Hardening off seedlings before transplanting
Last reviewed September 2026 · written from public-domain USDA/NOAA data and Cooperative Extension references — see how we source data
Seedlings grown indoors or in a greenhouse have soft tissue, no wind exposure, and filtered light. Moving them straight outside can sunburn or snap stems that were perfectly healthy an hour earlier. Hardening off is the gradual process that prevents that.
Why it matters
Direct sun, wind and temperature swings are all stresses a seedling has never faced under grow lights or a windowsill. Sudden full exposure causes sunscald (bleached, papery patches on leaves), wind damage, and transplant shock that can set a plant back weeks or kill it outright.
The 7–10 day process
Start with an hour or two in a shaded, sheltered spot, bringing plants back in afterward. Each day, add an hour and move toward more direct sun, until by the end they are outside all day in full sun with no wind break.
Once they are tolerating a full day outside, leave them out overnight for the final couple of days before transplanting, as long as there is no frost risk.
Weather to watch
Skip a day or bring plants back in during hard wind, hail, or an unexpected cold snap — a single bad day can undo a week of progress. There is no penalty for pausing the schedule; there is a real one for pushing through bad weather.
Signs you moved too fast
Bleached, curled or crisp leaf edges mean back off — return to a shadier, more sheltered spot for a day or two before resuming. Water well before and after the move, and transplant on a cloudy day or in the evening once hardening off is complete.