I could tell you to use the search function, but since I'm here I'll go ahead with it! They need to be at least 12 in. tall, and once you see preflowers forming(i think after 8 nodes grow they start to form, have to check my notes to be sure) You can put them into flower before you see the preflowers, but it will still grow until its ready to flower. But still do a search for when to flower and you'll find some more info. Hope I helped