As we all know cannabis is grown throughout early spring and late fall but I normally recommend transitioning your plants from outdoors into a greenhouse. Growing cannabis in a greenhouse removes the complex design of an indoor grow room and for many growers it is safer, and easier than growing outdoors. Greenhouses offer protection from the elements and provide a longer growing season.
Marijuana grown in a greenhouse can be set up two ways. Plants can be grown in separate containers or pots, or they can be planted straight in the ground. There are pros and cons for both options, but ultimately it comes down to the method that best fits your grow operation. Growing in containers makes it easier to move the growing plants whenever necessary. Cannabis planted directly into the high quality soil, on the other hand, will fare much better when it is left alone for a longer period of time.
Greenhouses are a popular way to cultivate cannabis. They harness the power of the sun, provide a warm climate and protect gardens from harsh environmental conditions. Past the basics, a greenhouse allows for year-round cultivation, climate control, and controlled exposure to sunlight. Greenhouses can be heated if needed and with a little bit of thought and planning growing cannabis in a greenhouse can offer better security. Greenhouses also have the benefit of allowing the grower to extend the end of their growing season. Cold and sunny days on the outside become warm and comfortable in a greenhouse and allow plants to ripen properly.
In terms of the design, there are 3 main types of greenhouses.
Attached greenhouses are connected to another structure. Because of their simplicity, these greenhouses cut down on design and material costs. The downfall of attached greenhouses, however is that they are limited by nature. They can only be placed where another building already exists.
These are a more common design. Because they are freestanding, detached greenhouses can be strategically placed to capitalize on the advantages of a specific area, like access to sun or water.
This is another example of a freestanding design, but it is geared towards large-scale production. Essentially, they are multiple greenhouses that are joined together. The large, undivided space allows for more efficient heating and cooling throughout the facility, but there are usually different sections to house and tend to seedlings, clones, and plants in different stages of growth.
Just like an indoor grow room, greenhouses do have space limitations. Whether growing in a small glass greenhouse or a huge and long poly tunnel, space needs to be planned to ensure your plants have room to catch light when they start to get big. When planning, remember that indica plants are shorter, yet bushy, with wide and fan-like leaves, whereas sativa strains can grow to great heights and are usually more on the slender side.
Cannabis like many crops likes different temperatures at different stages of growth. The air temperature in the greenhouse should be adjusted according to the level of light. In general, winter crops grow best at temperatures of about 65° to 85° F in the daytime and 60° to 75° F at night. On cloudy days, these temperature ranges should be somewhat lower, since the plant is not manufacturing as many sugars as usual. Your greenhouse heating and cooling systems need to consider the temperature needs and controls for each zone. Typically, the heat provided by the sun combined with the natural heat given off by the plants as a result of the greenhouse effect, should keep plants warm during cold weather in mild climates. No matter what kind of marijuana greenhouse growing techniques you use, you must be sure that you have proper ventilation. The temperature has to be kept at a steady level, even if you are using the hydroponics method. In extremely cold locations, it is advisable to equip your cannabis greenhouse with a heating system that controls the greenhouse temperature. Likewise, cooling and ventilation can be critical to preventing heat buildup during the warmer months.
Keeping your plants at the correct humidity levels is key to keeping your plants safe through the seasons. Most young plants and seedlings tend to react better to a slightly more moist environments or 60 – 70% humidity. As the plant matures, it is best to slightly dial down the humidity, shooting for around 40% by the time your plants are fully flowered (around week nine). Too much humidity invites disease and too little dries out the plant and hinders growth. Cannabis in the vegetative stage actually likes a higher humidity level and prefers lower humidity when in flower.
Every greenhouse needs to have a ventilation system and circulation fans installed to produce optimal growing conditions. Ventilation is the exchange of inside air for outside air to control temperature, remove moisture, or replenish carbon dioxide (CO2). Greenhouse ventilation breaks down into two main categories: 1) natural ventilation covers roof vents, sidewall vents and rollup sides, and 2) forced air ventilation requires mechanical systems like exhaust fans, and sometimes cooling pads. Exhaust fans should be sized to exchange the total volume of air in the greenhouse each minute. The total volume of air in a medium to large greenhouse can be estimated by multiplying the floor area times 8.0 (the average height of a greenhouse). In winter, 20% – 30% of one (1) air volume exchange per minute is sufficient for mixing in cool air without chilling the plants.
