 12 – 20 feet wide are a good choice for horticulturists who want to grow on more than just benches. Growers who want to incorporate raised beds or who have a collection of tropical plants which need additional head space will find a greenhouse in this width range more suitable to their needs. Greenhouses of this width are also nice for hobbyists who want to add a seating area, water feature, or some other unique addition to their garden spaces. All of these greenhouse extras will take up space so always be sure to plan your greenhouse layout accordingly.
20+ Foot Width
Larger width greenhouses (20+ feet wide) are mainly reserved for educational or commercial applications. These wide greenhouses can have two 3 foot aisles with 3 foot benches on the outer sides and a 6 – 8 foot bench in the center. When optimizing space for production, these greenhouses are the ticket.
Before purchasing or building a greenhouse, make sure you take the time to think about the space within and how that space will be utilized. In most cases, the grower’s intended application will determine the width of the greenhouse. A good greenhouse manufacturer will help you determine the most appropriate width for your application and help guide you to optimize the space within.
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Greenhouse Construction: Planning for Snow Loads and Wind Loads
With all the decisions to be made when contemplating purchasing or building a greenhouse, it may be easy to overlook snow and wind. However, a grower needs to seriously consider how a greenhouse will handle snow and wind loads. As you can imagine, heavy snow fall or intense wind gusts can cause serious damage to a greenhouse structure. Unfortunately, not all greenhouse manufacturers are straightforward with snow and wind load ratings or requirements. It can also get confusing because many of the smaller greenhouses are not required to meet the same codes as more permanent greenhouse structures. One thing is certain, potential greenhouse owners need to take it upon themselves to understand their geographical location’s building code requirements and which greenhouse designs will meet those requirements.
The very first thing a grower should do before constructing or purchasing a greenhouse is to check with the local building department (where building codes are issued) and see what the snow and wind load requirements are for his or her geographical location. Regardless of the type of greenhouse you plan to have (stand alone or attached) it is very important to check and see what the building code requirements will be. With this information you can go back to the greenhouse supplier or builder to see which greenhouse design will meet those requirements.
Small hobby greenhouses with a stamped aluminum construction generally will not require building permits but will rarely meet the requirements for snow or wind loads. Most of these greenhouses are not designed to last a lifetime and will quickly degrade. Growers who are looking for a more permanent structure should look for greenhouses that utilize extruded aluminum framing instead of stamped aluminum framing.
Higher quality greenhouses with extruded aluminum framing will most likely meet local building code requirements. These greenhouses are designed to be a more permanent structure. Generally speaking, greenhouses need to withstand 30 pounds of snow per square foot (an equivalent of about four feet of snow) and wind gusts up to 105 MPH to meet most of the code requirements throughout the United States. Greenhouses built with quality extruded frames will not only be able to meet the wind and snow load requirements but will also be considered a truly permanent structure that is designed to last a lifetime.
In locations where snow loads or wind loads exceed the norm, additional bracing or advanced technological designs can be used to create additional strength. Some of the older greenhouses use heavy cross ties to meet higher snow load requirements. Many of the newer greenhouses are utilizing sophisticated engineering designs, like the scissor truss. The scissor truss design not only adds strength but also, because it eliminates the need for cross ties, allows for additional head room in the greenhouse.
If you are in the market for a greenhouse that will handle heavy snow and wind loads and last a lifetime, you should seriously consider a greenhouse with extruded aluminum framing. Small hobby greenhouses are suitable for some applications but should not be considered a permanent structure and certainly shouldn’t be expected to withstand heavy snow loads or strong gusts of wind. A quick trip to your local building department will give you all the information you’ll need for snow and wind load requirements. With that information you can discuss all of your options with your greenhouse supplier.
Background information provided by Arcadiaglasshouse.com.
Photo Description: The innovative “Scissor Truss” design provides extra strength and higher headroom for larger spans in greenhouses that meet local building codes for snow load and wind loads specifications.
Greenhouse Lighting Options
Whether it’s to provide supplementary lighting or to extend the length of the photoperiod, some greenhouse growers are equipping their greenhouses with artificial light sources. Before purchasing a lighting system for a greenhouse, a grower should first determine which type of lighting system will best meet their needs.
T-12 fluorescent lights are the standard four foot fluorescent lights commonly used in garages and shops. In fact, many people refer to T-12 fluorescents as “shop lights”. Although T-12 fluorescents are just fine for starting seeds indoors or illuminating indoor houseplants, they usually lack the intensity and efficiency to provide light to plants in a greenhouse.
