How to Harden Off Seedlings Over Seven Days

Hardening off is the deliberate, gradual transition of indoor-grown seedlings to the outdoor environment so stems, leaves, and root systems develop tolerance to sun, wind, and fluctuating temperatures. Begin when seedlings have at least 2 true leaves, and aim to finish 7 days before transplanting into beds or larger containers. Ideal daytime temperatures during the process are 50–75°F; wait until nighttime temperatures consistently stay above 45°F before attempting overnight stays outdoors.

Set up a sheltered staging area close to the house that gets morning sun and afternoon shade. You need: pots or flats with drainage, a basic dial or digital thermometer, a 3×3 ft piece of 30–50% shade cloth, a lightweight windbreak (a board or fence panel), and a watering can or hose with a gentle spray nozzle. Place trays on a bench or table 18–36 inches off the ground to reduce slug attacks and to expose plants to gentler airflow similar to garden beds.

Day 1: Move seedlings outside for 1–2 hours in the morning sun and bring them back inside before midday heat; keep them in bright, indirect light for the remainder of the day. Day 2: 2–3 hours, including a short midday period behind shade cloth. Day 3: 3–5 hours, with direct morning sun and shade during peak hours. Day 4: 6 hours, remove shade for up to 2 midday hours if plants show no wilting. Day 5: 8 hours, include a couple of hours of direct sun and introduce a little wind by leaving them a few feet from a building edge. Day 6: 10–12 hours; if nighttime forecast is above 50°F, leave them out later but still bring them in if nights dip below 45°F. Day 7: Full day and, if nights remain above 50°F, a supervised overnight outside; otherwise bring in for the night and transplant the following day that meets safe-night criteria.

Check plants within 30–60 minutes after returning them indoors on days 1–3. If leaves wilt, shade and mist for 10–15 minutes and reduce next exposure by 25–50%. Keep potting soil evenly moist to 1 inch depth; that typically means adding 150–300 ml (about 1/3 to 2/3 pint) of water to small 3–4 inch pots or 0.5–1 L for 4–6 inch pots during midday checks, depending on mix and temperature. Delay any routine fertilizer until seedlings have completed hardening and been in the ground for 7–10 days; if you must feed, use a quarter-strength solution to avoid shock.

If seedlings wilt or develop sunscald (brown patches on leaves) during the process, move them into 50% shade and out of direct wind for 24–48 hours while reducing daylight exposure by about 30–50%. To revive a severely wilted seedling, water until the top 1 inch of mix is moist—typically 100–200 ml for a 3–4 inch pot—and place in filtered light for 6–12 hours before resuming the schedule at a reduced exposure by one day.

Protect against wind damage with a temporary windbreak during the first 3 days; set a board 1–2 ft upwind or place seedlings behind a fence. If midday temperatures exceed 80°F, use 30–50% shade cloth during the hottest 2–4 hours to prevent sunburn and excessive moisture loss. Secure shade cloth with clips or twine so it does not flap and abrade leaves.

Know when seedlings are ready for permanent planting: look for 2–4 true leaves for small greens, or 4–6 true leaves and a sturdier stem for larger crops like peppers and tomatoes. Check roots by easing a seedling from its pot—roots should hold the soil but not be tangled in a dense circling mass. Spacing at transplant depends on crop: lettuce 6–8 inches, brassicas 12–18 inches, tomatoes 18–24 inches. Dig a hole as deep as the root ball plus 1 inch of loosened soil for settling.

On transplant day, water seedlings 15–30 minutes before planting so the root ball is moist but not saturated. After planting, apply 1/2 to 1 liter (0.5–1 qt) of water per small plant to settle soil; for larger transplants use 1–2 gallons depending on size. Mulch with a 2 inch layer of straw or shredded bark, and consider a cloche or row cover for the first 1–3 nights if temperatures may fall below 50°F. Wait 7–10 days after transplant before applying a light feed; use a half-strength fertilizer at that time if growth appears slow.

Further reading: Seven Seedling Watering Mistakes That Drown New Plants · How to Grow Kitchen Herbs in Containers
Sources: UMN Extension