How to harden off seedlings before planting out

Why it matters

Indoor seedlings have soft growth. Moving them straight outside scorches and chills them. The RHS gives about two to three weeks, longer from a warm start, and quicker for hardy plants than for tender ones. Hardening off does not make a frost-tender plant hardy: do not plant tender crops out before your last frost date.

A routine with no cold frame

Based on the RHS method:

1. Week 1: put plants in a sheltered spot, for example in front of a south-facing wall or hedge, covered with two layers of fleece. Out by day, in at night.
2. Week 2: reduce to one layer of fleece, and remove it during the day towards the end of the week. If the night is mild, leave plants out, still covered.
3. End of week 3: leave uncovered, then plant out.

A gentler start

Penn State Extension suggests starting with one to two hours a day in shade and increasing over one to two weeks. Clemson suggests a fan blowing gently on seedlings once they have two true leaves, and withholding a little water towards the end, without letting them wilt. Bring plants in each evening until night-time temperatures stay at or above 50°F (10°C).

Frost surprises

Keep an old curtain or extra fleece to hand. If a sharp frost is forecast, cover plants overnight (RHS). If cold damage happens, cut out the damaged growth and carry on.