Curious about what "hardening off" means and why it's important? Here's a blog from our archives to help you prep your cold-season transplants for the great outdoors.
Plants need to be hardened off before being transplanted. What does this mean? It is the process you take them through to get used to direct sunlight, wind, cold, heat, dryness, and any other weather conditions they will be asked to live through during their lives. When plants are started in the greenhouse, in trays or pots, their world is sheltered and comfortable; food is readily available in the compost-fortified soil, and the temperatures are carefully regulated. Once it is time to go out into the harsh world of the garden, the goods and services become a little more scarce for those baby plants and they go through a bit of withdrawal known as "transplant shock." Hardening them off gives them a gradual adjustment to this new world. It is best to start out by putting the plants outside for a few hours, and then bringing them back in; this gives them a small taste of direct sunlight. In any greenhouse, the plastic film filters out 10 to 25% of the sun's rays and the tender plants are literally sun-burned when they first encounter that unmediated light. Here is how I do it at my house. When you buy plants that were grown by Red Wagon Plants, they have already been hardened off and you can put them right in the ground. The following method is important to follow if you are working with plants that you know are not hardened off yet.
Once they have gotten used to a few hours of sunlight at a time, they can stay outside overnight, but under a protective row cover.
The cover comes off in the daytime and the plants are acclimated to being where they will be planted for a few days.
If the temperatures drop down below freezing, cover them again. Cold hardy plants such as broccoli, cabbage, kale, hardy lettuces, hardy herbs, escarole, scallions, onions can take a frost without a problem, but they will do it with more grace and strength if they have been gradually introduced to those temperatures.
Once the plants have had a few cold nights, under cover, they can have a couple more nights without cover, and then are properly hardened off and ready to go in the ground.
It's also a good idea to give cold-weather transplants a little extra compost or a fish/seaweed emulsion watering at transplant time. It is hard for plant roots to uptake nutrients in cool weather and this will aid their transition as well.