Hardening off seedlings means moving indoor-grown plants outside a little longer each day, over about 7 to 10 days, so they can get used to full sun, wind and cooler nights before you plant them in the garden. Start with an hour or two in a shaded, sheltered spot, add time and sun each day, and leave them out overnight for the last few days. Seedlings that skip this step often get sunscald, wilt, or stall for weeks after transplanting. The day-by-day schedule below works for tomatoes, peppers, cabbage, lettuce and most other vegetables.
Starting your own transplants this spring? Our Tomato Seed Starter Kit comes with 8 heirloom varieties plus starter pots, soil disks and plant markers.
Why hardening off matters
Seedlings grown under lights or on a windowsill have never felt direct sun, wind or a cold night. Their leaves are thin and soft, with little of the waxy coating that protects outdoor plants. Put them straight into full sun and the leaves can bleach white or brown within hours. Wind dries them out faster than their roots can keep up. Hardening off thickens the leaves, toughens the stems, and slows growth slightly so the plants are ready for the move.
A 10-day hardening off schedule
Use this as a guide, not a rule. If a day is very hot, windy or cold, repeat the previous day's step instead of moving ahead.
| Day | Time outside | Where and how |
|---|---|---|
| 1 | 1-2 hours | Full shade, out of the wind (a porch or the north side of the house) |
| 2 | 2-3 hours | Shade or dappled shade |
| 3 | 3-4 hours | Dappled shade, with an hour of morning sun |
| 4 | 4-5 hours | About 2 hours of morning sun, shade in the afternoon |
| 5 | 5-6 hours | Half sun; some breeze is fine now |
| 6 | 6-8 hours | Mostly sun, with shade during the hottest part of the day |
| 7 | All day | Full sun; bring in at night |
| 8 | All day and night | Leave out overnight if nights are mild (see temperatures below) |
| 9 | All day and night | Full sun and outdoor conditions |
| 10 | Transplant | Plant on a cloudy day or in the evening, and water well |
Short on time? You can compress the schedule to about a week by doubling up the early steps, but don't skip the first shaded days. Some university extension services recommend up to two weeks, which is a good idea for tomatoes and peppers if your spring weather swings a lot.
Temperatures to watch
Hardening off doesn't make plants frost-proof, so watch the forecast and bring seedlings in when it turns cold.
- Cool-season crops (cabbage, broccoli, kale, lettuce, onions): can handle nights down to about 40Β°F once hardened. They usually go into the garden a few weeks before your last frost date.
- Warm-season crops (tomatoes, peppers, eggplant, melons, basil): keep them inside on nights below 50Β°F. Michigan State University Extension notes that temperatures below 50Β°F can stunt some warm-season crops such as melons. Plant them out after your last frost, once nights stay above 50Β°F and the soil has warmed.
Not sure when your planting window opens? Our seed starting calendar by last frost date lays out sowing and transplant timing, depending on your zone.
Tips that make it easier
- Use a tray or crate. Keep all the pots in one tray so you can carry them in and out in one trip.
- Check water daily, sometimes twice. Small pots dry out fast in sun and wind. Water when the top of the mix feels dry, but don't keep it soggy. A slightly drier routine is part of toughening plants up.
- Hold off on fertilizer. You want sturdy growth, not a flush of soft new leaves, during hardening off.
- Brush the tops. Running your hand lightly over the seedlings once a day indoors, or a small fan in the weeks before, helps build stronger stems.
- Raise them off the ground. A table or bench keeps slugs and curious pets away.
- Set a reminder. The most common way to lose seedlings is forgetting them outside on a cold or windy night.
Signs something went wrong
- White or papery patches on leaves: sunscald from too much sun too soon. Move the plants back to shade for a day or two. Damaged leaves won't recover, but new growth will be fine.
- Wilting in the afternoon: the pots dried out or the wind was too strong. Water, give them shade, and repeat that day's step.
- Purple-tinged leaves on tomatoes: often a sign of cold. Bring them in at night until it warms up.
- Broken stems: wind. Use a more sheltered spot.
Which seedlings need hardening off?
Anything started indoors: tomatoes, peppers, eggplant, cabbage and other brassicas, lettuce, onions and herbs. Store-bought transplants that were grown in a greenhouse benefit from a few days too. Crops you sow directly in the garden, like beans, peas, squash, corn, carrots and radishes, don't need it, because they grow up outdoors from the start.
New to growing transplants? Our guide to starting vegetable seeds indoors covers sowing, light and potting up, and the seed starting supplies checklist lists what you need. If seedlings are collapsing at the soil line before they ever make it outside, read what damping off is and how to prevent it.
Plan your transplants
For peppers, the Pepper Seed Starter Kit has 8 heirloom varieties with pots and soil disks. For cool-season transplants, try the Lettuce and Leafy Greens Starter Kit. To see every option, browse our seed starter kits. If you want a full garden's worth of seed for both transplants and direct sowing, the Survival Seed Vault Kit has 35 heirloom vegetables in one bag, and our 4x8 raised bed plan shows how to lay them out.
FAQ
How long does it take to harden off seedlings?
About 7 to 10 days for most vegetables. Up to two weeks is fine, and it's a good idea when spring weather is unsettled.
Can I skip hardening off?
You can, but seedlings often get sunscald or stall for a week or more after transplanting. On a cloudy, mild stretch of weather the damage is smaller, but it's still a gamble.
Can I harden off seedlings in a cold frame?
Yes. Open the lid a little more each day, and close it at night when it's cold. A cold frame makes hardening off easier because you don't have to carry trays in and out.
What if it rains during hardening off?
A gentle rain is fine and saves you watering. Bring seedlings in or under cover for heavy rain, hail or strong wind.
More questions about our seeds and kits are answered on our FAQ page.
Reviewed by Ryan, owner of Gardeners Basics, an heirloom seed company that ships from Spanish Fork, Utah.
