Elongated Stretch in Cannabis Stems
Controlling how much a cannabis plants stem stretches is usually a lighting, or nutrient issue. When they're in their seedling stage stems will stretch out for a variety of reasons, but generally because the light is too high and they are trying to "reach" up towards it. Lowering your lights can keep you from experiencing this problem.
But, first, let's talk about how to fix the problem if it occurs to you. The first thing you want to do is make sure the soil is dry. If it's wet, the roots will have a tendency to cling and you don't want them to be damaged in any way. Then, very carefully remove the marijuana seedling from its current container. Clean off the roots so that there is no more soil attached.
You may want to use a deeper container or just dump the soil out from the seedling's current container. Leave a little bit of soil in the bottom of whatever container you decide to use so that the roots won't try to grow through the drain holes. Place the seedling down in the container in such a way that you can bury the stem. It is ideal to bury the stem almost completely so that only the Cotyledon leaves are showing. After that, provide the marijuana plant with just a touch of water to get it going again.
At this point, your stem will be buried and it will eventually start to grow roots of its own. This is something that can apply to little seedlings and even incredibly long, emaciated stems. Again, as long as you're extremely gentle with the plant, you should cause no harm and the plant will not be stressed.
To avoid stem stretch make sure your marijuana seedlings get enough light an place them under a cfl bulb (3 watt per seedling). Distance between plant and cfl bulb should be around 2 inch. Distance between plant and HPS bulb should be around 40 inch for seedlings (20 inch for vegging plants) but make sure temperatures don't exceed 77 ºF.