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Article: Seed Starting 101: Seedling Heat Mats and Inexpensive Alternatives: Too hot heat mats

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Forum: Article: Seed Starting 101: Seedling Heat Mats and Inexpensive AlternativesReplies: 3, Views: 77
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Perham, MN
(Zone 3b)

February 1, 2010
4:02 PM

Post #7518537

I got some heating mats quite a few years ago, that I only tried a couple of times. They didn't have thermostatic controls on them, and I think they heated up the soil *too* much. I think they killed off the seeds I tried to grow over them. It seemed really, really hot in there. I've had far better luck putting seeds in flats under clear domes, with fluorescent shop lights overhead, very close to the domes. I keep the shop lights on chains, and adjust the hooks as necessary, for height.

Still, I'd love to actually use the heat mats! Heat from below seems like a good idea. Maybe I'm doing something wrong. Anybody have any suggestions? No directions came with the mats. I thought of putting a couple of towels between the mats and the flats. It just seems so hit-and-miss, and I wonder if I'm supposed to have some other piece of equipment. I have lots and lots of onion seeds I could experiment with, and dare to kill off. Other stuff is too expensive and precious. I think of trying a mat with one flat, and the old light method with another, and seeing which works better, but... suggestions or ideas welcome.
Frederick, MD
(Zone 6b)

February 1, 2010
4:17 PM

Post #7518575

Most mats say they'll raise the soil temp 15 to 20 degrees above ambient (room) temp... so if your seed starting area warms up to 75 or so, that can mean 95 degrees, and that will kill some seeds. You could get a controller or reduce the temp as you said... towels might work, elevating your seed flat a bit above the mat might help also (try a roasting rack, or just put a bunch of jar lids or canning rings under the flat). I'd suggest a test run with a thermometer -- set up your flat, then stick the thermometer in a pot of moist potting mix, with no seeds added yet. Check the temp a few times over the course of a day or two to see how it goes. 78 to 82 degrees is what you're shooting for, anything over 90 degrees F starts getting into the "too hot" zone.

Good luck!
Perham, MN
(Zone 3b)

February 2, 2010
5:31 AM

Post #7521270

Thanks, Critter. My house is usually around 68, in the seed-starting area, unless I use the shop lights over domes. That'll still be too high then, for most seeds. I particularly like the canning ring idea. I'll check out my "instant" thermometers and see if they measure temps that low. Might have some old mercury thermometers tucked away somewhere. I still think they're pretty good - maybe the most accurate. My soil thermometer bit the dust a couple years ago, but would have been too big anyway.

I wonder if this is anything like a typical experience with heating mats though. Maybe what I've got is really out of date. Or maybe they were meant to be fit into a sand bed in the greenhouse (I remember when I used to have one of those!), with the flats on the sand. Sigh. Once upon a time, I had a long cedar bench in an 8 x 12 greenhouse, and part of it was sectioned off, lined with plastic, had heat cables taped down onto the plastic, sharp sand over that, and seed flats on top. Those were the days! I couldn't wait to get out there and peer at things, and even more, smell the damp earth. (wish wish wish)
Frederick, MD
(Zone 6b)

February 2, 2010
5:34 AM

Post #7521278

I'd get all nostalgic over a sweet setup like that, too! Hopefully you can get your heat mats working well for you. :-)

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