Lower temperatures at this time of year will slow down the growth on my vegetable transplants. Also, the light emitted by my fluorescent lights is only about 25% as strong as sun light. Thus, the transplants must be acclimated to both cooler temperature and stronger light. This is accomplished by introducing the transplants slowly to lower temperatures and higher light levels.
Since I haven't put my cold frames together yet, but plan to prior to the weekend, I have moved my transplants outdoors on a table in a shaded area on the north side of my house. I will gradually move them into a sunnier location over the next couple of days. If temperatures are forecast to drop below freezing, I will move them indoors.
This picture shows broccoli, cauliflower, cabbage, bok choi, onions and tatsoi and various stages of development.
Once I get my cold frames set up, all of these cool season plants will go into them for about a week to finish hardening off. Last years picture.
The bags of leaves and soil around the base of the cold frame act as insulation and the gallon jugs of water in the back absorb solar radiation/heat during the day and radiate it at night, keeping the cold frame above freezing. On really cold nights, I will cover the top with a multi-layered tarp to keep things from freezing.
Can spring be very far away?