Hardening Off

Seedling Care

Hardening off gradually prepares indoor-raised plants for cooler temperatures, wind, sun and fluctuating moisture outdoors. Without it, tender foliage can scorch, wilt or stop growing after planting. A short, planned acclimatisation period produces sturdier plants that establish more smoothly in beds, containers and allotment plots.

How to do it

Start with healthy, well-watered plants that are not yet desperate for more root room. Put them outside in a sheltered, lightly shaded spot for a few hours on the first day, then bring them back under cover. Increase outdoor time daily over one to two weeks, exposing them gradually to brighter light and cooler conditions. A cold frame is useful: open it during mild days, then close it in the evening at first. Keep compost moist, as wind dries pots rapidly. Avoid leaving tender plants out during cold nights, strong winds or frost warnings. Once they stay outside day and night without stress, plant them into prepared ground and water them in.

When to do it

Harden off spring-raised tender plants during the one to two weeks before their final planting date, after local frost risk has passed. This may be later in northern, inland or exposed gardens. Hardy seedlings also benefit from a few days of acclimatisation when moved from a warm greenhouse to an open bed.

Tips

Do not harden off thirsty, leggy or freshly pricked-out plants. Check weather forecasts every day and bring plants in if a cold snap is expected. Shade is valuable at first, as direct spring sunshine can damage soft leaves quickly. Keep plant labels secure, since moving groups of pots can easily cause varieties to become mixed.

Hardening Off: frequently asked questions

How does hardening off work?

Start with healthy, well-watered plants that are not yet desperate for more root room.

When should I do hardening off?

Harden off spring-raised tender plants during the one to two weeks before their final planting date, after local frost risk has passed.

What are some tips for hardening off?

Do not harden off thirsty, leggy or freshly pricked-out plants.

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General guidance for UK gardens. Page updated 19 Sep 2026.