Greenhouse Gardening – hardening off

‘Hardening off’ is an essential part of greenhouse gardening and refers to the process of acclimatising plants that have been raised or over-wintered in the greenhouse to conditions outdoors, which must be undertaken by degrees or they will be shocked which means they will cease to grow, or actually killed. This means you will be exposing your plants gradually to the brighter light, cooler temperatures and windier conditions in the garden.

A cold frame in the garden is the best way to achieving this by increasing the plant’s exposure to the elements slowly from an hour at the warmest time of day to full exposure over around a fortnight. Depending on your location, you can use a sheltered area of the garden after about a week or so, to finish off their acclimatisation. Obviously even hardened half-hardy or tender plants should only be planted out in their final position once all danger of frost has passed. Similarly, if you plan to over-winter plants in the greenhouse, you need to follow the same method in reverse, by reintroducing plants to indoor conditions in September or early October, when night-time temperatures indoors and out are similar.

Alternatively, the system used by commercial growers is to change the temperature in the greenhouse, allowing it to lower gently over the same period by adjusting it with fans and vents – obviously this only works if all your greenhouse contents need to be hardened off at the same time.

Mastering these techniques is essential if the gardener is going to be able to take full advantage of the plant propagation opportunities that the greenhouse offers.

Greenhouse gardening hardening off photograph by Kitay, used under a creative commons attribution licence