Hardening off Transplants

Many gardeners have only the foggiest idea of what "hardening off" means. To get advice from a professional, I want to give you guidelines written up by Norma Rossel, Quality Assurance Manager for Johnny's Selected Seeds. Johnny's is in Maine and it specializes in short-season cultivars. They are very aware that, in order to get heat-loving crops to mature in places like Seattle, giving plants a head start as transplants is absolutely necessary. Transplants are also an essential element in intensive gardening, which is important to urban gardeners with limited land.

1. Harden off gradually, so that seedlings become accustomed to strong sunlight, cool nights and less-frequent watering over a 7-10 day period.
2. On a mild day, start with 2-3 hours of sun in a sheltered location.
3. Protect seedlings from strong sun, wind, hard rain and cool temperatures.
4. Use an automatically opening cold frame, if you have one.
5. Increase exposure to sunlight a few additional hours at a time and gradually reduce frequency of watering, but do not allow seedlings to wilt. Avoid fertilizing.
6. Keep an eye on the weather and listen to the low temperature prediction. If temperatures below the crop's minimum are forecast, bring the plants indoors or close the cold frame and cover it with a blanket or other insulation.
7. Know the relative hardiness of various crops. Onions and brassicas are hardy and can take temperatures in the 40's. After they are well hardened off, light frosts won't hurt them. Warm-season crops such as eggplants, melons and cukes prefer warm nights, at least 60° F. They can't stand below-freezing temperatures, even after hardening off. (See chart below for more detail.)
8. Gradually increase exposure to cold.
9. Root-prune plants in flats a week before setting out. Use a sharp knife and cut down to the bottom of the flat between the plants. Water thoroughly.
10. After transplanting to the garden, use a weak fertilizer solution to get transplants growing again and to help avoid transplant shock.

Note: Temperatures regularly below 50° F. can cause Chinese cabbage to bolt. Temperatures below 55° F. for 10 days or more causes celery to bolt. Harden by reducing water slightly for 7-10 days before setting celery out, but do not reduce temperature.

As I said above, these recommendations are a goal. Few gardeners, even the best ones, always follow these procedures to the letter. Do the best you can. A maximum/minimum thermometer is one of the first, best tools you can buy to help you. A self-opening cold frame would be a nice investment, but an inexpensive, plastic cloche or row cover can do the job.

One last note: the terms "hardy" and "tender" are possibly as poorly understood as "hardening off." They relate to whether a crop can withstand frost. Hardy plants can, tender crops can't and half-hardy ones may be able to take brief, light frosts.

Holly S. Kennell, WSU Extension Agent, King County

For more information contact your local WSU Extension Office.