How to Harden Off Seedlings
Harden off seedlings for 7 to 14 days before transplanting. Start in shade out of the wind, add sun each day, and bring them in if frost or storms threate...
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seed starting transplant hardening off
How to Harden Off Seedlings
Seedlings grown indoors have never felt full sun, wind, or cool nights. Moving them outside a little more each day toughens them up. This is called hardening off. Without it, leaves can scorch and plants can wilt or die (Minnesota).
Plan your date on the Seed Starting Setup Desk, then find your sow and transplant dates on the seed-starting calendar.
How Long It Takes
Extension sources give a range.
| Source | Length |
|---|---|
| Iowa State | 7 to 10 days |
| Nebraska | Start 10 to 14 days before planting |
| Minnesota | Start two weeks before planting |
| Clemson | Two to three weeks, starting when plants are about 3 inches tall |
Most sources land between 7 days and two weeks, and Clemson goes up to three. Clemson says growth may slow for a while during hardening, and the change must be gradual or plants can be damaged.
Before You Start
- Clemson says to put a fan near seedlings once they have two true leaves, so they sway gently. The fan dries plants, so check moisture often.
- Know your last frost date. Check what first frost means and your frost dates in My Garden.
Day by Day
- Pick a shady spot out of the wind. Set trays out for a few hours in the afternoon (Minnesota).
- Bring them in before it cools off at night (Minnesota).
- Each day, leave them out longer and give a little more direct sun (Minnesota, Iowa State).
- Check them once or twice a day. Water well when the soil surface feels dry (Iowa State).
- At the end, unless a freeze is forecast, plants can stay out in a sunny spot until planting day (Minnesota).
Clemson says to cut back on watering during hardening, but not let tomatoes wilt. Tomato leaves may develop purple veins as they harden.
Weather Watch
Iowa State says to move plants indoors if strong wind, a severe storm, or an overnight frost threatens them. See protecting plants from early frost.
A cold frame is an easy way to harden off. Vent the lid a bit farther each day so plants get used to wind and cold (Minnesota). See cold frames and winter sowing.
Transplant Day
Plant in the evening or on a cloudy day to ease the shock (Iowa State, Clemson, Minnesota). Hardened plants may wilt in full sun at first, and Minnesota says they usually recover within a day or so.
Next Steps
- Set your date on the Seed Starting Setup Desk.
- If seedlings are tall and thin, read how to fix leggy seedlings.
- Start the next batch with how to start seeds indoors.
Sources: Iowa State Extension, Selecting, Hardening, and Planting Bedding Plants. University of Minnesota Extension, Starting seeds indoors. Nebraska Extension G1650, Growing Tomatoes in the Home Garden. Clemson Extension HGIC 1259, Starting Seeds Indoors, and the Clemson tomato fact sheet.