A screen of netting is used to hold down all new growth until the plants form a mat. Appropriately called “screen of green,” one or two plants may fill an entire grow space. Letting your plants grow untouched during the vegetative phase, then topping just prior to flowering also controls height.
When your plants are about to enter the flowering stage, top all branches. This has the effect of reining in the flower stretch. The plants are urged to produce new flower growth instead of focussing energy on stretching out. Occasionally, pre-flower topping needs to be a full pre-flower pruning. If your plants fill the grow space during the vegetative phase, there is no way there will be enough room for flowering. Aggressively prune the plants to half their size and switch lights to the 12/12 flower cycle. Don’t worry, cannabis is very hardy and can even recover from being reduced to a stalk with only one or two leaves remaining.
Try not to beat yourself up about the loss of time and reduced final yields. All lessons are good lessons on the way to becoming a weed growing expert. Cannabis plants transpire a substantial amount of water. Marijuana prefers a relative humidity (RH) of 50-65%. When they get too big, humidity can become a problem as the plants constantly release water vapor into the air. Water then gathers on leaves and other surfaces in the grow room, potentially causing mould and other issues. If humidity is becoming a problem, the use of a dehumidifier or increasing exhaust fan power can help. Defoliating can also increase airflow to control humidity. This has the added benefit of increasing light penetration to the lower flowering branches. As with humidity, when plants get too big, temperature in the grow-op can rise. Reduced airflow and excessive height can drive up temperature and position plants too close to grow lights. Cannabis thrives when the temperature averages 25°C. When temps exceed this, plants are more likely to stretch. High temperatures can also affect leaf and flower formations, as well as final bud flavours and potency. If your plants are getting too tall or too hot and you have the room, simply raise the grow lights as high as possible. Remember, lights that are too far from the tops of your plants can also cause stretching, so make sure to achieve the ideal light positioning. Light intensity can be adjusted as a method for controlling growth speed and height. If your plants are getting unruly, reduce the intensity. If you are using more than one lamp, simply turn some off. During the flowering phase, growth can be controlled by reducing the photoperiod - but only if your plants have been flowering for at least six weeks. Reducing leaf production and stem stretching can be achieved by reducing the “lights-on” period from 12 hours to 11 or 10 hours. Appropriate strain choice can help avoid height issues before they even arise. If you have limited space, your dreams of cultivating a classic sativa will need to remain just that, dreams. Stout indicas and hybrids or autoflowering genetics can keep heights under control. Many autoflowering strains are ideal for indoor growing as they have relatively no vegetative period. Some strains will not grow taller than 50cm, but will still provide excellent yields per m².
With a little bit of consideration and forethought, height need not be an issue when growing marijuana.
Some research into strain characteristics is the first, very important step. Then, appropriate growing techniques will make sure you get lots of yummy buds with no drama. One of the best things about growing your own crops is the option to experiment with growing different varieties, often more flavourful than those found commercially. Genetic variation between different strains of the same species affect most of the variables in the final product including smell, taste, colour and strength of active ingredients.