Sowing seeds indoors is a good way for plants to become healthy, strong and get a good start before planting outdoors. However, many plants require a period of time to become acclimated to the outdoors before being planted into the garden, known as hardening off . In doing so, the plants are likely to be healthier.
Hardening off plants before moving them outdoors permanently helps them become acclimated to the wind, cooler temperatures and stronger light of their new environment. Seedlings raised under indoors growing conditions require gradual adjustment to the more strenuous conditions found outdoors. When plants are moved to the garden without first hardening off seedlings, they may suffer from stunted growth, sunburned leaves and wilting.
Young plants require hardening off in spring once they are large enough to be placed in the garden permanently and the outdoor temperatures are suitable for planting. Choose a location where the plants are sheltered from strong weather. The best place locations are on the north side of the home or under a shade tree. If only open areas are available to harden off seedlings, shade and protect the plants with wooden lattice or a cloth.
Gardeners can begin hardening off seedlings about 10 days before planting them in their permanent location. Start off by placing seedlings in their sheltered location for an hour, increasing the time the plants are left outside by an hour each day. While doing so, gradually expose plants to more in tense sunlight daily. Bring plants indoors at night so they are not exposed to cold each night, until the last day before transplanting, then leave them out all night.
It is important to keep in mind that even if plants are properly hardened off, they are not necessarily frost-hardy, so do not move frost-tender plants outdoors until the threat of frost has passed, and shelter plants from an unexpected late-season frost when necessary. Use row covers or cloches to protect plants. The seedlings will need more water than they did when growing indoors.