Hardening Off: How to Move Seedlings Outdoors Without Shock
Updated June 2026 · SowWhen
You've grown sturdy tomato and pepper seedlings under lights, the last-frost window has arrived, and it's tempting to plant them straight into the garden on a sunny afternoon. Don't. Seedlings raised indoors have never felt direct sun, real wind, or a cold night, and moving them out all at once can scorch their leaves and stall their growth for weeks. The gentle transition that prevents this is called hardening off, and it takes about a week.
Why indoor seedlings need it
Under grow lights, plants develop thin cuticles and soft tissue because they've never had to cope with UV, temperature swings, or moving air. Put them straight into full sun and the leaves can bleach white or brown within hours — a real sunburn. A stiff breeze can snap tender stems that never had to flex. Hardening off toughens the leaf surface, thickens the stems, and lets the plant adjust its water use before it has to survive on its own outdoors.
The one-week routine
Start on a mild, still, overcast day. Set the seedlings outside in a sheltered, shaded spot for just an hour or two, then bring them back in. Each day, add an hour or so of exposure and a little more direct sun, working up over about seven to ten days from deep shade to a full day out. By the end of the week they should be tolerating morning sun and light wind comfortably and even staying out overnight, provided no frost is forecast. Then they're ready to plant.
Practical cautions
Wind and water are the two things that catch people out. Small pots outdoors dry startlingly fast in sun and breeze, so check moisture at least once a day during the process — a wilted seedling loses days of progress. Protect plants from strong midday sun early on, keep them off the ground away from slugs, and never leave tender crops out for a night that might dip near frost. If a cold snap or storm rolls in mid-hardening, just bring everything back inside and resume when it passes; there's no harm in stretching the process over a few extra days.
Planting out
Transplant on a cloudy day or in the evening rather than into blazing midday sun, so the plants settle in without immediate heat stress. Water them in well, and consider a temporary shade cover for the first day or two if the forecast is bright. Hardened-off, properly timed transplants shrug off the move and keep growing; unhardened ones sulk for weeks — which is why this unglamorous week is one of the highest-payoff habits in the whole seed-starting process.
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