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Coldframes

How Does a Garden Coldframe Work?

Polycarbonate Modern Coldframe from Harrod Horticulture

Unlike a cloche that moves around the garden, being used for a relatively short period as plants establish or to extend the end of the season, the coldframe is more of a permanent feature in the garden.

The coldframe serves three main purposes:

First it provides a sheltered and relatively warm environment to germinate seeds and bring seedlings on.
Second, it provides a sheltered place to grow plants. If you do not have a greenhouse, you can grow cucumbers in a coldframe - hence the name frame cucumbers.
Thirdly, the coldframe provides an intermediate stage between greenhouse and planting out in the great outdoors. This process is known as hardening off.

Hardening Off in the Coldframe

If you just take a plant from a nice warm greenhouse and stick it into the cold outside the shock may well kill it and it will certainly check the growth. The hardening off process introduces the plant to the outside gradually, enabling it to adapt and thrive in the outside world.

Take the plant from the greenhouse and place in the coldframe and open the lid for a few hours in the warmth of the afternoon. After a few days, leave the frame open all through the day, just closing it up at night or if it gets really cold.

By the end of a week to ten days you should be fine to leave the frame open day and night, just sheltering the plants from the worst of the cold. After a fortnight the plant can come out and go into the ground.

Types of Cold Frame

There are three main types of cold frame:

The Wooden Sided Cold Frame.

This is actually quite easy to build and well within the capabilities of the average person. The wooden sides help keep the warmth of the day in the house overnight and are quite robust. See How to Build a Coldframe

The Aluminium and Glass Cold Frame

This type of coldframe has the advantage of having glass sides, which allows more light to enter, avoiding drawing the plants and spindly stems. The drawback is that they do not hold the heat as well overnight as the wooden sided coldframe. With both of the above coldframes care should be taken if children are around, glass can be dangerous.

Plastic Cold Frames

Available with wooden or aluminium frames, these are glazed with twin walled polycarbonate. This does not allow as much light through as glass but is far better as an insulator than glass and not as fragile or dangerous if broken. Because the light is diffused by the material, the risk of scorching in sunny weather is reduced.

My experience with wooden framed polycarbonate coldframes is that they tend to warp allowing the glazing panels to spring free.

My Coldframes

Coldframe

Cold Frame on Raised Bed
(photo taken in my back garden)

I owned a wooden framed polycarbonate coldframe but as I mentioned the frame warped. I have a basic glass sided frame that is very good for hardening off although I have had to replace some panes due to accident. We've no little children running around but the cat leaping down from the fence managed to break the glass lights.

I've actually got three polycarbonate glazed frames. 2 are upright types, like a glazed cupboard with shelving for seed trays and the third is a coldframe combined with a raised bed base.

This is brilliant for growing very early potatoes and for carrots protecting them from the fly. The potatoes are planted shallowly in the base and this is then earthed up into the deep bed. When the weather warms up the foliage is up to the roof panels, these are removed and stored away. The sides provide enough protection for a shallow frost. Excellent really early crop of potatoes. You can see details of this cold frame on the TwoWests site: Cold Frame & Raised Bed

The full range of TwoWests coldframes & cloches are here.