Hardening off plants from a greenhouse

What hardening off actually changes

Plants raised behind glass or indoors have experienced filtered light, comparatively still air and a narrower temperature range. Transplanting them straight into an exposed bed can scorch leaves, dry compost rapidly or stop growth. RHS hardening-off guidance recommends gradually acclimatising tender plants to lower humidity, air movement and cooler conditions over about two to three weeks. It also makes an important distinction: this process does not make a frost-sensitive plant hardy.

The exact pace depends on the crop and how warm its previous conditions were. Hardy brassicas raised in a cool greenhouse need less adjustment than peppers moved from a heated propagator. The moving seedlings into a greenhouse guide covers an earlier, related transition; going from greenhouse to open ground adds wind and direct outdoor exposure.

A practical staged routine

On the first mild days, place rooted, healthy plants outside in a sheltered position for a short period, away from midday glare and strong wind. Return them to protection before conditions deteriorate. Increase exposure gradually over subsequent days, including brighter light and more air movement. Then trial longer days and, only where appropriate for the crop and forecast, suitable nights. A coldframe can serve as an intermediate step if its lid is opened progressively.

Keep labels with trays and group plants by tenderness. Water pots before they dry out, but do not water simply because the surface looks pale; test the compost. Wind may remove moisture much faster than a sheltered greenhouse. Avoid placing trays directly on cold, wet ground if that chills roots or blocks drainage. Do not expose seedlings to a sudden midday sun just because the air temperature feels gentle.

Let weather interrupt the schedule

Hardening off is not a countdown that must continue regardless of weather. If a frost, cold rain or gale is forecast, pause or move plants back to a suitable protected position. A setback after several successful days is better than losing a crop. Compare the predicted outdoor low with the crop's requirements; check the greenhouse's own night minimum too. The RHS frost-avoidance guide notes that last-frost timing varies across the UK and between sites.

Tender greenhouse cucumbers, peppers and tomatoes should not be planted outdoors just because hardy salad seedlings are ready. Follow the relevant crop guide and the actual weather. If plants are intended to stay under glass for the summer, they do not need a full outdoor hardening-off process, although a gradual move from heated indoor conditions to an unheated greenhouse is still sensible.

Plant out and check the first week

Choose a calm, suitable day; water the root ball, handle it gently and plant at the depth appropriate for that crop. Add supports at planting so later work does not damage roots. Check for wilting, scorch and cold injury over the next few days. Continue to water based on root-zone moisture, not a fixed daily amount. If a late cold night threatens, use crop-appropriate temporary protection or delay planting rather than assuming hardened plants cannot be damaged. The greenhouse growing calendar helps position this transition in the wider season.