Hardening Plants

Transplanting can be a shock to young and rapidly growing seedlings when set out into a cold and windy garden in the spring. This is especially true for transplants started in the greenhouse, cold frame, hotbed or home. These young seedlings can be made somewhat resistant to heat, cold temperatures, drying and strong springtime winds, certain types of insect injury, injury from blowing sand and soil particles and low soil moisture by a process called "hardening".

Plants should be gradually hardened, or toughened, for 2 weeks before planting in the open garden. This is done by slowing down their rate of growth to prepare them to withstand such conditions as chilling, drying winds, shortage of water, or high temperatures.

Reducing the growth rate thickens the cuticle and waxy layers, reduces the percentage of freezable water in the plant and often results in a pink color in stems, leaf veins and petioles. Such plants often have smaller and darker green leaves than nonhardened plants. Hardening results in an increased level of carbohydrates in the plant permitting a more rapid root development than occurs in nonhardened plants.

Cabbage, lettuce, onion, and many other plants can be hardened to withstand frost; others, such as tomatoes and peppers cannot. Cool-season flower and vegetable plants can develop hardiness allowing them to withstand subfreezing temperatures. Unhardened cabbage seedlings have been reported to be damaged by temperatures of -2 degrees C (28 degrees F) while hardened cabbage will tolerate temperatures as low as -6 degrees C (22 degrees F).

Withholding water gradually - do not let the plant wilt - and lowering the temperature are the best ways to harden a plant. This may be done in a glass or plastic coldframe. Plants could be placed in a cold frame or other area that does not freeze during the night hours without lose of the hardening process. Expose plants to lower temperature than is reported as optimal for their growth.

Placing the plants outside during the day to encourage hardening and then bringing the plants back into the warm house during the night often reverses the hardening process.

Do not fertilize, particularly with nitrogen immediately before or during the hardening process. A starter solution or liquid fertilizer can be applied to the hardened transplants a day or two prior to transplanting into the garden, or at the time of transplanting.

About 10 days before being planted in the open ground, the young plants in beds or flats are blocked out with a large knife. Blocking, or cutting the roots, causes new roots to form quickly near the plants, making recovery from transplanting in the open easier. Blocking also makes it easier to remove the plants from the bed or flat with minimum injury.

Hardening is not necessary for all transplants. With the exception of tomatoes, plants that are susceptible to frost should not be hardened. Overly hardened plants while withstanding unfavorable outside conditions are slow to get started and may never overcome the stress placed on the plant during the hardening process. It is recommend that plants be hardened for no longer than seven to ten days before planting to the garden site.