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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.
| 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 |
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.
For more information contact your local WSU Extension Office.
