Indoor leaves have not experienced full sun, drying wind, rain, or large temperature swings. Sudden exposure can scorch, wilt, or kill them. Start hardening off about two weeks before the planned transplant date.
- On a mild afternoon, place seedlings outdoors for a few hours in bright shade, protected from wind.
- Bring them in before temperatures drop. Each day, lengthen the visit and add a little more direct morning sun.
- Check moisture often: small pots dry faster outside, but do not leave trays standing in water.
- Delay the process during frost, severe wind, hail, heavy rain, or an abrupt heat wave.
- By the end, let frost-tender seedlings remain outside only when no freezing temperature is forecast.
An open garage or window is not enough if it provides no gradual sun exposure, and a windy sill is a falling hazard. Fertilizer is not what makes a seedling “tough”; gradual environmental exposure does.
Transplant on a cloudy day or late afternoon and water immediately. A ventilated cloche or floating row cover can soften the transition, but it must be checked for overheating.