For cold weather veggies like kale and bok choi, you can probably start the seeds now. Those veggies will usually overwinter okay in an unheated cold frame -- they tend to just slow down and stop growing until it warms up again -- and they can stand light frosts. (Of course, as I say this, the forecast is predicting a low of 18 tonight in Eastern MA. Isn't it supposed to warm up in March?!) If you're worried, one way to keep seedlings warm without using a lot of electricity is to put a seedling heating pad right underneath the seed tray. You can get them at lots of places -- I think I got mine from Johnny's Seeds.

You might actually have a problem with too much heat. The sun's pretty intense already, and, even with auto-vents, my Harborfreight greenhouse is getting into the high 80s on sunny days. You can find plenty of advice and knowledge over on the greenhouse forum.