Marijuana plants that have stretching stems are problematic for grow rooms and outdoor gardens in which height is a major concern.
Marijuana plants stretch for a variety of reasons. Genetics is the most obvious reason. Some plants are programmed to produce long stems no matter how much you try to prevent it. Sativa strains that grow tall outside will do the same when indoors. Excluding genetic factors, there are some things that gardeners can do to decrease stem length between leaves.
Strong air circulation can create stem and leaf movement that strengthens and widens the stem while also slowing its vertical growth. Winds will bend the stem, creating infinitesimal tears in the plant's tissue. The plant will then quickly mend these small tears by growing new tissue. Simply brushing or bending the leaves and stem can mimic the benefits provided by air circulation and, thus, widen and strengthen the stem. Download my free marijuana grow bible and learn more about growing marijuana.
The light spectrum can also serve a purpose when it comes to controlling height. Blue light will promote shorter, sturdier stems while red and orange light will stimulate longer, skinnier stems. If exposed to metal halide lamps during vegetative growth, plants will have shorter stems. If exposed to high-pressure sodium lamps, they will have longer stems.
Infrared light, which is invisible to human eyes, can also be used to help maintain plant height. It is used when the lights are off and is produced by heat lamps (which also emit some red light). A black cotton cloth will cover the heat lamp reflector, allowing the infrared rays to produce the desired effect while also capturing red light to avoid negating the effect. Even so, infrared light can also induce flowering and should really only be used during flowering.
Heat can also affect stem growth. As the temperature rises, marijuana plants will naturally grow longer stems. At around 60*F (15*C), plant growth slows down and the stems tend to be thicker. At 80*F (27*C) you'll notice that both buds and lower stems will start to stretch.
Buds that extend too close to a light might become airy or lanky. Some growers will mistakenly associate this phenomenon with "light burn." But, this isn't the real problem. What really happens is that the lamp's heat creates an unsustainably hot atmosphere in the bud zone. Some solutions include using an air-cooled lamp that will prevent a large part of the meat from even entering the room. Likewise, water-cooled lamps will capture virtually all of the lamp heat so that buds can be very close to the light without the chance of burning.
Commercial greenhouses also use a method called temperature inversion. This basically entails keeping the temperature higher during dark periods and lower during light periods. Maintaining a temperature in the low 70's (21-23*C) during the light period and increasing that temperature to around 80*F (27*C) at night will slow vertical growth and will not affect yield adversely.
Lack of adequate light will cause stem elongation. Seedlings will grow long, thin stems in an attempt to reach more light. To prevent this, supply a much more intense light regimen or at least put the seedling closer to the light source. If the seedlings have already stretched, try to support them using wooden skewers. After providing much more intense supply of light, the stems will fill out and will be able to support themselves on their own. Older plants will also stretch toward the light source if they are suffering from light deprivation. In this case, buds grow airy, they won't tighten up, and they also have scant trichome coverage. Adding extra bulbs or moving the light closer to the plants can prove beneficial.
Pruning can also be utilized to decrease stem length. Excising the very top of the main stem forces the surrounding branch to grow. These ancillary branches will not grow as long or as tall as the main stem. You can also just bend the top branch until it snaps and hangs lower.
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