I just used a 400w on my first grow and I'd say one plant would prb thrive greatly as we started 10 and culled 6 males so we flowered 4 females the full time.. Your gonna wanna strip the lower stuff as it wont get much light I'm pretty sure no matter really how tall you get it, because unless the relector is air cooled your gonna have to have you light about 12" off the tops anyway, so I'd prb be stripping the bottom like 4-6" of branches... With my first grow I let it just be all natural and the lower shit was so popcorn-y and such it really was not even worth keeping..