T-5 fluorescents are a more powerful and efficient version of fluorescent lighting. Still available in four foot lengths, the skinnier T-5 bulbs are 54 watts each. Specialty horticultural enhanced bulbs are available for T-5 fixtures in both the warm (red) and cool (blue) spectrums. The warm spectrum bulbs are rated around 3000 K and the cool spectrum bulbs are rated around 6400 K. During the fruiting or flowering stage of growth most plants require a higher percentage of red spectrum light. During the vegetative stage of growth plants require more blue spectrum light. Many T-5 fixtures hold multiple bulbs and allow “banks” of bulbs to be turned on or off. In other words, it is possible to set up a T-5 fixture to customize the light spectrum in your greenhouse. During the vegetative stage, the bank with the blue bulbs should be on. When the transition or flowering stage begins the banks with the red spectrum bulbs can be activated in order to give the plants the more appropriate spectrum for that stage of growth. Generally speaking, T-5 fluorescents are the most cost effective option for the hobby greenhouse grower.
Metal halide is a type of high intensity discharge (HID) lighting that can be used in greenhouses. Metal halide lighting systems are more intense than fluorescent lighting fixtures and are a good choice when growing roses, tropical palm, or other plants that will require more intensity than fluorescents can provide. Metal halides generally produce a white or blue spectral output which makes them a good fit for vegetative growth. Metal halides are commercial quality fixtures that come in 400, 600, or 1,000 watts. Metal halide lighting systems require three components: the bulb, the ballast and the reflector or socket base. These lighting systems create a good amount of heat and can be quite expensive to operate. However, in some cases, metal halide is the only way to provide plants with the intense blue light spectrum they require for vegetative growth.
High Pressure Sodium
Like metal halide, high pressure sodium is a type of HID lighting. These lighting systems are also available in 400, 600, and 1000 watts. The biggest difference between high pressure sodium and metal halide is the spectral output. High pressure sodiums provide a good amount of red spectrum. This makes these lighting systems a great fit for fruiting or flowering plants. Since high pressure sodiums are also a type of HID lighting system, they will also require a ballast and reflector or socket.
If you have determined that your greenhouse will require a HID lighting system and you plan to provide artificial light during the plant’s vegetative and flowering stages, you should look into a convertible ballast. Convertible ballasts allow the grower to use both metal halide and high pressure sodium bulbs in the same fixture (but not simultaneously). A grower can use his or her metal halide bulb during the vegetative stage and then switch to a high pressure sodium bulb during the flowering stage.
There is certainly an emerging opportunity for LED lighting systems to be incorporated into the greenhouse. Consumers should be aware that LEDs are very spectral specific and they need to make sure the LED light system they purchase provides the spectral output required by their plants. Unfortunately, there are a lot of variables to consider when comparing LED lighting fixtures. The light spectrum, actual wattage, and type of lens used are all things to consider when purchasing a LED system. There is also a wide variance in overall quality. Many of the cheap LED fixtures will not stand up to the conditions found within a greenhouse. There is no doubt that LED lighting systems will continue to shape the future of horticulture but it is important for consumers to closely examine what they are purchasing.
Sizing Heating and Cooling Equipment for a Greenhouse
Maintaining consistent temperatures is one of the most important aspects of greenhouse gardening. During the hot, summer months, most greenhouses will require some sort of cooling to ensure the temperatures don’t get too hot. During the cold winter months, most greenhouses will require some sort of heating to ensure temperatures do not get too cold. Before purchasing any heating or cooling equipment, a grower should take a close look at his or her space and make a few sizing calculations. With these calculations and a few considerations, a horticulturist can be sure to get the properly sized heating and cooling devices needed to keep the greenhouses’ temperature in check.
The most effective way to cool most greenhouses is with a powered fan. A powered fan will actively draw fresh air through the greenhouse and exhaust it outside of the greenhouse. The cooler air, from outside along with a natural evaporative effect, help keep the greenhouse cooler. In fact, a powered ventilation system will typically make a greenhouse run 10 degrees cooler than if the greenhouse is passively cooled (vents only). To make sure there is enough cooling power, a gardener must “size” the fan that will be needed for the given garden space. Fans are rated by their CFM or cubic feet per minute. Ideally, a greenhouse should have all of its air exchanged in 1-2 minutes. A simple and straightforward way to determine the needed CFM is to multiply the length by the width by the wall height of the greenhouse as shown below:
Length x Width x Wall Height = Recommended CFM (Cubic Feet of air volume per Minute)
Granted, this is not an exact calculation of the greenhouse’s cubic feet because it does not take into consideration the roof pitch, etc. However, this measurement is accurate enough to properly size a powered fan for a greenhouse. Once a grower has calculated the recommended CFM, he or she can set out to find a fan that meets that criteria. For example, a greenhouse that is 20 feet long, 10 feet wide, and has a wall height of 10 feet will have a recommended CFM of 2,000 (20 x 10 x 10 = 2,000). The owner of this greenhouse should purchase a fan with a minimum of a 2,000 CFM rating.
