Re Vegging Your Marijuana Plant
Re vegging means putting a plant back into its vegetative state after taking a harvest from it. The advantage over starting from fresh seed is that you know what you're getting both in terms of sex (i.e. you know the plant is female) and in terms of product. Revegging can be used to create a mother plant from a plant which has given you particularly good results, or just to get another harvest (or even multiple harvests) from one plant.
If you are thinking of re vegging, you will need to plan ahead. Re vegging is a shock to the plant so it needs to be in a really healthy state to begin with. In particular, it needs to have plenty of nitrogen as it is growing up to ensure that it is able to keep its leaves after harvesting, otherwise it will simply die. You will also need to be prepared to leave at least the bottom 10th of the plant intact, a quarter to a third would be better. Once you have harvested, give the plant an extra dose of nitrogen-rich plant food to help keep the remaining foliage in good condition.
If you are growing in soil, you want it to be moist rather than completely dry. The aim is to slide out the plant with the root structure intact. Then you clean off the soil so that you can trim back the root structure, removing any brown or otherwise damaged roots as you go. You can reduce the overall root mass in line with the overall reduction in the size of the plant. The more plant you have left, the more roots you leave.
Repot the plant and give it plenty of care and attention. Start off with gentle feeding as it's going to be in shock and at least 18 hours of light a day, 20 to 24 is better. As you see your plant start to re-establish itself, go back to the feeding you used when you were growing it originally. The initial growth is likely to look a bit misshapen and peculiar, but after a couple of weeks, your plant will return largely to normal. It is however quite common for the regrowth to be somewhat spindly and unkempt, in which case you should look to train your plant into a healthier shape.