Now that I've migrated from T5's to HID's, I'm wondering how far above the plants these beasts should be. I'm running a 1000w MH dialled back to 75% above my 5x5 raised bed, with lettuce, kale and mustard greens. The lamp is 48' above the plants in a white umbrella reflector. I read that plants need between 30 and 50watts/sq ft of grown area, but cannot find out what the height should be. Do I need a special light meter for this. Any ideas?
T5s run fairly cool so you can have them very close to the plants, 1-2 inches is not uncommon. I have heard 25 watt/SF but I get away with 150 watts of HPS light for 4 square feet - but the edges are a little lacking in light. Grow lights are usually good at certain bandwidths of light which are better for for veg or fruit ...so just focusing on how much light without knowing the PAR spectrum of a light may not get you to a good result. T5s will be good for Vegetative growth while HPS would help with flowing and fruiting.
Place your hand at the tops of the plants. If it feels to warm, you are to close. It should feel luke warm to get you close to where you want to be.
Excellent, thanks everyone!
At 750 Watts...4 to 5 feet away (height) from the tops of those plants you mentioned...your fine for 25sq.ft coverage.
Thanks for that Vlad. Seemed that any closer it was getting fairly warm at the plants. I just did not want to cook them before their time :)