Growing food2 min read

Hardening off seedlings

A simple plan to toughen up seedlings before they go outside

Why harden off

Seedlings raised indoors have never felt direct sun, wind or a cold night. Move them straight outside and they can be scorched, shocked and flattened, and their leaves can sunburn.

Hardening off gets them used to outdoor life a little at a time, so they keep growing after transplanting.

When to start

Begin 1 to 2 weeks before your planting date; depending on the crop, it takes anywhere from 5 to 14 days. Warm-season crops like tomatoes and peppers shouldn't be planted out until after your last frost. Planted into warm soil, above about 60°F (16°C), they need less hardening.

A simple plan

Add a little more sun and time outdoors each day, and watch the weather forecast.

Start with an hour or two outside in a shaded spot, out of the wind, in the gentle sun of early morning or late afternoon.
Each day, add an hour or two more of sun.
Bring tomatoes, peppers and other warm-season plants in at night when it will drop below 50°F (10°C).
Keep tender seedlings indoors on windy days or below 45°F (7°C).
Bring everything in if a storm, strong wind or frost is on the way.

Care during hardening

Water a little less than usual so the plants toughen up, but never let them wilt. Hold off on feeding before and during hardening.

Don't overdo it, either. Seedlings held back too long can stall: cauliflower may form tiny heads, and cucumbers and melons can stop growing.

Planting day

Water the plants a few hours before you transplant. Plant on a cloudy, calm day or in the evening, which lessens transplant shock. Water them in well, using a weak liquid fertilizer if you like, following the label.

Shield them from strong wind and direct sun for a few days afterward. A cold frame, a sheltered porch or a wall makes the whole process easier.

Sources

General guidance, not a diagnosis. Written by PlantWiz in our own words from the sources above.