Professional Marijuana Grower


The Best Artificial Light Source for the Flowering Stage of Marijuana

Posted June 5th, 2017 by Eric Hopper in

The pinnacle of indoor marijuana production occurs in the flowering stage of growth. After waiting weeks for the plants to reach the desired height, a grower will introduce his or her plants to a 12 hour photoperiod, thus initiating the flowering stage. The flowering stage is the final stage of growth and is when marijuana plants produce the sticky, sought after buds. The flowering stage can last eight weeks or longer, which means the lighting system used in this stage is quite possibly the most influential factor over the way the buds develop. Multiple light technologies can be used successfully for flowering cannabis. Growers who understand the basics of the different lighting technologies will know when and where to use each and this knowledge will lead to a higher likelihood of success.

Light Intensity

Before discussing each individual lighting technology, it must first be noted that, regardless of the light technology being used, the secret to success is correctly utilizing the light energy. Light intensity, as it relates to indoor horticulture, is referring to the amount of energy available to the plants. One of the biggest mistakes a grower can make in the flowering stage is not providing the plants with enough light intensity. A general rule of thumb is to provide a minimum of 40 watts of light energy per square foot of garden space. After a marijuana plant enters the flowering stage, it will continue to grow so it is important to provide the light intensity that is adequate for the plant’s finishing size.

In other words, in the beginning of the flowering stage, the light energy may be somewhat disproportionate. The total light energy needed will be determined by the finishing size of the plant, not the size of the plant when it enters the flowering stage. Some growers will increase the light energy incrementally as the plant grows in the flowering stage, but many growers find that starting with the higher light intensity (corresponding to the plant’s finishing size) so the plants can “grow into” the space is the most effective way to achieve the best results.

Another aspect of lighting intensity is the distance between the artificial light source and the top of the plant canopy. The inverse square law states that the farther light travels from its source, the more it diminishes. Put another way, when the plant canopy is farther away from the light source there is less usable light energy available to the plants. However, due to the extreme heat and intensity of some artificial light sources, plants can become damaged if they are placed too close to the light source. The best way for a grower to make the most efficient use of his or her available light energy is to place the plants as close to the light source as possible without causing damage to the plants. Bleached or white flower tops are strong indicators that the plants are getting “over saturated” and are too close to the light source.

Artificial Light Technologies for Flowering Marijuana

There are a few different light technologies that can be used successfully to flower cannabis plants. Each technology offers its own advantages and disadvantages. Some garden set ups are simply more compatible with certain lighting technologies than others. In fact, the grow room and the type of growing system being used are both key factors when determining which lighting system would be the most efficient choice.

High Intensity Discharge (HID)

High intensity discharge lighting includes both metal halide (MH) and high pressure sodium (HPS). HID lighting consists of three components: the ballast, the reflector or socket, and the bulb. Ballasts can be specific to the type of bulb (MH only or HPS only) or they can be switchable, or “smart” ballasts, and have the ability to operate either type of bulb. There are also double ended HID bulbs which have increased efficiency and longevity over other bulbs in the HID category.

HID lighting is still the most popular choice for flowering cannabis for a few reasons. First, this technology has been around for a long time and is used in other industries. This means there are many manufacturers which helps lower the cost and creates newer, more efficient designs. Initial cost and reliability are not the only reasons marijuana growers continue to choose HID lighting. High wattage HID fixtures are very intense and are capable of penetrating deep into a plant canopy while also covering a large area. Due to state regulations, many medical marijuana growers are allowed a limited number of plants. To maximize production, many growers typically try to get the plants as large as possible. HID lighting is perfect for cultivating large monster plants because their intensity allows large plants to develop more fully. When using other lighting technologies on tall, monster plants the top portions of the plants will develop properly, but much of the undergrowth will never mature.

Light Emitting Diodes (LEDs)

Unlike other lighting technologies used in marijuana horticulture, LEDs are a solid state device which makes them extremely durable. LED fixtures usually consist of a panel of LEDs and a circuit board (generally housed within the lighting fixture). Many horticultural LEDs contain a heat sink and/or fan to help dissipate heat and

increase the fixture’s life span. The biggest advantages of LED technology are longevity and the ability to customize the light spectrum. Plants have a heightened response to particular light wavelengths and LED technology has the ability to provide higher amounts of the particular wavelengths plants desire.