A gardener planning to heat his or her greenhouse with gas or electric must first determine the size of the heater. The best way to determine this is to figure out how many BTUs will be required to heat the space. This is done by first calculating the total square footage of exposed surface area. In other words, the exposed wall and roof surface area. Remember that the roof area, due to the pitch, will not be equivalent to the area of the floor. In fact, the exposed surface area of the roof will be larger than the floor area footage. After determining the total square footage exposed, the grower must determine the maximum desired temperature in the winter (the temperature at which the gardener wants the greenhouse to operate during the winter months) and the minimum temperature outside of the greenhouse. Remember to be realistic when making the temperature determinations. In other words, base your calculations on average temperatures rather than the extremes. The BTU calculation is completed when you multiply the total exposed surface area by the difference between the desired temperature and the outside minimum temperature, and then divide that number by the R-value of the greenhouse material. It’s important to remember when calculating the required BTUs that gas heaters operate at 80% efficiency compared to electric heaters which operate at 100% efficiency. In other words, a grower must take the lower efficiency into consideration in his or her calculations or he or she must find the gas heater’s output BTU rating (which already takes the 80% efficiency into consideration).The following is the formula that should be used to calculate the needed BTUs:
Sq. Ft. Exposed Surface Area x ( Tmax – Tmin ) ÷ R Square Feet of Glass or Poly Surface Area x (Desired Temperature Inside – Minimum Temperature Outside) ÷ R-value
Here are the R-values of some commonly used greenhouse glazing materials:
Glass – Single Pane
Polycarbonate – 8mm Twinwall
Polycarbonate – 8mm Triplewall
Glass – Double Pane
Polycarbonate – 16mm Triplewall
Polycarbonate – 16mm Five-wall
Glass – Double Pane Low-E
Although all of these R-values may seem low when compared to a home or commercial building, there is a significant difference when comparing these materials to each other. For example, a greenhouse with single pane glass will require twice the BTUs (and cost twice as much to heat) as a greenhouse with a triplewall polycarbonate. In other words, when compared to each other, the R-values of these materials are quite significant, especially when considering how the required heating load is affected.
With a few simple calculations, any greenhouse grower can determine the appropriate size cooling fan and heater for his or her greenhouse. Greenhouses that have properly sized heating and cooling equipment will not only allow a gardener to extend the growing season but will also efficiently control temperatures in the greenhouse.
Background information provided by ArcadiaGlasshouse.com.
Grow All Year Round with Artificial Heat
A gardener who owns a greenhouse has the capability of extending the growing season by a few months each year. There are even a select few greenhouse gardeners who can get away with growing year round without adding a heating system. In most climates, however, during the winter months the nighttime temperatures become too cold for most plant species. In order for most greenhouse hobbyists to keep the greenhouse operational year round, some sort of heating system will need to be added. The three most common ways to heat a hobby greenhouse are with gas, electric, or passive solar heat.
Heating a greenhouse with gas is the least expensive option. The cost of heating a greenhouse with gas is the main advantage of gas heating and is the main reason people choose gas heating over other options. Beware of open flame heaters for greenhouse applications. These heaters will emit ethylene gas which can affect budding plants, such as orchids, to the point where their blossoms will not form or will fall off altogether. The other problem with open flame heaters is they have safety oxygen sensors which will automatically turn off the unit when depleted oxygen levels occur. This is a great safety feature for a heater within a home but in a greenhouse it can cause some problems. Many of the newer greenhouses are practically air tight which means they can get depleted oxygen levels fairly easily. If the heater turns off during the night, all the plants in the greenhouse could perish. Gas heaters that have a flue and are power exhausted are the best for greenhouse applications. These heaters will need to have a hole punched into the side of the greenhouse for the exhaust. All in all, an average gas heater for a greenhouse runs at 80% efficiency (mainly due to the heat loss through the exhaust ports).
Because there is no need for ventilation with electric heat, this type of heating is the most efficient. Electric heating runs at 100% efficiency. However, electricity is expensive and heating a greenhouse year round with electricity alone may be too expensive for many gardeners’ greenhouse budgets. However, there are some ways a gardener can supplement heat naturally to offset some of the cost of heating with electricity. For example, passive solar heat can be used to offset some of the cost associated with electric heating. If you do choose to go with an electric heater, it is best to get a 240V heater which will have plenty of heat capacity. Over the long run, 240V units save money and are very reliable. If you must go with a 120V unit, try to avoid infrared heaters or heaters that transfer heat to objects instead of heating the air. For greenhouse applications, a gardener will need a heater that will heat the air within the greenhouse. The best inexpensive 120V heaters for greenhouses are the oil-filled radiator-style heaters. If placed near a fan, these small heaters can adequately heat some small hobby greenhouses during the winter months.