CO2 enrichment in greenhouses allows crops to meet there photosynthesis potential. Enriching the air with CO2 can be done by means of the combustion of natural gas or with liquid CO2. The supply of extra carbon dioxide is an often applied method to increase the yield of greenhouse crops. The amount of carbon dioxide in the outside air is, depending on your location, 350 parts per million. This amount is sufficient for plants to grow, however when a high number of plants are placed in a greenhouse, the carbon dioxide levels drop as all plants are using carbon dioxide for photosynthesis. By adding CO2 (CO2 enrichment), it is possible to increase the photosynthesis potential of the crops, especially on sunny days. Keep in mind that yields will only increase if CO2 is a ‘limiting factor’. This means that if all the other variables are not optimal (light, fertilizer, temp/humidity, pH, etc.) you will not achieve the benefits of increased CO2 levels.
CO2 enrichment will not be as effective if the grow area is not sealed because the CO2 will be exhausted before the plants can use it. Ideally, air should not be exchanged in and out of a grow room. If you have an air-cooled reflector, the air drawn through the fan to cool the bulb must enter and exit the grow area without coming in contact with the air in the grow room. Sealing the grow space allows plants to more completely use the CO2 that is enriching the environment. During the night cycle plants actually give off CO2 which causes a gradual increase in CO2 levels until the lights come on and the plants resume absorbing CO2. You can save CO2 by waiting an hour or so into your daylight cycle to turn on the CO2 device.
Typically an indoor grow room will have wall mounted oscillating fans blowing back and forth with no real ‘direction’ of the air. A greenhouse uses Horizontal Air Flow or HAF which is fans mounted above the canopy that create a directional flow. One side of the greenhouse will have the fans point in one direction while the other side of the house will have the fans blowing in the other direction. This creates a vortex of air movement that is more effective than an oscillating fan.
Electric fan heaters are particularly good for moving the air around the structure which helps to prevent cold spots and reduce the risk of disease. If the greenhouse doesn’t have power then a paraffin heater is a better option. Save money and energy by only heating greenhouses when necessary. Most electric greenhouse heaters come with a built in thermostat that can be set to come on only when the temperatures drop below a certain point. Heat circulation is important when using an electric heater. Moving the warm air around will prevent hot spots (and their contrasting cold spots) as well in reducing condensation that heating encourages.
Large, commercial-sized greenhouses are finding wood as an alternative to compensate for expensive gas and petroleum products. When installing a wood stove in the greenhouse be sure to follow all your local code requirements. Standalone pellet stoves are especially easy to load and operate and most come with some kind of temperature control. Some have blowers to circulate heat.
Optimize the soil. This can mean replacing all the soil with quality compost or just adding some manure, organic fertilizers, bat guano etc. In dry areas some people add water absorbing crystals to the soil which allow the soil to hold more water than normal. Soil testing kits should be used to determine soil quality, pH, etc.. The most successful greenhouse cannabis growers know that a small amount of time spent optimizing the soil conditions will encourage outstanding cannabis plants with heavy harvests of potent buds.
Most greenhouse cannabis growers prefer to grow cannabis directly in the soil. This allows unrestricted root growth and the potential to grow massive cannabis plants which can be eight to ten feet high and just as wide. A single plant like this can often produce over two pounds of cannabis. However some cannabis growers prefer the flexibility of growing their plants in containers, allowing them the flexibility to move the plants if required. The biggest problem with growing in containers is the need to continually water/feed them while plants grown directly in the soil need much less attention.
Victor Krantz is a master grower for The Bucket Company. He has been involved with the agriculture industry for 30 years producing a wide variety of food crops and setting up commercial greenhouses in many different states. You can find them on Instagram @thebucketcompany or @cookiesandcreamfarm. Victor may be contacted directly at 727.424.0132.