How to Harden Off Seedlings
Seedlings raised indoors have never felt direct sun, wind, or a cool night. Hardening off introduces them gradually over 7 to 10 days so they don't scorch, wilt, or stall when you plant them out.
Why seedlings need it
Indoors, seedlings grow under soft, steady light, still air, and constant temperatures. Moved straight outside, they meet full sun many times brighter than a grow light, drying wind, and nights much colder than the house. Unprepared leaves can sunburn (bleached, papery white patches) within a few hours, and the shock can stall growth for weeks.
Hardening off thickens leaf surfaces and toughens stems so the plants can handle real weather. It takes about a week to ten days.
When to start
Count back 7–10 days from your planned transplant date. That date depends on the crop and your last frost date: cool-season crops like broccoli and onions go out a few weeks before the last frost, while tomatoes, peppers, and basil wait until after it. Your city page lists the transplant window for each crop.
A day-by-day schedule
| Days | Time outside | Conditions |
|---|---|---|
| 1–2 | 1–2 hours | Full shade, sheltered from wind. Bring in afterward. |
| 3–4 | 3–4 hours | Bright shade or gentle morning sun. |
| 5–6 | 5–6 hours | Some midday sun; light breeze is fine. |
| 7–8 | Most of the day | Full sun. Bring in at night if it's cool. |
| 9–10 | Day and night | Leave out overnight if night temperatures allow (see below), then transplant. |
The schedule bends with the weather. A cloudy, mild stretch lets you move faster; a hot, windy, or cold spell means holding steady or stepping back a day. If you miss a day, pick up where you left off rather than jumping ahead.
Night temperatures to watch
- Warm-season crops (tomatoes, peppers, eggplant, basil, squash, melons) do best kept in on nights below about 50°F. Cold nights won't always kill them, but they can stunt growth.
- Cool-season crops (broccoli, cabbage, kale, lettuce, onions) can stay out on nights down to about 40°F once they're a few days into the process.
- Bring everything in if frost is forecast. See frost vs. freeze for what each crop can take.
Tips for a smooth transition
- Water carefully. Pots dry out much faster outside. Check them daily, and twice on sunny, windy days, but don't keep them soaking wet.
- Stop fertilizing during hardening off. You want the plants toughening up, not pushing soft new growth.
- Use a tray or wagon so you can move everything in and out in one trip.
- A cold frame makes it easier. Prop the lid open a little more each day instead of carrying trays back and forth.
- Transplant on a cloudy day or in the late afternoon, then water well. This gives roots a night to settle before the next full day of sun.
Which plants need hardening off
Anything raised indoors or in a greenhouse: your own seedlings and, often, nursery transplants that came straight from a greenhouse bench. Seeds sown directly in the garden don't need it; they've lived outside from the start.
New to seed starting? See when to start seeds indoors for the full timeline, and how to protect plants from frost for a cold snap after planting.