LEDs are the only technology that has the potential to manipulate the ratio of these wavelengths into the perfect ratio for photosynthesis. In other words, LEDs are more efficient at converting electricity into the light energy most used by cannabis plants. When flowering, marijuana plants prefer a higher ratio of red light to other spectrums and LED lighting systems are capable of providing the ideal light ratios for the final stages of growth. Although large plants can be grown under LED fixtures, the most efficient use for this lighting technology is a SOG, or sea of green, method. This method utilizes many small plants which never reach a tall stature. Instead, the entire light system’s footprint is filled with small flowering plants. SOG growing is one of the most efficient ways to maximize the given light energy.  When teamed with LED lighting systems, the overall efficiency of the yield (gram of finished material per watt of energy consumed) is virtually untouchable by any other lighting technology.


High efficiency fluorescents, like the T5 style, are very popular among indoor horticulturists for vegetative lighting or for seedlings and clones. However, in particular situations, they can be used as an effective flowering lighting system. As with LEDs, if a SOG style garden is implemented, T5 fluorescents can produce extremely impressive yields in terms of grams of finished material per watt of energy consumed. However, if large plants are grown, fluorescents typically lack the punch to deliver enough light energy to the lower sections of the plants. One of the most efficient marijuana garden operations I ever saw utilized T5 fluorescents with a series of shelving racks. This allowed the grower to have multiple gardens stacked upon one another and was an extremely efficient use of floor space.

Induction Fluorescents

Unlike HID and standard fluorescents, which rely on electrodes to bring electricity into the bulb, induction lighting utilizes microwave or radio frequencies to pass through the sealed bulb and excite the metals and gases sealed within. Induction fluorescents can be used to flower marijuana in the same way as standard fluorescents, but they have one large advantage: longevity. Because of their unique design, induction fluorescents break down very slowly. This allows them to retain a high CRI (color rendition index) and high amount of PAR (photosynthetically active radiation) for an extremely long time. What this equates to is more uniform growth in the garden over the course of multiple years. Like LEDs and T5 fluorescents, induction fluorescent are best for SOG type gardens.

Sulfur Plasma

Sulfur plasma is an induction technology that emits all of its light from a small quartz sphere. The most promising thing about sulfur plasma is that it has the intensity of HID lighting with a more suitable light spectrum for plant growth. In fact, of all the artificial light sources, sulfur plasma’s spectral light output is the closest to that of the sun. In time sulfur plasma may just become the ultimate lighting technology for growing marijuana. As with HID lighting, coverage and plant canopy penetration are not issues for sulfur plasma. However, sulfur plasma is the newest lighting technology to be introduced to the horticultural industry and has a very high initial cost. As more manufacturers produce sulfur plasma lighting, there will be a reduction in cost and, hopefully, a continued increase in efficiency. There is no doubt that the future of cannabis horticulture will include sulfur plasma lighting systems.

Supplemental UV

Ultra violet light (UV) has been shown to increase essential oil production in cannabis plants during the flowering stage. The trichomes which house the majority of the THC in a cannabis plant actually act as a protectant against too much UV light. As levels of UV light increase, a plant will increase its production of essential oils as a defense. In order to increase resin production, many cannabis growers are supplementing UV lights in their flowering rooms. Although this can be done throughout the entire flowering stage, it is usually most effective in the mid to late parts of flowering.

The lighting system used in the flowering stage of marijuana production is perhaps the most important piece of equipment in an indoor garden. A grower’s particular desires or the type of grow room will influence which is the most suitable lighting system for flowering. The flowering stage is the final stage of growth in the life of a marijuana plant. Growers who provide enough light energy during this critical time of development will be rewarded with a healthier growth and larger yields.

Growers with limited space or those using the SOG method can use the gram of finished product per watt of energy consumed equation as a gauge to determine how well their lighting system is performing in the flowering stage. One of the most important things a marijuana grower can remember is that every garden has a limited amount of light energy. Making the most efficient use of that light energy is the key to achieving a garden’s full potential.

Eric Hopper is a Professional Marijuana Grower Senior Editor.

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