Hardening Off Transplants

by Don Janssen, UNL Extension Educator

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Do not abruptly transfer seedlings from indoors to the uncertain outdoor climates. Gradually harden or toughen plants before you plant them in the garden. This process is called "hardening-off".

As a result of the hardening process the plants will accumulate carbohydrates, and cell walls will thicken, thus changing soft, succulent growth to firmer, harder tissue. Do this for plants you purchase as well as ones you grow yourself.

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.

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 40 degrees F are forecast, bring the plants indoors or place them in an unheated outbuilding.

Know the relative hardiness of your plants. Hardy cool season plants can take temperatures in the 40's. After they are well hardened off, light frosts won't hurt them. Warm-season plants prefer warm nights, at least 60 degrees F. They can't stand below-freezing temperatures, even after hardening off. After these minimum night temperatures are not exceeded go ahead and place transplants in the garden.

After transplanting, use a weak fertilizer solution to get transplants growing again and to help avoid transplant shock.