Cultivation Depending on location, outdoor growers may need to force their cannabis plants to flower. Forcing cannabis plants to flower is particularly useful to growers outside the warm-temperate zones. In the warm-temperate zones, forcing is rarely necessary, as the natural seasonal light cycles are optimal for cannabis growth.
Depending on location, outdoor growers may need to force their cannabis plants to flower. Forcing cannabis plants to flower is particularly useful to growers outside the warm-temperate zones. In the warm-temperate zones, forcing is rarely necessary, as the natural seasonal light cycles are optimal for cannabis growth.
Why do extreme-climate growers force-flower cannabis?
Growers in the extreme northerly and southerly regions of the planet (such as Alaska, Scandinavia, southern Chile and Argentina) may force their plants to begin flowering early. Allowing plants in these areas to naturally commence flowering (when day length decreases to around twelve hours) will not give them enough time to complete flowering by the time the winter cold sets in and plants die off.
Growers in the equatorial zones may also force their plants to flower, in order to make them complete their life cycle more rapidly. In some cases, this can allow for multiple crops to be harvested in one year, rather than just one. Equatorial cannabis varieties will naturally go through a long period of vegetative growth before flowering, and can reach giant proportions. Thus, forcing flowering can not only speed up the harvest, but can also ensure plants remain smaller and easier to conceal.
Why do temperate-zone growers force-flower cannabis?
In the temperate zones, forcing plants to flower early is generally unnecessary. However, it may be highly advantageous for growers closer to the tropics—in the warmest reaches of the temperate zones, temperatures and growing season may allow for two or even three harvests per year. This is the case in many parts of the Southern USA, as well as in some Mediterranean regions and parts of Central Asia.
Growers in the colder reaches of the temperate zones may be advised to force flowering a little early, as often the growing season in such regions falls two or three weeks short of being ideal. Much of northern Europe, including the UK and the Netherlands, falls into this zone.
Urban growers may also need to force flowering to commence if there is a lack of total darkness during the night-time hours. People growing on balconies on well-lit streets are particularly prone to light pollution in the night-time hours.
How can outdoor cannabis plants be forced to flower?
To force outdoor cannabis plants to flower, they need to be given at least twelve hours of total darkness per day. This is usually achieved by “capping” plants with lightproof material of some kind—Commando cloth or even simple cardboard boxes are common choices. The plants are “capped” at the same time every evening, and “uncapped” at the same time each morning.
If possible, using one large cap which contains the entire crop is advisable, rather than capping each plant individually. This makes the job far easier, allows for greater circulation of air, and makes it possible to position fans or dehumidifiers inside the cap. It may also make it possible for the process to be mechanised. Some growers will repurpose blackout tents typically used for photography or camping in order to cap multiple plants at once, but it is also possible and perhaps cheaper to construct a cap by fixing blackout fabric to a simple frame.
When should cannabis plants be “capped” and “uncapped”?
In most locations, providing twelve solid hours of darkness is sufficient to force cannabis plants to flower. However, force-flowering in the equatorial regions may require a longer period of darkness (up to thirteen or fourteen hours) to be successful, as night length is typically around twelve hours the whole year round.
Most growers will uncap at dawn, but in warm temperate zones this may not be advisable—if dawn occurs at 5am, and plants are recapped at 5pm, there may still be several hours of intense sunlight hitting the outside of the cap and increasing temperatures to dangerous levels. Thus, uncapping and capping later in the day (around 10am-10pm) takes advantage of the typically-cooler morning, while roughly corresponding on the onset of night-time. Capping times may need to be adjusted as the season progresses.
Potential disadvantages of forcing cannabis to flower
Capping plants with heavy black material during the daytime can send temperatures within the cap soaring—to get around this it is advisable to use pale-coloured fabrics that reflect heat. Some white and beige blackout fabrics provide up to 94% opacity, which should be sufficient to force flowering while also reflecting significant amounts of thermal energy. It is also possible to use two layers of fabric, black on the inside and white on the outside, to ensure near-total blackout while also reflecting heat. If the grow site is supplied with electricity (balcony grows, for example) then use of fans may also prevent serious overheating.
Another issue caused by capping plants is mould. Capping plants severely reduces the airflow around the leaves and soil, and good airflow is crucial for carrying away excess moisture—when left to condense and settle on the leaves and soil, excess moisture can provide the ideal environment for mould growth, particularly as night-time temperatures drop. To minimise this risk, fitting dehumidifiers if possible is a good solution. Choosing a blackout material that is as breathable as possible may also slightly reduce mould growth. If budget allows, specialised breathable blackout fabrics are available.
Another downside of forcing cannabis to flower early may be decreased plant size and a corresponding reduction in yield. This only really applies to warm-temperate and tropical growers—indeed, not forcing plants to flower in the extreme north and south will be ultimately more damaging to yield, and may even jeopardise the chance of getting any harvest at all. But for growers weighing up the pros and cons of multiple harvests per year, this becomes a serious consideration.
As with most aspects of cannabis cultivation, trial and error—and the benefit of experience—provide valuable insights into the requirements of a particular strain, and how best to successfully flower it in a given climate zone.