When the leaves come out, they need light. Lots of light. Seeds and plants will do fine without heat and humidity if they're kept above freezing (and ideally above 50F) and watered. It's plenty moist underground. Warmth will make them grow faster, but is not necessary for growth or survival.

Common question on the Growing from Seed forum and the answer is always if you must use a dome then be sure to remove it as soon as there is the first sign of seedling germination. Same goes for heat mats. Off the heat ASAP. The only alternative is to cut the 6 packs you used apart.

Also recommended you not mix varieties much less different vegetables in the same 6 pack to avoid the issue of mixed germination times.

Growing from Seed forum

Dave

Thanks. Perhaps I’m having a low germination rate. Each cell pack contains a single variety.

Iunderstand removing the seedlings from the dome, but what about keeping them with bottom warmth is harmful? Just curious, since several mentioned this. I had thought about putting the cell packs back on the heat mat at night.

Thanks for your replies!

(oops, guess I should go to the link digdirt2 provided, too)

Keeping seedlings on a heat mat causes them to grow at a faster rate than necessary which results in lankier, weaker seedlings. Growing seedlings at a cooler temperature is better for stockier growth and you get plants that are more easily hardened off and transplanted.

Rodney

Like Rodney said, heat mats only promote top growth but sacrifice root development. Young rootlets are quite sensitive to heat and easily killed. That unbalanced type of growth results in weak seedlings and even weaker plants as the root development cannot support all the top growth adequately.

Heat for germination. cool for plant growth.

Dave