i started off with a few clones that were already rooted when i got them 4 weeks ago. I'm using soil and a bunch of CFLs. I stayed away from nutes based on the advice of a friend until a couple days ago. i am using some 8-7-6 miracle grow. i got the clones from a friend that grows so i assume they are pretty good genetics. the plants look very healthy (aside from a light burn or two) and they are approx. 18" tall. they're still on 24/0 and I was wondering how i can tell when they are mature enough or big enough to start blooming. they have been preflowering for about 2 weeks now i'd guess. Also, as an aside, when is the best time to clone these beauties?
To answer the second question first; you can take cuttings pretty much any time that the plant has enough branches with several nodes on them. The main criteria for successfully rooting cuttings is to make sure you have at least one, but preferably two or more nodes that will be in the dirt (or other medium) and a couple of sets of leaves to support the cutting until the roots develop. (Note: you can not root fan leaves) While I don't know how your plant(s) look, I'd say that 18" tall is plenty big to begin the flowering cycle...especially if it is bushy as well as tall. Once the plant enters the flowering cycle for real, it will put on a growth spurt and possibly double in height. I veg my clones for about 5 wks before putting them out to flower.