wait until a few true leaves pop up.. and even then, do it like.. every other watering and keep an eye on them.. and I would start with like 1/8, then go from there.. like I have some 10-10-10 indoor MG fert that I got a while back, and it says use 7 drops per plant per liter of water.. so I started using 1 drop and now I use 2 drops every other watering.. and I also use something close to... 1/6th or something like that of Cal-Mag solution in there.. I take a straw and just dip it in so the straw is filled up about 1/2inch... working pretty well so far, but I don't plan on going any higher till they go into actual party cups or you next larger containers.