What's the ideal height to hang a 400W lamp over two week old plants?

well in the veg stage you really want to use a fluorescent light it is the best becuse with the hps it gets to hot and can burn them... the fluorescent light well not burn them becuase it do not give off that much heat... but if you want to go and buy that fluorescent light it is about eight to fifteen dollars for the lamp and then it is about five dollars for the light that are made for plants... if you want to do that then place the lamps 2 to 3" away from the plant becuase if you don't it will cause it to grow really tall and reach for the light and the stems will be weak and not be able to grow big buds... the hps light are better for flowering becuase the light dosen't have to be as close becuase it not growing as much it is more flowering and it needs more heat and the hps lights give off more heat.

keep the light 12-24" away, I keep mine about 18" away, you are useing a mh for veg right

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supercalavagalisticexpialadankbitch... I do not grow marijuana anymore

The light I'm referring to is Metal Halide, meant for vegagtive growth. At the moment, it's hung about 3' above the tops of the plants. If you were to place your hand next to the tops of the plants, you wouldn't feel much heat from the lamp at all. Luke-warm at best. If it's not too hot to my hand, is it safe to assume the plants would feel the same?

that is correct you can probably move it down a lil bit 3' seems kinda far you dont want em to stretch to much try movin it down like 12" than check on it again after an hour like you did with your hand if it still feels good try movin it down a few more inches like i said i have a 400w and keep it at 18" I also suggest a fan blowing accross just under the light. but if you dont have one you will be ok just keep checkin it with your hand

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supercalavagalisticexpialadankbitch... I do not grow marijuana anymore

At two weeks healthy seedlings can handle a little heat. If they haven't been getting any at all since the light is 3' away , start by moving it down about a foot today, six inches more tomorrow, then maybe an inch or so a day. The plants will grow up to meet it, and they will 'tell' you if you get it too close.
If you keep it to far away they will stretch. If that happens stop by, I have some skewers I use to prop them up. No i never use them. Well, maybe once..... or maybe a few more. VV