I know the goal of hardening off is to get the young plants gradually used to hotter and colder temperatures. The other night, I left the plants out until it reached 50F. The tomato plants looked like they were cringing:
They did recover, and I've been putting them outside and keeping them there until the temperature hit 48F. When I brought the seedlings in, the eggplant seedlings, cantaloupe seedlings and cucumber seedlings didn't look so hot. They looked very pale, and like they were losing their green color.
I went on line to see if I could find out more about hardening off. I learned that certain plants should not be kept outside in temperatures below 60F, because they are tender. I also read that the hardening off process should begin a week or 2 before planting day.
I have decided to bring the plants in when it gets down to 60F, and take a couple of weeks to fertilize the plants a bit. I am starting with Super Thrive.
- Harden off gradually, so that seedlings become accustomed to strong sunlight, cool nights and less-frequent watering over a 7-10 day period.
- On a mild day, start with 2-3 hours of sun in a sheltered location.
- Protect seedlings from strong sun, wind, hard rain and cool temperatures.
- Use an automatically opening cold frame, if you have one.
- 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.
- 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.
- 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.)
- Gradually increase exposure to cold.
- 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.
- After transplanting to the garden, use a weak fertilizer solution to get transplants growing again and to help avoid transplant shock.
|Hardy||40° F.||Broccoli, Brussels sprouts, kohlrabi, cabbage, onions, leeks, parsley|
|Half-Hardy||45° F.||Celery, Chinese cabbage, lettuce, endive|
|Tender||50° F.||Squash, pumpkin, sweet corn|
|60° F.||Cucumber, muskmelon|
|65° F.||Basil, tomatoes, peppers|
This chart has been taken from Washington State University