Have you thought of installing PVC hoops over your containers and covering with row covers, leaving them open at the ends? It would give them a little frost protection, as well as slowing moisture loss as spring warms up. Calistoga Al

Al, I think it's the soil my neighbors who sold me the containers used. I have used a cover when planting so the seedlings weren't in the full hot sun. The winter greens seem to do OK, but summer things that I planted, even heat lovers seemed to burn up, where as the same plants in the regular raised beds (on the ground) did fine.
Last winter WAS rather cold, with 2 weeks of 19 *s!
I almost always plant on April 15th (last day of frost in this area). I've had a couple of surprise frosts, where I had to re-plant, but not many!
I'm near you in Santa Rosa. Nancy

Nancy I am pretty sure the soil is not the best, but growing in a container even a large one will not have the same condition as in the ground. Your soil even a foot deep does not compare with the ground soil, infinitely deep. Al

Well, I'll just stick to shallow rooted things, then. Nancy