Passive Solar Heat
Passive solar heat refers to collecting heat from the sun’s radiation during the daytime and releasing that heat into the greenhouse during the nighttime. This is usually done with water or some other thermal mass that is capable of absorbing and holding heat for a duration of time. One technique is to line the north wall of a greenhouse with black water containers. When placed on the north wall, these containers will not shade light from reaching the plants in the greenhouse; instead, they will absorb light and heat that passes through the greenhouse to the north wall. When the sun sets, the heat that is held by the water will slowly dissipate into the greenhouse environment. When passive solar heat is used in conjunction with electric heat, it can significantly reduce the annual cost of heating a greenhouse.
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Foundations for Greenhouses
When setting up a greenhouse, one of the most important aspects of the construction process is the foundation. There are a few different types of foundations that can be used for greenhouses. The foundation used will be determined by the type of greenhouse, the building codes and, in some cases, personal preference. Essentially, the foundation is the complete system on which the greenhouse structure sits. One of the key components of a foundation is the footing. The footing refers to the point at which the structure meets the soil. This is the section that the structure rests upon. Footers are not always necessary for a standard hobby greenhouse. In situations where they are necessary, footers are typically poured concrete and their exact depth is determined by local building codes and the location’s frost levels. Footers help to prevent sagging or the dropping of the structure’s walls and/or floor into the ground. The choices for a greenhouse’s foundation are typically dependent on the type of greenhouse structure that will be built.
If you have an attached even-span or a lean-to greenhouse there is a need for footers with the foundation. Local building codes will determine the exact depth needed for the footers. Basically, attached greenhouses require frost free footers that will not shift or move. As the ground freezes and thaws structures without the proper foundation could move or sag. If you plan on building an attached greenhouse, you should also plan on a more extensive foundation. In most cases, the gardener is already planning on the expense of a foundation for the attached greenhouse so the cost of pouring a concrete pad for the floor is seen as a minimal addition. However, some gardeners still prefer the moist, earthy smell of gravel and will have a gravel floor installed. A limestone screening and weed barrier placed beneath the gravel will ensure no weeds start growing from the floor. Some gardeners prefer a gravel floor for its natural drainage properties as well. Regardless of the floor material chosen, the footing requirements will not change.
Freestanding Hobby Greenhouses
Smaller, freestanding greenhouses do not have the same requirements as an attached greenhouse and, therefore, give the grower more options when it comes to the foundation and floor. Freestanding greenhouses can have a concrete slab poured without a deep foundation. In fact, if the greenhouse doesn’t have a knee wall, it can be placed directly on a cement slab or it can be placed on a 4’ x 6” treated lumber base. The best wood choices for a lumber base are usually cedar, redwood, or cypress. Rebar can be added to the wooden base for additional support if desired. For a freestanding greenhouse with a knee wall (a brick or cinder block wall that stands roughly knee high), a 12” x 12” surface footer is required. This is because the additional weight of the brick or cinder blocks could otherwise sink in to the soil and affect the structural integrity of the greenhouse. Knee walls are usually installed to add design appeal and for their natural high thermal mass. During the day, the stones absorb heat from the sun and during the night, they act as a passive heater as the heat is released into the greenhouse.
Generally speaking, greenhouses that are attached to the home or freestanding greenhouses that are more than 120 square feet will need building permits or will have to be up to code. However, it is always a good idea to check the local city codes to see what permits and footings are required before beginning construction on any greenhouse.
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Keeping Your Greenhouse Cool in the Summer
In order for the plants in a greenhouse to continue growing rapidly during the summer months, a greenhouse gardener needs to make sure that the temperature within the greenhouse doesn’t get too hot. There are a few different ways that greenhouse growers can maintain cooler temperatures during the heat of summer. A grower usually chooses a cooling method based on the size of the greenhouse and his or her budget. The most common types of cooling devices used in greenhouses are based on water evaporating which cools the air within the growing space. The most commonly used systems are wet wall systems, portable evaporative coolers, foggers, and humidifiers.
