How to Harden Off Seedlings
Harden off seedlings over 7–14 days before transplant so trays survive sun, wind, and cool nights.
Published by ThriveThyme
seed starting transplant winter
How to Harden Off Seedlings
Move indoor starts outside gradually so sun, wind, and cool nights do not shock them before transplant.
Published draft 2026-09-29 Slug: how-to-harden-off-seedlings Sources: Live ThriveThyme plant copy (“harden off… a little longer each day for about a week”); frost guides for light frost vs hard freeze; /api/seeds tender warm-season crops (tomato, pepper) vs cool crops (broccoli, lettuce); New Bot seed-start v1 (a578c06) crop-specific harden days (tomato/pepper 14, broccoli/brassicas 10, lettuce/chard 7, cucumber/melon family 7). Expanded here to a practical 7–14 day schedule. Live: seed-start calendar. No live /guides/how-to-harden-off-seedlings in sitemap (2026-09-29). Does not invent a yard frost date—use My Garden.
How to Harden Off Seedlings
Hardening off bridges indoor light and outdoor weather. Skip it and leaves scorch, stems flop, or growth stalls. Start from How to start seeds indoors and the live seed-start calendar, then use What to plant now and the Winter hub for timing around your frost settings.
Before You Begin
- Seedlings should have true leaves and be nearly transplant size.
- Know your last frost in My Garden—not a national zone average. See What is first frost and first frost date.
- Have shade (cloth, porch, north side of a building) and a way to bring trays in overnight early in the schedule.
- Check crop-specific harden days on the seed-start calendar—most fall in the 7–14 day range below.
7–14 Day Schedule
Use the overall 7–14 day shade-to-sun schedule. Stretch toward the crop’s harden day from the seed-start table:
- 14 days: Tomato, Pepper, Eggplant, Onion, Celery, Leek
- 10 days: Broccoli, Cabbage, Cauliflower, Brussels Sprouts, Kale, Collards, Basil, Pansy, Parsley, Snapdragon, Marigold
- 7 days: Lettuce, Swiss Chard, Zinnia, Cucumber, Melon, Watermelon, Squash, Pumpkin
Cool broccoli and lettuce often settle in closer to a week if nights stay mild. Live plant notes often say “about a week”; use the longer end when plants were under soft indoor light, and always match tender warm-season crops to the 14-day end.
1. Days 1–2: 1–2 hours outdoors in bright shade, then back inside. 2. Days 3–4: 3–4 hours; still mostly shade; add a little morning sun if leaves stay firm. 3. Days 5–7: Most of the day outdoors; increase direct sun; bring in if wind is harsh or nights drop near freezing for tender crops. (Stop here for 7-day crops when nights are mild.) 4. Days 8–10: Full sun for sun crops; leave out longer; water when pots dry faster outdoors. (Typical end for 10-day brassicas and similar.) 5. Days 11–14: Overnight outside only if nights are safely above what that crop can take. Then transplant into the bed. (Full stretch for tomato, pepper, and other 14-day crops.)
After harden-off, transplant on the calendar’s transplant-after offset (for example tomato and pepper about 7 days after last frost; many cool crops at last frost / 0 days after). Exact values are on the seed-start calendar.
Shade → Sun → Overnight
Indoor leaves are soft. Sudden noon sun burns them. Shade first, then morning sun, then midday. Wind is part of the toughening—use a sheltered spot early, not a wind tunnel. Overnight is the last step, not the first.
Frost Risk
A light frost (about 32 F) can damage tomato, pepper, basil, and beans. Cool brassicas and lettuce often tolerate more chill once hardened, but a hard freeze still hurts tender growth. If a freeze is forecast during harden-off, bring trays in or cover them. See Protecting plants from early frost.
When Not to Harden Off
- Do not start the schedule if a hard freeze is due within a few nights and you cannot bring plants in.
- Do not harden off leggy, diseased, or damping-off seedlings—fix trays first.
- Do not leave tender crops out overnight before your last frost without protection.
- Do not jump from a dim room to all-day full sun in one afternoon.
Next Steps
Finish harden-off, then transplant into your zone spring window—for example patterns on zone 6 calendars. Return to indoor seed starting or the seed-start calendar if you need another wave of trays, or use cold frames and winter sowing for cool crops without a full indoor setup. Hub: Winter.