Or, put a light on them. (sodium, metal halide, etc.)
I'm trying to get a jump on the growing season this year.
It has taken a month, and I think that the plants are getting less 'suspicious' of the early warmth, and are slowly coming up.
If they are a few feet tall by the time you can put them out post-frost, they are still manageable. Mine will be on the twine by then. I hope.