Wet Wall Systems
Wet wall systems are the most popular and efficient way to cool large, commercial greenhouses. These systems consist of cooling pads encased in aluminum housing. The aluminum housing appears similar to a honey comb. The honey comb design allows air to pass over the cooling pads which are kept drenched in water. The air is cooled significantly as it passes through the wet pad on its way into the greenhouse, much like a radiator cooling an engine. After entering the greenhouse, the evaporative effect cools the growing space. The powered fans (placed on the wall opposite the wet wall system) are the driving force for the air movement. The motorized shutters which allow air to enter the greenhouse are usually thermostatically controlled to open and close as determined by the temperature. Wet wall systems are fairly expensive and more elaborate in their installation. This is because the water used in a wet wall system is continuously recirculated and requires a fair amount of plumbing. Because of their expensive and more elaborate design, wet wall systems are usually only used in larger commercial greenhouses.
Portable Evaporative Coolers
Portable evaporative coolers are a cooling device which is much more commonly used by home hobbyist greenhouse growers. Portable evaporative coolers are completely self-contained, movable, and relatively inexpensive ($500-$1000 depending on size). Portable evaporative coolers contain a similar pad to that used in wet wall systems. Like wet wall systems, air is drawn in and forced over the wet pad to create an evaporative cooling effect. Portable evaporative coolers usually contain a float valve and are connected to a garden hose to maintain the water level in the tank as the water evaporates. Portable evaporative coolers work best when they have access to fresh air. In other words, for the best effect, portable evaporative coolers should be placed near the greenhouse entry or a fresh air intake vent. Portable evaporative coolers are thermostatically controlled and can easily be stashed under a bench or out of the way when not in use. Portable evaporative coolers are a great way for home hobbyist greenhouse gardeners to maintain cooler temperatures during the hot summer months.
Fogger or Humidifier
For small hobby greenhouses, a fogger or humidifier can serve as an evaporative cooling system. A fogger is a device that emits water in a fine fog. These devices can be placed directly behind a circulating fan to blow the water-cooled air around the greenhouse. As the water evaporates, it creates a cooling effect. A humidifier found at a big box store can be set up in a similar fashion. When placed behind a circulating fan, the cool moist air can be blown across the greenhouse for an evaporative cooling effect. Both foggers and humidifiers emit water droplets so small that the plants within the greenhouse will not get wet. Instead, the water quickly evaporates and cools the greenhouses climate in the process.
When discussing cooling systems within a greenhouse, some gardeners ask about mist systems. Although a mist system will cool a greenhouse, they are generally better suited for irrigation. The water droplet size from a mist system is much larger than that of a fogger or humidifier and this causes the plants to actually get wet which can create a whole new series of moisture related problems during the hot summer months.
By maintaining temperatures within the desired range for plant growth, a greenhouse gardener can ensure that he or she maintains vigorous growth even during the peak heat of the summer months. Whether a gardener is attempting to cool a large commercial greenhouse with a wet wall system or a small hobbyist greenhouse with a portable evaporative cooler or a humidifier, evaporative cooling is the best way to ensure that the plants beat the heat of summer.
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Size Matters in a Greenhouse
They say that everything is bigger in Texas. Well, Texas is a fine state but you don’t have to live there to enjoy the benefits of a big greenhouse. A good general rule is to figure out what size footprint you need for growing things and then double it. That should be about the right size for your greenhouse. Here’s why I suggest this approach to sizing:
Easier Heating and Cooling
Smaller structures heat up too quickly and cool down too quickly. Think of how temperamental a cold frame is. If you don’t quickly vent them, they’ll cook your plants in a matter of a couple of hours. The same hold true for a small greenhouse, so bigger is better.
Room to Work and Grow
You need room to work in addition to the space allocated for growing. A greenhouse footprint based on sufficient room for growing plants is a bit like space planning for an office. It’s nice if the desk fits well in the room, but you also need room for your large office chair, a bookcase and a filing cabinet. And, you’d like to be able to push back from the desk and lean back in your chair as well. All of this requires “swinging room.”
Room to Relax
Don’t forget about room to relax. When you’re taking a break from gardening, you’ll want some space where you can sit down and enjoy the splendor that you’ve created. What could be more enjoyable than relaxing in a comfortable chair and enjoying the sights and smells of your herb garden or tomato jungle?
Room for Storage
Lastly, you need room for features like tool storage, a hose rack, walkways, a sink for rinsing off your root vegetables and perhaps a work table or space dedicated to starting seeds in covered, lighted and heated seedling greenhouse within your greenhouse. Also, don’t forget that a drum wall for passive solar energy storage takes up about four square feet per drum. When you start to think about it, there’s plenty of reason to have room in a greenhouse for things that aren’t green. You simply can’t expect to “dance” in a hall closet. You need room to spread out.
