Will my plants begin to stretch if their 10 and a half inches away from my light? I'm using a 400w HPS in an Air cooled hood. Thanks.
Put your hand around the level of your plant's canopy, and if it feels uncomfortable raise them lights. I'm using a 600 watt HPS grow lamp (sealed off and air cooled as well). I keep it 12 inches away and have seen no detrimental effects.
Plants usually stretch when they get the wrong type of light or not enough light. A 400w is plenty enough to help the plants grow healthy and normally