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Too Hot??

Posted by honnat 4 MN (My Page) on
Mon, Feb 6, 12 at 19:43

I have just started to learn about growing under lights. I have a 4 bulb, 4 foot fixture with T5 grow lights. I set it up, filled a tray with seed starting mixture, watered it, put on the dome and turned on some bottom heat. The Fixture hangs just above the dome. Before I put any seeds in, I wanted to see how warm the soil was going to get. I don't have a thermostat for the heat mat; and I'm not sure I want to invest in one now.
So I let it be over night; and measured the soil temp in the morning with a water/meat thermometer that I trust. It read 114 degrees! So I unplugged the heat mat and let it stay set up like that for the day. I got home this afternoon and measured the soil temp at 94 degrees. Yikes! Now, I've got a small fan running on the area; but I'm not super hopefull because the dome is going to block most of that air movement.

All my seed packs (all flowers), say to have temps at about 75 degrees for germination.

Is it normal for things to be running this warm?? This is the fixture I have:

Follow-Up Postings:

RE: Too Hot??

Hi honnat,

I thought I answered this post yesterday but it looks like my answer didn't get posted, so I'll try again :-)

The fixture you have is way more than you need for seed germination. I think those fixtures are really designed more for supporting larger plants (up to a couple of feet tall) or even plants in the vegetative stage. If you have that light close to the dome, 216 watts is bound to create too much heat for seedlings. You could try raising the light higher above the dome until you can maintain something closer to a 75 degree soil temperature. Normally, you would not want the lights over about 4 inches above the seedlings but with that light, I think you could safely raise it higher.

Hope this helps,


RE: Too Hot??

Thanks for the response. I was begning to think i was doing something wrong - or something was wrong with my light!

With my little fan on high I've got the temp to a consistent 75 degrees. I put some cone flower seeds in there this morning.

Is there any advantage in raising my light fixture and turning the bottom heat back on? Otherwise, I figure now the soil temp is about right and I've got the lights nice and close to the soil.

RE: Too Hot??

No advantage to raising the light as long as it's not causing too much heat. Normally, you'd want the light to be about 2 to 4 inches above the seedlings anyway. But raising the light would sure be better than frying the seedlings. I don't know how that fixture is wired but sometimes you can remove two of the bulbs and the other two will stay lit. You might consider trying that. That would eliminate depending on a fan to cool things off and save on the electric bill at the same time. You could always put the other two bulbs back in if you feel they are needed later on.

Regarding the fan, once the seedlings grow a little, a fan (usually on a timer) is a good idea, it strengthens the plants (which is done by the wind in nature). But you shouldn't have to use a fan to counteract the effects of the lighting being too hot.


RE: Too Hot??

thanks again for the info. My fixture has two switches to turn on 2 lights at a time. I don't have a timer (again - already invested too much $ in seeds, heat mat, trays, starting mix, potting soil, plugs....) I think what i could do is turn off two of the lights in the evening and also turn off the fan. So it will get full light for about 16 hrs, and half light for 8. That would save me a bit of energy.

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