I’m not advocating a huge greenhouse, but we need to be aware that there are people, accessories, resources, tools, walkways and activities that need to be included in our indoor growing spaces – along with all the plants. If you forget about room for these types of things, you’ll soon come to the conclusion that you built a structure that simply isn’t big enough for your needs and longer term interests.
The Different Types of Attached Greenhouses
Attached greenhouses are exactly what they sound like: greenhouses attached to another structure. In many cases, the other structure is the gardener’s house. Attached greenhouses include lean-to greenhouses, window-mounted greenhouses, and attached even-span greenhouses.
Lean-to greenhouses are built against an existing building using that structure for one or more of its sides. In most cases, lean-to greenhouses are attached to a house. Lean-to greenhouses can be as long as the length of the building they are attached to or as short as desired. Because of their particular construction, lean-to greenhouses are typically limited in width to roughly 12 feet wide. The advantages of a lean-to greenhouse are accessibility to electricity and water (when placed against a house with electricity and water) and decreased cost of materials (since one of the main walls is an existing structure). Lean-to greenhouses typically require less heating than freestanding greenhouses. This can save a gardener a significant amount of money especially if he or she is located in a colder climate. Another advantage of lean-to greenhouses is accessibility. Since the greenhouse is so close to the home, the gardener has easy access to culinary herbs and vegetables. Accessibility also makes it easier for gardeners to keep up with greenhouse maintenance. The last advantage of lean-to greenhouses is that they are relatively simple to construct or inexpensive to purchase. Building a custom lean-to greenhouse can be an easy project for hobbyists who are not usually considered handy at construction. In fact, dozens of basic plans for homemade lean-to greenhouses can be found on the internet. Building one’s own greenhouse can give gardeners a feeling of self-reliance. For gardeners who don’t wish to build their own greenhouses, a lean-to greenhouse kit is an inexpensive way to get into home horticulture. The biggest disadvantages of lean-to greenhouses are their limited space and, in some cases, limited light. Whenever possible, lean-to greenhouses should be constructed on the side of the home that will receive the most sunlight (south side). Even when perfectly placed, a lean-to greenhouse will not receive as much light as a freestanding greenhouse.
Window-mounted greenhouses are special structures built into a window frame of a home, usually on a south facing wall. These handy, little greenhouses are a great way for hobbyists to keep their thumbs green year-round. Window-mounted greenhouses give hobbyists the opportunity to have fresh culinary herbs and ornamental plants at their fingertips while in the comfort of their own homes. The advantages of window-mounted greenhouses are they are relatively inexpensive and allow growers to harvest plants from inside their homes. In some cases, window-mounted greenhouses can help with heating a home as they let in passive solar heat. The disadvantage of a window-mounted greenhouse is space. These greenhouses are relatively small which limits the species and number of plants a gardener can grow.
Attached Even-Span Greenhouses
Attached even-span greenhouses are less common than lean-to greenhouses but also share a wall with an existing structure. The difference is that an attached even-span greenhouse appears very similar to a freestanding greenhouse except it is attached to an existing structure at one gable end. In other words, it doesn’t “lean” against the existing structure, but instead has its own symmetrical roof. Attached even-span greenhouses can be much larger than lean-to greenhouses and there are countless design possibilities. The largest advantage of an attached even-span greenhouse is they are less expensive than a freestanding greenhouse and can provide a lot of growing space. As with lean-to greenhouses, water and electricity are more accessible. Disadvantages of an attached even-span greenhouse are increased cost compared to other attached greenhouses and reduced light from shadowing of the attached structure (usually a home).
Greenhouse Glazing Options
When purchasing a greenhouse, a gardener is faced with a few options in regard to the type of material the transparent panels are made from. Although all greenhouses are designed to allow sunlight to reach the plants, there can be a difference in the way a garden performs due to the materials that make up the greenhouse. Hobbyists looking to set up a greenhouse have essentially three choices when it comes to the greenhouse’s glazing options: single pane glass, double pane glass, or multi-wall polycarbonate. Each glazing option has its own advantages and disadvantages which means growers should examine each before making a final decision.
