by Sama’a Djomehri
As medical and recreational cannabis is passed into law in more states across the USA, greenhouse cultivation is emerging as the preferred and ideal way to grow. This is because it preserves the best of both worlds- harnessing sunlight, while maintaining a climate controlled and contaminant free growing space.
There are many different greenhouse designs and technologies which have developed due to the varied climates found in the different regions of the US. As the cultivation of high-THC, pharmaceutical cannabis in Greece is now permitted only within greenhouses, we can use the experiences and knowledge of these US cultivators to design greenhouses for our climate conditions which will maintain an ideal plant environment and produce top quality medicine.
The most fundamental difference in greenhouse design is a seasonal production facility, utilizing the cannabis plant’s natural growing cycle with no supplemental grow lamps, versus a year-round cultivation, requiring artificial light.
This Greenhouse style is used predominantly in California, because of the abundant sunshine and warm and dry climate. California is the largest producer of cannabis in the US. Soft polyurethane plastic has better light transmission and light diffusion factors than glass or polycarbonate, hence this material is preferred as a greenhouse cover. Seeds are usually germinated in early March, in a greenhouse with a heat distribution system regulated by a temperature sensor keeping the levels around 30°C. If clones are used, they are usually started inside. Because they are cut from a larger plant, they have a sensitive photoperiod and may go into flowering prematurely if they do not receive a long enough period of light. It is safe to put most varieties of clones in a greenhouse with no supplemental light only after March 21st, the Spring Equinox; to be certain, most growers in the US only remove clones from supplemental light after April 1st.
A warm and humid environment is created inside the greenhouse; this is ideal for young, vegetative plants, which also do not require intensive light levels compared to later stages. Fresh air exchange is also important during the day to discourage detrimental molds and other plant diseases. Exhaust fans extracting stale air from the interior work well; a low-tech and inexpensive option is simply to have screened ventilation windows which are opened during the daytime and closed at night.
Some seasonal greenhouse cultivators go for just one round of plants; starting them in March and harvesting at the beginning of October. These plants will become huge: many greenhouses in California have had plants reach as high as 5 meters. There are numerous creative methods of pruning, trellising, tying down and supporting the branches to use the space inside the greenhouse as efficiently as possible, and prevent the tops of the plants from touching the ceiling. Growers using this technique usually have less plants, but each plant can potentially yield up to 6 kilos.
Other seasonal greenhouse cultivators aim for 2-3 harvests per season. This involves applying a technique called Light Deprivation. Seeds or clones that are started in early March are placed into another greenhouse at the beginning of April; this greenhouse is equipped with “black-out” curtains, which when drawn, block out all outside light. The curtains are placed on a pulley and cable system, which is operated either manually or automatically, and the sunlight the plants receive is restricted to 12 hours a day. This triggers the plants to begin their flowering cycle early. Without this deprivation of light, the plants start to flower naturally only in the first week of August. The plants undergoing light deprivation are ready to harvest after 8 weeks of this artificially imposed 12-hour light cycle.
Meanwhile, the grower has started a new set of seeds or clones in the nursery greenhouse. When the first batch of plants is harvested at the beginning of June, this new set of plants is put into the light deprivation greenhouse, and the cycle begins again. The second harvest is ready by the beginning of August, and the grower can choose to have a third round of plants ready to put in the greenhouse at this time. Because this third group of plants is placed in the greenhouse at the time when the shorter days naturally trigger them into flowering, no black-out curtain is required; they will start to flower on their own, and will be ready at the beginning of October, the traditional harvest time.
In areas of the US which have less sunshine, and colder, wetter weather, seasonal greenhouses will not provide high-yielding or good quality medicine. To produce the same quantity and quality of medicine, growers in these areas opt for year-round cultivation in climate-controlled, insulated spaces. The vegetative and flowering cycles of the plants must be artificially manipulated for year-round cultivation; this requires both supplemental light systems to extend the day to keep plants in vegetative growth, as well as a light deprivation system to trigger the plants into flowering as needed. Using these methods, it is possible to obtain between 5-6 harvests per year, depending on how large the grower wants the individual plants.
In mountainous areas like Colorado, with a short sunlight season, and lots of snow and wind in winter, greenhouses are mostly constructed out of glass or polycarbonate rather than soft plastic. They can be dug into the ground on the northern side, using the earth as insulation, or feature a solid insulated northern wall. This helps retain heat, especially since temperatures can go below freezing regularly. Heating systems, such as air heaters, radiant heat from warm water pipes, and Ground to Air Heat Transfer, can all be used to maintain appropriate temperatures during the cold months.
During hot months, cooling systems must be implemented. Evaporative cooling techniques work well for dry climates such as Colorado, California, and the Southwestern US, these include wet pad and fans, and high-pressure fog systems combined with powerful extractor fans providing continuous ventilation. Other equipment such as refrigerant-based chillers, do not rely on water vapor. These are a necessary option for regions such as South Carolina or Michigan, which are extremely humid during the summer. During the flowering stage, cannabis requires relative humidity levels of 60% or below to prevent mold and rot issues.
In light of the low humidity requirements for flowering cannabis, another vital system for year-round cultivation greenhouses is dehumidification; this is important in cold, wet weather, as well as hot, humid weather. Many greenhouses in high-precipitation areas such as Oregon, Washington, Michigan, Maine, and the Southeastern part of the US, implement dehumidification in both winter and summer, areas such as Colorado and California mainly during the winter.
Greece is a country with many diverse micro-climates as well. By studying techniques used in areas of the US which closely resemble climates here, Greek medical cannabis growers can apply these ideas as a guide to designing greenhouse facilities ideally suited to their specific regions, producing high-yielding plants with good structure and maximum medicinal potential.
The Green Greeks Magazine