Twit once suggested that you take as much of the stolon as you can, when taking off the chick. So I've been doing that, and notice it does seem to make a difference. As far as when, I feel like that depends on the type of semp. You just have to experiment. I noticed that with rollers, they seem ready when they're smaller/younger. I also read on one of the threads that leaving the chicks attached helps them get a better start. Personally if I know I'm going to be around and able to water, shade or whatever else they may need, and I want to increase the amount of a certain type, I take the chick whenever I want to!
