Excerpt From: California Master Gardener Handbook, Second Edition.
“Hardening is the process of slowing plant growth to withstand changes in environmental conditions that occur when transplants are transferred from a greenhouse or home to the garden. A severe check in growth may occur if plants produced in a controlled environment are planted outdoors without a transition, or hardening, period. Hardening is critical with early crops, when adverse climatic conditions can be expected.
Hardening can be accomplished by gradually lowering temperatures and relative humidity and reducing water, causing an accumulation of carbohydrates and a thickening of cell walls. A change from a soft, succulent type of growth to a firmer, harder type of growth is desired. The hardening process should be started at least 2 weeks before planting in the garden. If possible, plants should be moved to a temperature of 45° to 50°F indoors or outdoors in a shady location. A cold frame is excellent for this purpose. When put outdoors, plants should be shaded, then gradually moved into sunlight, increasing the length of exposure each day. Reduce the frequency of watering to slow growth, but do not allow plants to wilt. Tender seedlings should not be placed outdoors on windy days or when temperatures are below 45°F. Even cold-hardy plants will be hurt if exposed to very cold or freezing temperatures before they are hardened. After proper hardening, however, they can be planted outdoors and light frosts will not damage them."