Hardening off seedlings: how long and how
Oklahoma State says 7 to 14 days. Utah State says to begin at least about two weeks before transplanting.
What the sources say
- Oklahoma State: hardening off takes 7 to 14 days. Start with about 1 hour in shade, then increase sun exposure. Use a protected spot and avoid windy days, which can dry out and kill seedlings. Include about one week for it when counting back from your last frost date.
- Utah State: start soon after emergence, and begin at least about two weeks before transplanting. Start in shade with short outdoor periods, then gradually increase sun and time outside until plants tolerate full sun all day. Some yellowing or browning of leaves is normal.
- Cold limits (Utah State): avoid freezing temperatures. Keep tomatoes, peppers, squash and melons above 45 to 50 degrees F. Hardy and semi-hardy seedlings exposed to temperatures below 40 degrees F may flower prematurely.
- Cornell Monroe: protect plants from cold and wind for a few days after setting them out.
A simple schedule
- Day 1: about 1 hour in shade, out of the wind.
- Following days: add time and a little more sun each day.
- By the end of one to two weeks: a full day in sun, if the temperature allows.
- Bring plants in if the temperature will fall below the limits above.
This schedule is our simplification of the sources, which give the first day and the total length but not a day-by-day plan.
Sources
- Oklahoma State University Extension: Tips for starting seeds at home (2025)
- Utah State University Extension: Starting vegetable seeds indoors, seeding culture and transplanting (2025)
- Cornell Cooperative Extension, Monroe County: Starting vegetables from seed