seeds
Hardening off
Last reviewed 2026-10-04
Hardening off is the gradual process of moving seedlings from protected indoor conditions to the brighter light, wind, temperature changes, and drier air outdoors. It reduces transplant shock by giving leaves, stems, and roots time to adjust before a seedling spends a full day in the garden.
What it means
Indoor seedlings grow under relatively steady conditions. Their leaves are not prepared for direct sun, and their stems may be tender because there has been little wind. Hardening off is not simply putting plants outside once. It is a staged exposure followed by time back in shelter when conditions are harsh.
How it affects the garden
Begin when seedlings are healthy and outdoor weather is suitable for the crop. Put trays in a bright, sheltered location for a short period, then increase exposure over several days. Start with indirect light and light wind. Extend the time outdoors gradually, while watching for wilting, leaf scorch, cold injury, and drying mix.
Watering needs can change quickly outside. Check the containers rather than following an indoor schedule. Bring warm-season crops in when temperatures approach their tolerance limit, and protect all seedlings from unexpected frost. A calendar based on the seed starting calculator helps, but the plant and weather still determine whether a particular day is safe.
Example
A tomato seedling raised under lights might spend its first outdoor session in bright shade for two hours. Over the next several days it can receive more morning sun and longer exposure. It should be ready for transplanting when it remains firm and upright after a full day outdoors.
Common confusion
Hardening off does not make a frost-sensitive crop frost tolerant. It prepares a plant for ordinary outdoor conditions. Tomatoes and peppers still need protection from cold even after the process is complete.
Sources
University of Minnesota Extension covers moving seedlings outdoors and the need to acclimate them first.
