the end of there 3rd week after sprouting the plant has finished using up all the nutrients within the seed from sprouting to that point from then on u can begin a feeding regime always starting out at least 1/2 to 1/3rd strength the recomended dosage on the nutrients instructions after that it should be good anytime to switch off secondary growth is a good indicater of vegitative growth taking off so keep an eye out once they begin veging they take off good luck