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Most people say not to start with the hps until the plants are at least a couple of weeks old. Some think it will burn the plant or dry out the soil to fast. I used one from the beginning on my first grow and didn't seem to have any adverse effects.
You should always keep the lamp about 12-18 inches away from the plant so you don't burn it. A test that some people use also is to hold your hand just over the plant.... if your hand starts to get hot than the lamp is to close.
I also use a 400W hps, and usually keep it about 16 inches above the plant, and keep raising it as the plant grows.
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