Harden off
/ˌhɑɹdən ˈɔf/
Definition
Harden off refers to the gradual acclimation of seedlings or other protected plants to outdoor conditions before transplanting. This horticultural technique exposes plants over several days to stronger light, wind, cooler temperatures, and less stable humidity, allowing leaves and stems to adjust and reducing transplant shock, sunscald, and wilting. It differs from transplanting itself because the goal is adaptation to the environment, not moving the plant to its final growing place.
Use in Gardening
- Reducing transplant shock: This step helps seedlings move from sheltered conditions to sun, wind, and cooler nights with less wilting, leaf scorch, or stalled growth after planting.
- Building stronger outer growth: Gradual exposure encourages tougher leaf cuticles, sturdier stems, and better control of water loss through stomata, the tiny pores that regulate gas exchange and moisture.
- Most important for indoor starts: Plants raised under lights, on windowsills, or in greenhouses are especially vulnerable because they have not developed the same tolerance as plants grown outdoors from the start.
- Timing with outdoor conditions: It matters most in spring, when light intensity, temperature swings, and drying wind can change quickly. Tender vegetables, annual flowers, and newly rooted cuttings often benefit the most.
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