Last Updated on October 8, 2026

Hardening Off Seedlings: A Day-by-Day Schedule

Harden off seedlings over 7 to 10 days. Start with 2 to 3 hours outside in shade, add a few hours and a little more sun each day, and leave them out day and night for the last day or two before you plant. Keep them inside on any day that will not reach 45°F. Keep warm crops like tomatoes, peppers and squash inside at night until nights stay above 50°F. Start about a week before your transplant week.

Seedlings raised indoors have never felt full sun, wind or a cold night. Rushed outside, their leaves can scorch in the sun or wind, and they may even wilt and die (University of Minnesota Extension). Hardening off is the slow version of that move.

The 10-day hardening off schedule

Print this and write the date next to each day. If the weather is bad, skip the day and pick up where you left off.

The schedule spreads Nebraska Extension’s method over 10 days: 2 to 3 hours in a shaded spot out of strong wind, a few more hours of time and sun each day, and after several days of 10 to 12 hours outside, a couple of days left out around the clock. The first outing goes in the warmest part of the day, as South Dakota State University Extension advises. The hours in between are even steps from that start to that finish.

• Short on time? Nebraska’s range starts at 7 days. Combine days 1 and 2, days 3 and 4, and days 7 and 8.
• More time? SDSU allows 7 to 14 days, and my guide to transplanting seedlings uses 10 to 14. Repeat a day whenever the plants look stressed.
• Water less, but never let them wilt. Cutting back on watering slows growth, which is part of hardening (University of Maryland Extension). Check them every day.
• Hold off on feeding. Skip fertilizing before and during hardening off (Nebraska Extension).
• Skip windy days early on. Tender seedlings should not go out on windy days (University of Maryland Extension).

Minimum temperatures for cool and warm crops

How much cold a seedling can take depends on the crop. The Plan My Garden planner sorts every crop into hardy, semi-hardy or tender; on this page, hardy and semi-hardy crops count as cool-season crops and tender ones as warm-season crops. The day column is for days 1 to 8, when the trays come in at night. The night column is for days 9 and 10.

• 45°F to start the day outside. Keep every tray inside on a day forecast to stay below 45°F (SDSU Extension). Hardy crops handle more cold than tender ones, but even cold-hardy plants are hurt by freezing temperatures before they are hardened (University of Maryland Extension).
• The 40s at night for cool crops. Hardy cool-season plants can take temperatures in the 40s, and once they are well hardened, a light frost will not hurt them (Nebraska Extension).
• Not too cold for too long. Too much time below 40°F can make hardy and semi-hardy seedlings flower early (Utah State University Extension).
• 50°F nights for warm crops. Keep tomatoes, peppers, squash and melons away from temperatures below 45 to 50°F (Utah State). Cucumbers and melons can stay out overnight once it will not drop below 50°F (University of Maryland).
• Wait for warm nights before you plant for good. Warm-season plants prefer nights of at least 60°F before they go into the garden, and they cannot stand freezing even after hardening off (Nebraska Extension). During hardening itself, the 50°F overnight line above still applies.

The last column comes from my vegetable frost tolerance chart, which has the line for each crop once it is planted.

When to start hardening off each crop

Start hardening off the week before your transplant window opens. The 10-day schedule then finishes inside the window. These transplant windows are the ones the Plan My Garden planner uses, counted from your average last spring frost.

To turn these weeks into dates, look up your last frost date by zip code, or enter it on my seed starting calendar, which uses the same planner windows. The weather still has the last word: for tender crops, also wait for the warm nights described above.

The cold frame shortcut

A cold frame saves you carrying trays in and out every day. Closed, it protects plants from cold evenings; opened, it lets them feel outdoor conditions (Nebraska Extension).

1. Put it in the sun. My cold frame tips call for at least six hours of sun a day, a southern exposure if you can, and a fence or other windbreak in a gusty yard.
2. Shade it for the first days. A piece of burlap or light-colored fabric over the frame cuts the sun and heat, the same trick my cold frame tips use on hot days.
3. Open the lid wider and longer each day. Leave it open at night too if nights are not too cool, until the lid comes off for good (University of Missouri Extension).
4. Prop it open on sunny days. A closed frame can get hot enough on a sunny day to make plants wilt (University of Wisconsin Extension). An automatic vent opener, which my cold frame tips suggest, opens the lid as the frame heats up.
5. Close it on cold nights. Be ready to put the lid back on for a sudden late spring freeze (University of Missouri Extension).

A cold frame only buys a few degrees. Inside, it is generally no more than 5 to 10 degrees warmer than the outside air, and a mat or blanket over it adds only a few degrees more (University of Missouri Extension). So a 35°F night is only about 40 to 45°F inside, still below the 50°F that tender seedlings need. Bring them in.

