Horticulture Magazine

How To Use A Cold Frame In Gardening

pak choi being grown in a cold frame

Gardening in the UK is all about making the most of the relatively short growing season and a cold frame is something that can help you do that.

A cold frame can be a great addition to a garden, especially where there is not sufficient space for a greenhouse or polytunnel. Even if you do have a greenhouse, a cold frame can be a useful extension to the space for undercover growing, and meet further specific needs.

What is a Cold Frame?

a timber cold frame in a garden with surrounding vegetable patches
Cold frame

A cold frame is basically a low structure, with a glass or clear plastic top which is angled towards the sun. It is primarily used to catch the sun’s heat and offer some protection from outdoors weather conditions within it.

These simple structures can be purchased, but are also commonly made at home. They can be constructed from a wide range of different materials, and using reclaimed materials can be an eco-friendly choice.

Using a cold frame can make your gardening more resilient, as plants can be protected from weather extremes – especially during the spring, late autumn and winter. These can be useful whether you are growing your own edible plants, or concentrating primarily on ornamentals. A cold frame is a very useful structure to consider buying or making for your garden.

Making a Cold Frame

It is common to make the sides of a new cold frame from reclaimed timber, or reclaimed bricks, and the sloping top from an old window, section of old glass, or sheet of hard plastic where glass is not available. Glass will be better for keeping the cold frame warm during the shoulder seasons and winter months. Some cold frames have clear sides as well as a clear top, but these are not as useful for keeping out the cold.

man in camo jacket using a hammer to construct a cold frame
Constructing a cold frame

As well as considering timber and bricks when constructing the sides for a cold frame, you can also consider a range of other materials. For example, some gardeners have used straw bales to form the sides of a simple cold frame, with a window over the top. Others have been inventive with other materials, from logs/branches, to rocks, and other items sourced within the garden or surrounding area.

There are many ways to make a cold frame from natural materials or from waste materials that might otherwise have been thrown away.

When choosing materials for a cold frame, it is of course important to consider whether a cold frame will remain in place, or be moved at various times throughout the year.

Where To Place a Cold Frame

A cold frame will usually be placed in a south-facing position, often against a wall. A brick or stone wall has good thermal mass, and will collect the sun’s heat energy during the day, releasing it slowly at night. This will help to keep conditions warmer when it is chilly outside. Usually, you will want to make sure that plenty of light makes its way into the cold frame.

However, for certain applications, such as when using it for cuttings, you may also wish to place a cold frame in a somewhat more shaded situation.

Cold frames can be positioned over soil, or on an area of hard paving if containers are placed within.

If you want to create a new bed within a cold frame placed on soil (or grass), lay cardboard over the area, then build up layers of organic material to make a lasagna bed, topping with a layer of rich compost or topsoil.

Another interesting option is to place a cold frame on top of a raised bed. A hot bed could even be made, which takes advantage of the heat given off by decomposing materials (such as manure and straw). A cold frame will help you retain the heat given off by a hot bed.

So, how exactly will you use your cold frame? Here is a brief guide to the main different ways in which a cold frame is used:

Extend Your Growing Season

A cold frame can be used to extend your growing season, both in the spring, just before the last frosts, and in the autumn, as the first light frosts arrive. Temperatures within a cold frame will typically be between 3 and 5 degrees warmer than they are outside it. This means that it can be used, when placed over soil, as a seed bed. Under a cold frame, you can typically start cool season crops a couple of weeks earlier than you will be able to outside in your garden beds.

A cold frame placed a couple of weeks before sowing will warm the soil sufficiently for early sowings.

A cold frame can also give you a little wiggle room in autumn, as the weather gets colder. The cold frame will stave off light frost and protect plants before winter well and truly arrives.

Harden Off Seedlings Sown Indoors

Of course, extending the growing season can also involve sowing seeds indoors a little earlier in the year. And all those seedlings grown indoors need to be hardened off before you can plant them out in your garden. This process is a lot easier if you have a cold frame in your garden.

Cold frames are either vented, with a vent which can be opened and closed incrementally. Or have a mechanism to hold the lid open, partially open, or closed. One of these structures, therefore, allows gardeners to easily increase exposure of the plants to outdoors conditions (breezes, winds and lower temperatures) over time.

Hardening off is crucial for all tender plants started off indoors or in a greenhouse. The process typically takes a couple of weeks. It ensures that your plants are sturdy and are not shocked into a check in their growth.

If you have a greenhouse, plants will typically be moved into it from indoors first. After a week or two, plants can then be moved into a well-ventilated cold frame. The cold frame serves as a kind of half way house between the greenhouse and the outdoors garden.

If you do not have a greenhouse, plants can be placed directly into a cold frame. The lid should be opened slightly during the daytime for a week or so, and closed at night. You will then gradually raise the lid a little more, for longer periods, a bit at a time, before you open the cold frame entirely prior to planting out.

Hardening off is possible without a greenhouse and without a cold frame. But having a cold frame makes the process a lot easier.

Speed Ripening of Summer Crops

tomato plants growing in frames with a gravel driveway
A cold frame with tomato plants

Placing summer crops such as tomatoes and peppers, for example into a cold frame can help keep them safe and protected while increasing yield and speeding up ripening of summer crops. Tomatoes and peppers are typically sown indoors in February, or even late January, to get them off to a good start and increase the chances of a worthwhile yield by the end of the season. These seedlings are potted up and grown on.

Placing them into a cold frame allows you to provide them with suitable conditions before it is warm enough to place them in their final growing positions. And it can clear space indoors so that other seeds can be sown.

Once these plants go outdoors, more quick growing leafy crops can be placed in a cold frame and successionally sowed throughout the summer. This means that in essence, the cold frame can become an additional growing area for edible produce over the summer months.

For Cuttings and Plant Propagation

A cold frame can also be used to provide some extra protection for softwood cuttings, semi-ripe and hardwood cuttings, and to provide optimal conditions for them to put down roots.

melon plant growing in a cold frame
Growing melon in a cold frame

As mentioned above, if a cold frame is to be used for cuttings and plant propagation, then it may sometimes be beneficial to position it out of full sun, in a slightly more shaded position.

For Winter Growing / Overwintering

A cold frame can also be used during the winter months. It can be used to provide a little protection for frost-hardy edibles like kale and other brassicas, winter lettuces, Asian greens, etc.. Inside a cold frame, these can usually make it all through the winter in the UK.

You can also use a cold frame to overwinter plants that can survive at relatively low temperatures, but which cannot tolerate winter wet. Many alpine plants and several succulents, for example, can be moved from outside in the garden into a cold frame for overwintering. A cold frame can also be used for overwintering a wide range of other half-hardy or slightly tender plants.

Additional Tips for Cold Frame Gardening

  • Take care regarding ventilation. Good airflow is key, especially in warmer weather. Make sure the lid can be propped open at different angles, or a vent can be opened and closed to regulate airflow.
  • Remember that plants in a cold frame will not receive natural rainfall. It is important to make sure you have a good and consistent watering regime in place.
  • Over winter, and in the early spring and late autumn, you can maintain temperatures in a cold frame that stay above freezing by thinking about thermal mass. As well as placing a cold frame against a wall (and using materials with good thermal mass like bricks, rocks or clay for the sides of the frame), you can also add thermal mass by filling black containers with water and placing these in a cold frame during the day. They will heat up in the sun, and keep the cold frame a little warmer during the night.

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