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Old February 12, 2012   #13
SEAMSFASTER
Tomatovillian™
 
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Join Date: Jun 2010
Location: American Fork, Utah
Posts: 160
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I've met several people who assume that metal halide lights are very hot. Halogen bulbs are hot, while MH's are not. I've measured the temperature several times - 4" under the bulbs is a steady 80° F, very nice for tomato plants which are kept at least 12" away.

In my "nursery" room, I run up to five 400-watt bulbs simultaneously. Yes, if I kept the windows and doors closed all day long, the heat might build up to 90°, but it's not a good idea to stop air circulation anyway, especially for tomatoes.

I keep the lights on timers, 16 hrs. on, 8 hrs. off. The lights are off at night so there would be no accumulation of heat anyhow. It's my understanding and experience that tomato plants really benefit from cooler night time temperatures in the 50-60° range. This helps prepare them for outdoor conditions and (I'm assuming) helps prevent spindly growth.
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