What hardening off does
Indoor light, even from good grow lights, is much weaker than the summer sun. Indoor air is still and the temperature is steady. Seedlings grown in those conditions have soft, thin leaves and stems. Gradual exposure to the outdoors lets them build a thicker waxy layer on their leaves, sturdier stems and better control over water loss. The result is less sunscald, less wilting and less transplant shock.
Bought transplants from a garden centre may or may not have been hardened off. If they came straight from a greenhouse, give them a few days of the same treatment.
When to start
Begin about one to two weeks before your planned planting date. For tender crops, that planting date is after your last frost. For hardy crops, it can be several weeks earlier. The seed-starting calculator puts a hardening-off date on your schedule automatically. A useful rule of thumb: don’t put warm-season seedlings such as tomatoes, peppers and basil outside when it is below about 50°F, and bring them in on cold nights throughout hardening off.
A simple schedule
This is a starting point. Stretch it out if the weather is harsh, and move faster if it is mild and overcast.
| Day | Where | How long |
|---|---|---|
| 1–2 | Shade, out of the wind | 1–2 hours |
| 3–4 | Shade or dappled light | 3–4 hours |
| 5–6 | Morning sun, afternoon shade | Half a day |
| 7–8 | Full sun | Most of the day; bring in at night |
| 9–10 | Full sun | Day and night, if night temperatures are suitable for the crop |
Hardy crops such as cabbage, broccoli, kale and onions can stay out on cool nights earlier in the schedule, as long as there is no hard freeze. Tender crops should come in whenever nights drop toward the 40s °F.
Watering during hardening off
Pots dry out much faster outside, especially in sun and wind. Check them every day, and twice a day on hot or windy days. Many gardeners slightly reduce watering during hardening off so plants are not lush and soft. Let the surface dry a little between waterings, but don’t let seedlings wilt badly.
Signs of trouble
- White or pale, papery patches on leaves usually mean sunscald from too much sun too soon. Move the tray back into shade for a day or two. Damaged leaves won’t recover, but new growth will be fine.
- Wilting means the plant is losing water faster than it can take it up. Water and move it into shade.
- Purple tinge on tomato and pepper leaves often appears in cold weather. It usually fades once plants warm up.
- Broken stems mean the wind was too strong. Find a more sheltered spot.
Shortcuts
- A cold frame makes hardening off easier. Put seedlings in and open the lid a little more each day.
- Row cover over transplants for the first few days after planting softens sun and wind and gives a few degrees of frost protection.
- Overcast, mild, still days are ideal for moving faster through the schedule, and for transplanting.
If you are out all day
The schedule assumes someone can move trays in and out. If you work away from home, a few adjustments help:
- Start on a weekend, when you can do the first short, shaded sessions yourself.
- Use a permanently shaded spot on weekdays, such as the north side of the house or under a table or bench, so seedlings get outdoor air and indirect light without full sun.
- Use row cover or shade cloth over the trays to soften sun and wind while you are away.
- Allow a few extra days. A slower, gentler schedule is safer than a fast one you can’t supervise.
Whatever the schedule, check the forecast every morning. A sudden hot, sunny or windy day is the most common cause of damage during hardening off.
Planting out with minimal shock
- Pick the right day: overcast and calm is ideal. Otherwise plant in the late afternoon so plants have the night to settle.
- Water seedlings an hour or two before transplanting so the root ball holds together.
- Prepare the hole first, then slide the plant out by supporting the root ball. Hold seedlings by a leaf, not the stem. A torn leaf is replaceable and a crushed stem is not.
- Plant at the right depth. Most plants go in at the same depth they were growing in the pot. Tomatoes are an exception. They can be planted deeper, and roots will form along the buried stem.
- Firm the soil and water well to settle soil around the roots.
- Shade and cover if needed for the first few days in hot or windy weather.
Check frost and soil first
Hardening off makes seedlings tougher, not frost-proof. For tender crops, check that you are past your last frost (the 10% risk date if you prefer to be safe) and that the soil has warmed. See soil temperature for planting. Keep row cover handy in case a late frost is forecast.
Related
- Starting seeds indoors
- Protecting plants from late frost
- Soil temperature for planting
- How frost dates work
- Frost dates for your ZIP
Written for SowByZip. Last reviewed 2026-10-20. See about this site for how pages are made.