Hardening off seedlings before transplanting
Last reviewed September 2026 · written from public-domain USDA/NOAA data and Cooperative Extension references — see how we source data
Seedlings grown indoors or in a greenhouse have soft tissue, no wind exposure, and filtered light. Moving them straight outside can sunburn or snap stems that were perfectly healthy an hour earlier. Hardening off is the gradual process that prevents that.
Why it matters
Direct sun, wind and temperature swings are all stresses a seedling has never faced under grow lights or a windowsill. Sudden full exposure causes sunscald (bleached, papery patches on leaves), wind damage, and transplant shock that can set a plant back weeks or kill it outright.
The 7–10 day process
Start with an hour or two in a shaded, sheltered spot, bringing plants back in afterward. Each day, add an hour and move toward more direct sun, until by the end they are outside all day in full sun with no wind break.
Once they are tolerating a full day outside, leave them out overnight for the final couple of days before transplanting, as long as there is no frost risk.
A 10-day hardening-off schedule
What you need
A sheltered spot in light shade - the north side of a house, under a table or a tree - for the first few days. A tray or crate makes carrying seedlings in and out quick, which matters because you will do it every day.
A cold frame is the easiest shortcut: seedlings sit in it with the lid open during the day and closed at night, and you simply open it further each day.
Weather to watch
Skip a day or bring plants back in during hard wind, hail, or an unexpected cold snap — a single bad day can undo a week of progress. There is no penalty for pausing the schedule; there is a real one for pushing through bad weather.
Signs you moved too fast
Bleached, curled or crisp leaf edges mean back off — return to a shadier, more sheltered spot for a day or two before resuming. Water well before and after the move, and transplant on a cloudy day or in the evening once hardening off is complete.
Which plants need it
Anything raised indoors or in a greenhouse: tomatoes, peppers, eggplant, squash, cucumbers, and cool-season transplants such as broccoli, cabbage, lettuce and onions. Seeds sown directly outdoors are already adapted and need nothing.
Cool-season seedlings can be hardened off earlier in spring and tolerate nights in the 40s°F. Warm-season seedlings should not be left out overnight until nights stay above about 50°F.
Store-bought transplants
Plants from a garden center often come straight from a greenhouse. Give them at least two or three days of gradual exposure before planting, especially if they were displayed indoors or under shade.
Short on time?
If you have only four or five days, keep the plants in bright shade or under row cover for the first half and build up faster, watching closely for bleached leaves. A cloudy, calm spell is the ideal time to compress the schedule.
Common questions
How long does hardening off take?
About 7-10 days for most seedlings. Start with an hour or two in shade and add time and sun each day until the plants are outside all day in full sun, then leave them out overnight for the last couple of nights if there is no frost risk.
Can I skip hardening off?
You can, but indoor-raised seedlings moved straight into full sun often get sunscald - bleached, papery patches - and can stall for weeks or die. A few days of gradual exposure is far less work than replacing plants.
Can seedlings stay outside overnight while hardening off?
Cool-season seedlings like broccoli, cabbage and lettuce can once nights are above about 40°F. Tomatoes, peppers and other warm-season crops should come in until nights stay above about 50°F.
Do nursery plants need hardening off?
Usually yes, for a few days. Many are grown and displayed under cover, so they are as soft as home-grown seedlings. Set them in light shade first and build up to full sun.