Hardening is a critical element of gardening. It exposes the plant set to the same conditions it will face when it is transplanted: wind, rain, and insects. Harden a plant set for two weeks in a cold frame or screened sunny porch and you will be rewarded with a strong, viable plant more resistant to such conditions. If you purchase commercial sets, harden them for several days if they were stored in a greenhouse.
A cold frame can add as much as 10 degrees to the air temperature if the sun is shining on it. We made ours, using Crockett's Victory Garden instructions, from plywood and clear, corrugated solar panels. The total cost was under $75.
Brassicas, perennials, greens, and other cold-weather crops are hardened in early spring. Temperatures below 40 or above 68 degrees may cause brassicas or other cold-weather plants to bolt later on, or go to seed before reaching maturity. Growing them as spring crop is a problem for us in Zone 4-5 for two reasons: cutworms, which feed in June, and extreme weather -- either too cold and rainy, or too hot and dry. For Garden 2005, we will try our neighbor's approach and plant 8-week-old sets in mid-May, before our frost-free date of May 20th. By the time the cutworm larvae are feeding, the broccoli is too large to do any damage. He was harvesting broccoli in early June!
Tomatoes, peppers, and other warm-weather plants should be hardened off when nighttime temperatures reach 55 degrees, optimal for warm-weather plants.