I flush or clear all my crops. I have done it both ways & found it does make a big difference. Weed that is not cleared is harsher & has a stronge nute taste, which I dont like. Some growers dont mind that. I just use pure PH adjusted water from 1 to 2 weeks, than harvest. When you are clearing, use your ppm meter & keep track of the rise of the ppm's in the res.. When they stop going up, replace the water in the system with ph adjusted water & repeat the process as many times as needed until the ppm's dont change at all from what they were with fresh water. I like to clear a few days longer after that to make sure all of the nutes are out, than harvest. It sound hard but it's not. You will be happy with how smooth it is & how tasty the smoke is. Later Man.