Single Pane Glass
The older-style single pane glass greenhouses with overlapping glass are less than desirable due to the fact that many of them are not adequately sealed. This can cause a series of problems including inefficiencies with heating and cooling and humidity/moisture problems. The good news is that most of the newer-style single pane glass greenhouses have full length glass panels and are sealed which solves many of the problems associated with the outdated, leaky designs. One great thing about single pane greenhouses is that they look fantastic. Single pane glass greenhouses are very aesthetically pleasing and will only cost a little more than a polycarbonate greenhouse. However, single pane greenhouses should probably be avoided by gardeners in northern climates. Single pane glass greenhouses are the least energy efficient type of greenhouse. In fact, a single pane greenhouse will have double or even triple the heating costs when compared to a double pane or multi-walled polycarbonate greenhouse. Single pane glass greenhouses allow for 92% light transmission. This can be an advantage or a disadvantage depending on your climate and the location of the greenhouse. Greenhouses should let in as much sun as possible, right? In some cases, a glass greenhouse can allow too much light to reach the plants which may create “hot-spots” and cause significant damage to the plants. The gardener may need to install a shade cloth in the greenhouse to rectify this problem. All in all, single pane glass greenhouses are a good choice for gardeners who will not be heating the greenhouse or who strongly desire an aesthetically pleasing greenhouse.
Double Pane Glass
Double pane glass greenhouses are the bee’s knees when it comes to greenhouses. Double pane glass greenhouses are designed to be the most energy efficient and offer gardeners half the heating costs associated with single pane glass greenhouses. In other words, gardeners can look at a double pane glass greenhouse as a long term investment that will pay for itself in the form of energy savings. Double pane glass can also include a special low-e reflective coating on the inner pane of glass which reflects heat and acts as an insulator. For a serious hobbyist searching for the nicest looking and most energy efficient greenhouse, a double pane glass greenhouse is for you. The only real disadvantage of a double pane glass greenhouse is the initial cost. Double pane glass greenhouses are by far the most expensive option but, again, when examining the long term energy savings, the increased initial cost of the greenhouse would be paid back over time.
Another glazing option for a hobbyist looking to set up a greenhouse is polycarbonate. Multi-wall polycarbonate greenhouse material comes in a variety of thicknesses (8mm triple-wall, 16mm 5-wall). The construction of the material varies slightly but all include multiple walls inside of the material which form channels that hold air. This airspace is important because it creates insulation value. A polycarbonate greenhouse offers similar energy efficiency to a double pane glass greenhouse. One advantage of polycarbonate material is that it naturally diffuses the sunlight. By diffusing the sunlight, polycarbonate greenhouses reduce the likelihood of “hotspots” and give the plants in the greenhouse an even distribution of light energy. Polycarbonate panels that are slightly tinted can also be installed in the roof of the greenhouse to reduce or eliminate the need for shade cloth. This is a great option for gardeners in hot, sunny locations. Although polycarbonate is not as aesthetically pleasing as glass, these greenhouses offer the best upfront value. Polycarbonate can also be used to retrofit older greenhouses to increase efficiency.
Please remember when making a decision regarding your greenhouse glazing options that efficiency and greenhouse performance are dependent on many factors including your geographical location and the orientation of the greenhouse. Where you live and what you grow will make a big difference in your selection of a greenhouse and greenhouse glazing. Take the time to explore all of your options to find the greenhouse design that will best serve your purposes.
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Choosing an Attached or Freestanding Greenhouse and Greenhouse Orientation
When setting up a greenhouse, one of the first things a gardener needs to decide is whether the greenhouse will be a freestanding or an attached greenhouse. A closer look at some of the advantages and disadvantages of each will give a gardener a better idea of which type of greenhouse will best suit his or her needs.
Advantages of an Attached Greenhouse
One of the biggest advantages of an attached greenhouse is that they are generally more energy efficient. This is because one wall is already heated by the home. Another advantage of an attached greenhouse is accessibility. Many attached greenhouses are accessible from the living space which makes entering the greenhouse especially convenient. Attached greenhouses can also add aesthetic value to a home. In many cases, a greenhouse will enhance and/or complement the architecture of the home.
Disadvantages of an Attached Greenhouse
The disadvantages of an attached greenhouse are that attached structures will require a frost-free footer and a foundation. This can be an added cost to the project. Also, building codes are generally more stringent for structures attached to living spaces.
Advantages of a Freestanding Greenhouse
Perhaps the biggest advantage of a freestanding greenhouse is that there are fewer limitations regarding size, height and style. Another significant advantage is that freestanding greenhouses usually have better lighting conditions because they can allow light to enter on all four sides of the greenhouse.
Disadvantages of a Freestanding Greenhouse
A disadvantage of a freestanding greenhouse is an increased heating cost during colder months. Due to all four sides being exposed to the elements, freestanding greenhouses are less energy efficient compared to an attached structure. Freestanding greenhouses are also less convenient to access because the gardener must go outside to enter the greenhouse.
