I don't think it is necessary to raise soil temperature to 80F. That is actually excessively hot. 60-65F will be fine. You only need 80-86F soil temperature for optimum germination. The thins is that at 50-60 air temperature, tomatoes will grow very slowly. But I think anything cooler than 60 is too cold. But that is fine, you can compensate for that by starting early. But more than temperature, they will need light to grow stocky.

Another question - re: heating mats, if I did use one...although I may not need to...I might experiment with a batch using one and a batch not using one.

How "deep" can the mats' heat penetrate? I have a bunch of seed starting containers that are about 4" deep. Would the mat warm that deep of a volume of soil thoroughly?

Recommended soil germination temp is 70-75 (80-85 for peppers) thus the recommendations for the use of heating mats or hot boxes, etc. Once germinated they are removed immediately to the cooler tem area of the GH.

If interested you can find a detailed FAQ about this over on both the Growing from Seed and the Growing Tomatoes forums here.

Will a heat mat effectively warm 4" of soil? No. That is why shallower containers are used for germination. You can use your 4" deep ones if you wish but only put 1 1/2 to 2" of soil in them. Then fill the pot as the plant grows. Better yet, germinate all of them in one shallow tray and transplant them to the individual containers once they develop true leaves.

Dave

If you are putting together seed-starting equipment, keep in mind that if you use a light fixture with a metal hood, you may be able to use the warmth from the light as your heating mat. The hoods on my grow lights are nice and sturdy, so I can set planted flats on them for bottom heat, then move the babes under the lights when the sprouts show.

I agree with planatus. I have measured , the space between the top of hood and what you put on it, can get up to 92F. I cushion it a bit an put my germinating baggies on it. Another place is the top of a lamp shade, covered with a dish towel. With a fluorescent lamp it can get close to 90F up there. Again, you can control it by cushioning more. So far I have done about 10 test germination the way I mentioned. So no more heating mat for me.

Good stuff. Thanks!