Seedlings grown indoors under lights live in a controlled environment with no wind, no temperature swings, and no direct sun. Moving them straight into the garden is a shock that can stunt or kill them. Hardening off gradually acclimates plants to outdoor conditions.
About 7–10 days before your planned transplant date, begin setting seedlings outdoors in a sheltered, shaded spot for a few hours. Each day, increase the time and light exposure. By day 5–6, they can handle full sun for most of the day. By day 7–10, they are ready to transplant.
Wilting (bring them back into shade), sunburn on leaves (white or bleached patches), and cold damage. Do not put seedlings outdoors if night temperatures drop below what the plant tolerates. Cool-season flowers like snapdragons and stock handle cool nights; tropical flowers like celosia and lisianthus need warm nights.
A cold frame is a hardening-off shortcut—set trays inside with the lid propped open during the day and closed at night. A covered porch provides partial protection. The key is gradual exposure over at least a week.