Hi DigiMan,

Well, I guess basically, the closer the lights the better. However, other factors must be taken into consideration. The heat put out by the lights is one of those things; you don't want to burn the seedlings up. Also, when the lights are too close, the surface of the soil can get dry very quickly! I usually try to keep the lights between about 2 and 3 inches above the seedling. I grow flowers only, but I would assume vegetable growing would be about the same.

Something you might want to keep in mind is that light is one of the things that obey the general "Inverse Square Law". Let's say, for example, that you normally keep a light 3 inches above your seedlings. If you double that distance, by moving the light to a position that's 6 inches above the seedlings, the seedlings would then get only one fourth as much light (2x2=4, inverted is 1/4). Moving the light three times farther away would result in only 1/9 as much light (3x3=9 inverted is 1/9) and four times farther (12" for our example) would be 1/16 as much light (4x4=16 inverted is 1/16).

As for how long to leave the lights on, I leave mine on for 16 hours. But as I say, I'm growing flowers so I can't really make any suggestions regarding vegetable growing.

Hope this is helpful,

Art

I used shop lights for years and always got them as close as I could. This year we picked up t5ho 8 bulb units, they put out a lot of heat and caused problems at first, even with fans. I have since raised them up to 12" inches from the plants and everything is growing great although we still need to watch the drying out. Tomatoes have pencil sized stems after 5-6 weeks and peppers are flowering (bad thing but growing great). I think it all depends on light intensity and heat balance. The bulbs I have are meassuring 20000+ lux at plant level. If you have enough light you can back away..... all in the balance of heat/light. I;m a newbie with lights so not sure if this is good advice, but it's working for me.

i have used this set up for 7 years now, you can see results for yourself.
T12, 4-bulb lights ..two bulbs are cool white and two are plant/acquarium bulbs from lowes, standard home builder store. Touching plants does not hurt them...i keep as close as i can without touching keeps them moving. you want tall stringy plant raise light, you want thick healthy, keep low. A fan also makes a big difference in strength of plant and stem.
lights, 8 off 16 on for mine. i'm sure there are many other ways but i like big strong tomato trees :)

Here is a link that might be useful: http://photobucket.com/albums/p577/superlight27