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A gradual transition helps young plants adjust to sun, wind, and changing temperatures.
A seedling that looks sturdy on a sunny windowsill can still scorch outdoors. Indoor plants are unaccustomed to direct sun, moving air, and sudden temperature changes.
Hardening off is a gradual introduction to outdoor conditions before transplanting. Giving leaves and stems time to adjust helps seedlings handle stronger sunlight, airflow, and temperature shifts.
Begin when seedlings are healthy and established, with roots holding their potting mix together. Check frost risk and crop tolerance; wait if frost or strong winds are expected.
A typical hardening-off period is 7 to 14 days, not a deadline. Start with a short session in bright shade, then add outdoor time and light in small steps as plants cope.
Start near a wall, porch, or windbreak where you can reach the plants quickly. Move trays toward brighter light and more open air, avoiding places that trap intense heat.
Small pots can dry quickly in sun or wind, but saturated mix deprives roots of air. Check before each session, water when the mix begins to dry, and ensure excess water drains.
If seedlings wilt, move them to shade and shelter, then check the mix before watering. Bleached or brown patches can signal sun injury; wait for recovery before increasing exposure.
Plant after seedlings tolerate a full day outdoors and the weather suits the crop. For tender vegetables, wait until frost danger has passed and nights are warm enough.
Usually, bring seedlings indoors overnight, especially early in the process or for tender crops. Leave them out only after daytime adjustment and when the forecast is safe for that plant.
An open window may provide some airflow, but it cannot reproduce outdoor sun, wind, or temperature changes. Build exposure gradually, and slow down whenever seedlings show stress.
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