Hardening Off Seedlings – The Ten-Day Move Outdoors
A seedling that spent six weeks in still, filtered air is not ready for weather. Hardening off is the ten days that build the leaf and stem for it — and the step most often skipped.
TL;DR
- Start seven to ten days before planting out, in shade and shelter, then build up.
- Bring in at night until lows stay above about 45F and the frost date has passed.
- Sunburn and windburn both show up within hours — a cold frame or row cover removes the risk.
The ten-day schedule
| Day | Where | How long |
|---|---|---|
| 1-2 | Sheltered, full shade, out of wind | 1-2 hours |
| 3-4 | Same spot, an hour of morning sun | 3-4 hours |
| 5-6 | Half a day of sun, still sheltered | Morning to mid-afternoon |
| 7-8 | Full sun most of the day | All day, in at night |
| 9-10 | Full sun and wind exposure | Out overnight if lows allow |
Two adjustments matter more than the exact numbers. On a bright, breezy day, halve the exposure — sun and wind together do more damage than either alone. And if the forecast turns cold mid-schedule, hold at the previous step rather than pushing through; the plant keeps the hardening it already has.
What goes wrong, and how it looks
Sunburn appears as bleached, papery patches on the leaves facing the sun, within a couple of hours of the first unprotected exposure. The leaf will not recover, but the plant usually will: shade it for three or four days and restart from a lower step.
Windburn is subtler — the leaves curl, the edges go crisp, and the plant wilts even though the mix is damp. Wind pulls water out of the leaf faster than the roots can replace it, and a plant that grew indoors has thin cuticle to resist it. Shelter, not water, is the fix.
Cold check shows up overnight: leaves go purple or blue-green and growth stops for a week or more. That is the plant's metabolism slowing, and it is what happens when trays go out before the frost date for that crop.
The shortcuts that are worth it
A cold frame does the whole schedule passively: trays go in with the lid propped, the lid opens wider each day, and the plants get sun, wind and cool nights without you carrying anything. A row cover over the first nights outside does the same for the last step and doubles as frost protection if the forecast turns.
If you have neither, park the trays against a north wall for the first two days — bright but without direct sun — and move them a few feet a day toward where they will be planted. It is slower and less controlled, but it beats trusting an afternoon.
Tools for this job
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Frost protection row cover
A light fleece over the first nights out, when the forecast turns colder than expected.
Garden shade cloth
Filters the first days of direct sun so leaves harden rather than burn.
Cold frame with vented lid
The passive version of hardening off, with no carrying trays in and out.
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Frequently asked questions
What does hardening off actually change in the plant?
The leaf cuticle thickens, stems stiffen and the plant adjusts its water use. Without it, a seedling grown in still air and filtered light burns or wilts within an hour of real sun and wind — the tissue simply was not built for it.
How long does hardening off take?
Seven to ten days for most seedlings, longer for tender crops like basil and cucumber started early. Ten days is the honest number if the weather is mixed; a rushed four-day version works only in mild, overcast weather.
Can I skip hardening off?
Only with a cold frame, which does the same job passively: put trays in with the lid propped for a week, opening further each day. Moving plants straight from indoors to the garden is the most common way a healthy tray is lost in an afternoon.
What does sunburn on seedlings look like?
Bleached, papery patches on the leaves facing the sun, appearing within hours. They do not heal, but the plant usually survives and pushes new growth. Shade it for a few days, then resume the schedule more slowly.
What about wind?
Wind dries leaves faster than roots can replace the water, so a plant that was fine indoors collapses in ten minutes of a breezy afternoon. Start in a sheltered spot and use the first days to build up, not to test the plant.
Do I bring plants in at night the whole time?
Until the overnight low is above roughly 45F, yes. After that, and once the frost date has passed for that crop, they can stay out. Hardy crops like kale and broccoli can go out permanently a couple of weeks earlier than tomatoes.