Again - there is no rule of thumb if I don't know your ventilation setup. I have a 410m3 fan plus passive intake, which means that I can keep the temperatures okay - my 400W HPS is placed app. 35cm away from the tops, and they are doing great. Your biggest risk is heat.
pinkchick Reviewed by pinkchick on . HPS LAMPS I have a 400 watt hps lamp and wondered what is the ideal distance between the top of the plants to the lamp ? I cannot get my lamp very close to my plants as they curl upwars and i posted this on here and LIP said this was because of the heat. I don't want the plants to stretch so if anyone knows the ideal distance that would be much appreciated. Cheers Bob Rating: 5