Location of the Greenhouse
Many gardeners believe that a greenhouse should always be located facing south so it receives the most light possible. However, this is not always the best location for a greenhouse. Although the southern exposure will receive the most light, the truth is that most plants do not need that much light and, in some ways, a southern exposure can be counterproductive. During the summer months, a south facing greenhouse can easily get too hot and require a shade cloth to help protect the plants. An overheating greenhouse can hinder plant growth and be a nightmare for the grower. For most hobbyist applications, a greenhouse with western exposure is the way to go. Think “west is the best”. This is not to say that other orientations will not work. In fact, an eastern exposure will work just fine for most plants. A northern exposure may be too shady for any tropical varieties and is the only orientation that should be avoided if possible. Since virtually any orientation will work, greenhouse hobbyists should choose a location that will best complement the home or garden. Just remember that greenhouses do not need to be exposed to the maximum amount of light at all times. In fact, most horticulturists will want sunnier and shadier locations in their greenhouses to accommodate the needs of the various plant varieties.
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A Quick Guide to Greenhouse Structure and Frame Materials
There are several frame types being used to construct greenhouses. And Depending upon your needs some are better than others. The primary materials are galvanized steel, aluminum, PVC, fiberglass and wood.
Steel is one type of framework used on kits. It is strong, and galvanized steel will be rust resistant, but in time will rust a bit. Because of the weight, it will cost more to ship than other materials. Galvanized steel tends to be used on the lower end greenhouse kits, but it can be found on the more expensive ones too.
Aluminum is strong, will not rust and is lightweight. Round or square tubing is used, depending upon the covering material that is being used. Square tubing is available from building supply stores and this material could be used to construct a greenhouse frame. Drilling holes in it is fairly easy, so fiberglass panels or polycarbonate sheets could be fastened to the structure.
PVC pipe can be used to construct a Quonset type greenhouse frame. It is readily available, inexpensive and easy to work with. Up to one inch diameter can be bent to form hoops over which Polyethylene film can be stretched. Ends can be constructed from either plywood or fiberglass sheets. Fiberglass sheets can also be screwed to the hoops to form a cover over the frame.
Treated wood can also be used to construct a greenhouse. If you are do-it-yourself grower, this type of structure can be a bit harder to build. Just about any type of covering could be installed over a wood frame. It is strong enough for glass, Polyethylene films can be stapled to it and polycarbonate or fiberglass sheets can be screwed to it. Wood is the most versatile material available for a greenhouse structure. Besides pressure treated lumber, cedar and redwood can also be used for greenhouse construction. Both are lightweight and strong. Redwood is the most readily available and tends to be used in high end kits. Greenhouses constructed from redwood are both attractive and long lasting.
Fiberglass tubing is also used as a greenhouse framing material. An advantage to this material is that it will not conduct heat or cold easily, thus helping keep heat in and cold out.
Tips for Choosing the Right Size Greenhouse
The size of a greenhouse can be determined primarily by the quantity of plants you plan to grow during the period of cold weather when additional heat and protection from frost is required. Plants that prefer warmer climates can continue to grow inside the greenhouse to produce flowers, fruits, or vegetables through cold and warm seasons. Plants, requiring shade during both cold and warm weather, can be protected from direct sun with a shade cloth. Size also will be influenced by the amount of space and money you have available.
The square footage required can be determined by laying out a floor plan that includes growing areas, walkways, and work and storage spaces not provided for in other structures. Enough space should be provided to prevent plants from touching greenhouse walls during freezing weather and to allow adequate air circulation. Plants should be reachable from walkways or from between benches and beds to allow for regular watering, fertilizing, and insect and disease control.
Walkways must accommodate the movement of workers, customers, equipment, plants, and vegetables in and out of the greenhouse. However, you want to minimize your walk, work, and storage areas as much as possible, because only the growing areas provide income or plants and vegetables for your own use.
If you are a commercial grower your greenhouse will need to accommodate trucks backing into (or at least close to) the greenhouse to load and unload. If this is the case make sure the greenhouse doors are large enough to accommodate them.
If money is tight, you can make the permanent greenhouse smaller by using less expensive temporary structures to raise early and late vegetables. Bedding plants can also be moved outside early to harden off, using temporary covers to protect them when frost is predicted.
Double and triple uses of greenhouse space allow for increased production per square foot. A simple wooden or metal frame can be built over a supply-storage area, creating more room for flats of plants. Temporary benches, filled with plants, can be set between rows of transplanted vegetables and moved as space needs change. Lettuce, spinach, radishes and other quick maturing crops can be planted early between tomato and pepper plants and harvested before later crops grow large enough to block the light.
Greenhouse rafters should be high enough to hang baskets above the heads of workers and customers, especially in walkways, yet low enough to water and fertilize regularly. They will need to be spaced far enough apart to grow without crowding and to allow light penetration to plants below.