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Move cool-season seedlings outside two to four weeks before your average last frost date, and warm-season seedlings one to two weeks after it, once night temperatures stay above 50°F and soil is above 60°F. Neither goes out without seven to ten days of hardening off first. Find your local average last frost date from your state Cooperative Extension service or the USDA Plant Hardiness Zone map, then count backward and forward from it — the date is a statistical average, not a guarantee, so watch the forecast.
When does each crop go out?
Timing is set by cold tolerance, and vegetables sort into three fairly clean groups. All timings below assume the seedlings have been hardened off.
| Crop | Move out | Min. night temp | Min. soil temp |
|---|---|---|---|
| Onions, leeks | 4 - 6 weeks before last frost | 28°F | 45°F |
| Peas, spinach | 4 weeks before | 28°F | 45°F |
| Broccoli, cabbage, kale | 2 - 4 weeks before | 28°F | 50°F |
| Lettuce, chard | 2 - 3 weeks before | 32°F | 50°F |
| Celery | 1 week after | 50°F | 60°F |
| Tomatoes | 1 - 2 weeks after | 50°F | 60°F |
| Peppers, eggplant | 2 - 3 weeks after | 55°F | 65°F |
| Cucumbers, squash, melons | 2 weeks after | 55°F | 65°F |
| Basil | 2 - 3 weeks after | 55°F | 65°F |
Soil temperature is the number most gardeners skip and the one that matters most for the warm-season group. A soil thermometer pushed 3 to 4 inches deep at mid-morning for three consecutive days gives you a real reading. Peppers set into 55°F soil sit still for a fortnight and are frequently overtaken by peppers planted two weeks later into warm soil. Sowing dates by zone are in when to start seeds by zone.
How long does hardening off take?
Seven to ten days for most crops, and up to two weeks for peppers, eggplant and anything grown under a very bright indoor light. The purpose is not cold acclimation alone: indoor seedlings have thin cuticles and soft tissue, and direct sun, wind and low humidity will scorch them within an hour if introduced all at once.
- Day 1 to 2: one to two hours in full shade, out of the wind, then back in.
- Day 3 to 4: three to four hours, with an hour of morning sun.
- Day 5 to 6: most of the day, half of it in sun. Let them feel a light breeze.
- Day 7 to 8: full day outdoors in sun, brought in at night.
- Day 9 to 10: left out overnight if temperatures allow, in a sheltered spot.
- Then transplant on a cloudy day or in the late afternoon.
Reduce watering slightly through the process — mildly stressed plants harden faster — but never to the point of wilting. The full method, including a cold-frame variation, is in hardening off seedlings.
What are the signs a seedling is ready?
Age is a weaker guide than condition. Look for two to four sets of true leaves, a stem thick enough that the plant stands without support, and a root ball that holds together when lifted but has not begun circling the base of the cell. Color should be even; a seedling that has gone pale or purple is short of light or nutrients and will transplant poorly.
Equally, know the signs of a seedling held too long. Roots spiralling at the bottom of the cell, water running straight through, growth that has stalled, or flower buds forming on a tomato still in a plug tray all mean the plant is root-restricted. Pot up into a larger container rather than pushing it out into cold soil, because a plant checked twice rarely recovers fully. If your seedlings have gone leggy while waiting, the fix is in fixing leggy seedlings, and if they are collapsing, start with why seedlings fall over.
What if a late frost is forecast after you plant?
Cover, and cover before sunset rather than after dark, so you trap the day's warmth in the soil. Floating row cover over hoops gives roughly two to four degrees of protection, doubled if you use two layers. Cloches, upturned buckets and cardboard boxes all work for individual plants provided you remove them in the morning. Wet soil holds more heat than dry soil, so watering the afternoon before a frost genuinely helps.
Direct-sown crops sidestep the whole question, and several vegetables strongly prefer it — carrots, beets, radishes, beans and most root crops resent transplanting. The comparison is in direct sow versus transplant, with the list in best vegetables to direct sow. For staggered plantings that spread the risk across several weeks, see succession planting schedule, and for bed preparation, best raised garden beds.
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