What is a cold frame?

A cold frame is like a mini greenhouse. It is used by greenhouse gardeners to be an interim between the greenhouse and the garden. Plants that have been raised in the warmer climes of a glasshouse need to adapt to garden conditions. You can plant them straight out into the garden but you take a huge risk. Instead gardeners place their greenhouse plants into a cold frame for a week or two to acclimatise the plants to the outside conditions. This is called hardening off.

A cold frame is also used by gardeners who do not have a greenhouse to raise seeds and cuttings in a more protected environment earlier in the season. In the same way that a greenhouse can extend the growing season a cold frame can do the same in a lesser way. The structure protects the plants from the worst of the weather, such as heavy rain, the weight of fallen snow and wind. It is usually unheated and provides a little protection against cold.

A cold frame can be something as simple as a box with a pane of glass over it. It must be possible to lift the glass or lid for ventilation and access to your plants. It can be a homemade structure created from wood and old windowpanes (double glazing is popular) or you can invest in a specially made creation from a number of sources. Sometimes cold frames are made from brick and become permanent garden features. They can also be made of an aluminium frame, which is glazed with glass or polycarbonate. Do your research; choose something that suits the style of your garden as well as the depth of your pocket.

Usually a cold frame is sited next to a wall, a fence or even the greenhouse, ideally protected from the prevailing wind. If the cold frame is placed next to the greenhouse then it makes it easier moving plants between the two.

Normally a cold frame is taller at the back than at the front, this allows plants of different height to be protected. In the summer, the cold frame gardener usually removes the glass completely to allow the plants to grow freely. By this time the plants are often too tall to be enclosed anyway.

The lid/glass is usually angled to allow rain to run off and to capture the rays of the sun.

Important considerations

Keep your cold frame free from slugs and snails, either by creating an impassable barrier or using a slug control.
Make sure that water can drain away freely or you may encourage fungal rots within your cold frame.
Keep a bundle of fleece handy for cold snaps, it won’t protect tender plants but it will stave off extreme cold.
Make sure you have a watering regime for your cold frame. If you are going to be away, install an automatic micro irrigation system that will water your plants in your absence.

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