Just spoke with Ed Person from Ledgewood Farm in NH, the greenhouse guru. Important tips:
1. Once seedlings emerge, take domes off and let soil dry out as much as possible without killing the plants. The humidity will cause the legginess and create other problems. Transplant in a couple weeks after first true leaf appears.

2. Plants will be happiest at around 60 degrees, growing much faster than at 50. The amount of fuel used is doubled to keep the temperature at 60 as opposed to 50.

3. Spinach will bolt if the diiference between the soil and air temperature is too great. In winter, the soil temperature is generally around 45 degrees, so you don’t want to let the daytime temperature get higher than 55 or 60. The difference shouldn’t be more than 10 degrees in low light conditions(winter). As the light increases, the delta can increase.

4. Humidity in the greenhouse is a problem. Creates enviroment for fungus/mold.

Needless to say, I just took the plastic domes off the sprouted seedlings.

Thanks, Ed!