Sunscald, wind burn and other warning signs

• Sunscald. Scalded leaves turn papery white or tan only where the sun hit them, and the tissue next to the scald stays bright green. Once leaves are damaged, all you can do is support the plant until it grows new ones. That comes from Jerry Brust, a University of Maryland vegetable specialist, writing for commercial growers in the University of Delaware Weekly Crop Update.
• What makes it worse. Transplants started indoors that have not been hardened off, or that are short on water, are more prone to this sunburn, and it shows first as bleached patches on exposed leaves (MOFGA, the Maine Organic Farmers and Gardeners Association). Keep the cells moist.
• Wind. Strong spring wind can rip leaves and snap stems, and even a fairly low wind throws soil at leaves and stems like sandblasting (Nebraska Extension).
• Normal changes. Hardened plants may wilt when first set out in full sun but generally recover within a day or so (University of Minnesota Extension). Some leaves may yellow or dry and brown during hardening off, which is normal (Utah State).
• Freeze injury. Look for water-soaked or see-through tissue, limp growth after thawing, and blackened or browned areas (SDSU Extension).
• Over-hardening. Carried too far, the cold stops growth: cauliflower makes thumb-sized heads that never develop, and cucumbers and melons stop growing (University of Maryland Extension).

What to do in a cold snap

• Bring them in and wait. Keep plants inside on days forecast below 45°F, or when strong wind or rain is on the way (SDSU Extension).
• Keep them under light. Seedlings left in an unlit garage for long can stretch or turn yellow (SDSU). Put them back under their lights or in a bright window.
• Add days, do not skip steps. Pick the schedule back up at the last day that went well. SDSU’s range runs to 14 days, so a few lost days still fit.
• Hold the planting date. If a frost advisory is in the forecast, wait a few more days to plant rather than lose the plants (SDSU).
• Already planted? Hardened plants may still need protection from high winds and extreme cold (SDSU). My guide on how to protect plants from frost covers row cover and other covers step by step.

Hardening off tomatoes and peppers

Tomatoes and peppers get the same 10-day schedule, with two rules. First, they stay inside on any night below 50°F. Second, the planner does not transplant them until 2 to 3 weeks after your last frost, so the table above starts their hardening off the week after your last frost. There is no rush.

My tomato transplanting guide puts indoor-grown tomatoes outside for a week or two first, a little longer each day, and out of direct sun for the first couple of days. My pepper growing guide gives peppers 7 to 10 days, with no direct sun for the first day or so.

Planting day

• Pick a cloudy day or the evening. A plant in shock does not want full sun, as my tomato guide puts it.
• Water the seedlings first. My transplanting guide waters seedlings well before transplanting, and again once they are in the ground.
• Bought plants? Ask the nursery whether they were already hardened off; if they were, my transplanting guide says you may be able to skip this. If they came straight out of a greenhouse, give them at least a week, the minimum SDSU Extension sets for purchased plants too.

Quick questions

How long does it take to harden off seedlings?

Plan on 7 to 10 days, the range Nebraska Extension uses. Give it up to 2 weeks if cold or windy days interrupt the schedule; SDSU Extension allows 7 to 14 days.

What temperature is too cold to harden off seedlings?

Keep seedlings inside on days that will not reach 45°F. Leave tomatoes, peppers, squash and other warm crops out overnight only when the night stays above 50°F. Cool crops like broccoli and cabbage can take temperatures in the 40s, but long stretches below 40°F can make them flower early, and even cold-hardy seedlings are hurt by a freeze before they are hardened.

Can you harden off seedlings in a cold frame?

Yes. Open the lid a little wider and longer each day, prop it open on sunny days so it does not overheat, and close it on cold nights. A cold frame is generally no more than 5 to 10 degrees warmer than the air outside, so bring tender seedlings in on cold nights.

Do you need to harden off plants from a nursery?

Ask first. My transplanting guide says that if the nursery already hardened them off, you may be able to skip it. If they came straight from a greenhouse, give them at least a week, as SDSU Extension does for purchased plants.

What happens if you do not harden off seedlings?

Their leaves can scorch in the sun or wind, and the plants may wilt or even die. Sun-scalded leaves turn papery white or tan and do not recover, so the plant has to grow new ones.

Get your transplant week

Your transplant week comes from your frost dates, and the planner shows it for every crop. Plan My Garden is free to start: enter your zip code and it finds your frost dates, draw your beds, pick tomatoes, broccoli and everything else you want to grow, and it places each plant at square-foot spacing and builds a planting schedule (start indoors, transplant and direct sow dates) from your frost dates.

Related: How to Transplant Seedlings into the Garden · Seed Starting Calendar by Last Frost Date · Vegetable Frost Tolerance Chart