I did a little experiment. I started my lettuce bowl, putting seeds on the ‘bowl’ and lightly covering with sand. I didn’t have enough room to put the whole bowl on my seed heating mat (I’ve ordered another one) so I put half ON my mat and half OFF the mat as in the picture. The heat comes from the bottom. The temperature of half ON the heat mat was 77°F and the half OFF the mat was much cooler at 70°F. The half on the mat germinated in 3 days while the half off the mat had only a few seeds germinate. Amazing how the temperature affects the germination rate. Lesson here? If you plan to start seeds indoors, get a seed heating mat. Next: Review of my Seed Starting Class last Saturday.
giantveggiegardener 
Jannine,
Thanks for this information concerning your seedling heat mat experiment. When you complete your greenhouse you will discover your seeds will germinate outside at a ground temperature of 47 degrees here in the high desert in northern New Mexico. With a min/max thermometer you will find the temps will swing at this time of the year, with no external heat source, from 100 degrees ( air temp at the roof apex of the GH) to about 12-15 degrees at night. Ground temp will be about 47-50 degrees.
The seed germination % is of course less than when being germinated inside in a heated environment, but the germinated seeds do not have to be transplanted. I am just learning all this after reading the Coleman “4 season” gardening book. I am happy its working for me.
I congratulate you on your growing skills. Do keep us informed, you area wonderful source of gardening information.
Gene S.
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