Hardening Off Seedlings: A 7-Day Schedule

Hardening off means getting seedlings used to outdoor conditions gradually, over about a week, before planting them in the garden. Plants raised under lights or on a windowsill have never felt direct sun, wind or a cool night. Moved straight outside, they can scorch, wilt or stop growing for weeks.

Start hardening off about 7 to 10 days before your planting date. The Seed Starting Calculator gives planting dates for each crop from your last frost date.

The 7-day schedule

Day	Time outdoors	Where
1	1–2 hours	Full shade, out of the wind
2	2–3 hours	Shade or dappled light
3	3–4 hours	Morning sun, afternoon shade
4	5–6 hours	Some direct sun
5	Most of the day	Sun, sheltered from strong wind
6	All day	Full sun
7	Day and night	Full sun; overnight if temperatures allow

Move faster on cloudy, mild days and slower on hot, sunny or windy ones. If a cold snap interrupts the schedule, keep the plants inside and pick up where you left off.

Overnight temperatures

• Cool-season crops such as onions, lettuce, broccoli and kale can stay out overnight once frost is no longer forecast.
• Warm-season crops such as tomatoes, peppers, eggplant, basil, cucumbers and squash should come in if the night will drop below about 50°F. Cold nights stunt them even without frost.

Check your average and safer frost dates with Frost Dates by ZIP Code.

Tips for a smooth week

• Water well before each outing. Pots dry out fast in sun and wind. Check them every day; small cells may need water twice.
• Use a tray or wagon. Moving all the plants at once makes the daily routine quick.
• Protect from wind. Set trays against a wall or fence, or inside an open cardboard box for the first few days.
• Watch for pests. Slugs and birds find seedlings fast. Keep trays off the ground on a table or bench.
• Stop feeding. Pause fertilizer during the week so plants toughen up rather than putting out soft new growth.

Signs of trouble

• White or papery patches on leaves are sunscald. The leaf is damaged but the plant usually recovers. Go back to shorter, shadier sessions for a day or two.
• Wilting in the afternoon means the plants are drying out or getting too much sun at once. Water and give them shade.
• Purple tinge on tomato leaves often appears in cool weather. It fades once nights warm up.

Planting day

When the week is done, plants are ready to go in the ground.

1. Plant on a cloudy day or in the late afternoon so they settle in before facing full sun.
2. Water the seedlings an hour before planting, and water the planting holes too.
3. Disturb the roots as little as possible. Plant tomatoes deeper, up to the first leaves.
4. Water again after planting, and keep the soil moist for the first week.
5. Keep frost cloth or old sheets handy in case of a late cold night.

Shortcuts

• Cold frame: a box with a clear lid makes hardening off easy. Open the lid a little more each day.
• Bought transplants: plants from a garden center that were kept outdoors are already hardened off. Plants sold from inside a greenhouse still need a few days.
• Direct sowing: beans, squash, cucumbers and many other crops can be sown straight into the garden and need no hardening off at all.