How to Harden Off Seedlings
Seedlings raised indoors or in a greenhouse are tender and unaccustomed to sun, wind, and swings in temperature, so moving them straight into the garden often shocks or kills them. Hardening off is the week-or-so process of gradually toughening those plants to outdoor conditions so they can settle in and keep growing without a stall.
Start hardening off about seven to ten days before your intended transplant date, once the plants have a couple of sets of true leaves and the outdoor weather has genuinely settled for the crop you are growing. Cool-season vegetables like lettuce, kale, and broccoli can begin earlier and tolerate light chill, while warm-season crops such as tomatoes, peppers, and squash should not go outside until nights are reliably mild and any danger of frost has passed for your area.
The core idea is a slow ramp-up in exposure. On day one, set the seedlings outside in a spot that is sheltered from wind and gets only an hour or two of morning sun or bright shade, then bring them back indoors. Each following day, add roughly an hour of outdoor time and a little more direct sun, working up to a full day and eventually an overnight stay. Rushing this is the most common mistake, and the plants will tell you they were pushed too fast with bleached or scorched patches on the leaves.
Wind matters as much as sunlight. A breeze that feels pleasant to you can shred or flatten a seedling that has only ever known still indoor air, so give plants a protected location at first and let them meet stronger wind gradually. That same wind, along with gentle handling or a light brush of your hand across the tops, actually helps stems grow thicker and sturdier, so a little movement is good once the plants have started to acclimate.
Water and warmth need closer attention during this stretch. Pots and trays dry out much faster outdoors in sun and wind than they ever did indoors, so check the soil at least once a day and sometimes twice, watering when the top of the mix feels dry. At the same time, ease off on fertilizer and do not push tender new growth, since soft, lush foliage is more easily damaged by sun and cold than slightly leaner, hardier growth.
Keep an eye on the forecast and be ready to adjust or pause. Bring plants back inside or into a garage if a cold snap, hard rain, hail, or strong storm is coming, and hold off on leaving warm-season crops out overnight until nighttime lows stay comfortably above chilly. A cold frame, an unheated porch, or even a cardboard box you can cover the plants with makes a handy halfway house on borderline nights.
When the seedlings can spend a full day and night outdoors looking healthy and unbothered, they are ready to transplant. Choose a calm, overcast day or late afternoon rather than blazing midday sun, water the plants well before and after moving them, and firm the soil gently around the roots. Even well-hardened plants may droop a bit for a day or two as they settle, so keep the bed evenly moist and give them a little grace while the roots take hold.
Frequently asked questions
How long does hardening off take?
Plan on about seven to ten days for most vegetables and flowers. You can move a bit faster with tough cool-season crops and should go slower with heat-loving plants like tomatoes and peppers, especially if the weather is unsettled. The goal is a gradual increase in sun, wind, and outdoor time, so let the plants' response set the pace rather than forcing a strict schedule.
What happens if I skip hardening off?
Seedlings planted straight from indoors into full sun and wind often suffer transplant shock. You may see sunburned or bleached leaves, wilting, stunted growth, or plants that simply collapse, and even survivors tend to sit still for weeks instead of taking off. A short hardening-off period is far quicker than nursing damaged plants back to health or restarting from seed.
Can I harden off seedlings if I work during the day?
Yes, you just need a setup that does not require constant tending. Place trays in a spot that gets gentle morning sun and afternoon shade, or use a cold frame or a shaded porch where light and wind are naturally limited, and water well before you leave. On weekends you can give closer attention and lengthen the exposure, and moving plants a little more slowly over ten to fourteen days works fine for a busy schedule.
General guidance for US home gardens. Conditions vary by region and season; check with your local Cooperative Extension office for site-specific advice. Page updated Aug 4, 2026.