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A Beginners Guide to Cold Frames

Jo.Baker
Published on January 28th 2021
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A close up of a large cold frame in the garden
Cold frames can easily be overlooked when deciding which garden equipment you need, but they are indispensable for those of us in the know. If you are new to gardening and haven’t discovered the benefits of using a cold frame, this article is perfect for you.

What do you use a cold frame for?

A cold frame’s primary use is to 'harden off' - acclimatise - your tender indoor-grown young plants before planting outdoors in their final growing position.

Hardening Off Plants

Aldetha

It can also extend the growing season by protecting the early and late sowings of crops.
Indeed, some plants need cold, but not wet conditions to germinate and a cold frame will be the perfect place to leave biennial and perennial seed trays over winter.
You can also use a cold frame to house half-hardy plants over the winter, increasing their chance of survival if you live in colder areas.

Isn’t a cold frame just a small greenhouse?

Well, yes and no.
The main difference between a cold frame and a greenhouse is predominantly size, but greenhouses tend to be equipped with heating and ventilation systems, whereas cold frames generally don’t.
A close up of the glass panels of a timber framed cold frame.
The timber-framed cold frames at The Newt in Somerset are beautiful to look at and have a mechanised opening system to speed up ventilation

What is a cold frame made from?

Traditionally they were made with bricks and topped with glass lids; examples can still be seen in some large public gardens or nurseries.
However, they can be made from a whole variety of materials. Aluminium frames holding plastic perspex sheets are a very lightweight moveable version, or you can make your own with recycled wood and old windows, or concrete/cinder blocks and polycarbonate sheets.
Make the back 15 cm (6") taller then the front to create a slope that will take full advantage of the lower angled winter sun.
A wooden framed cold frame made by Alex_StatusGrow
Alex_StatusGrow shared his homemade cold frame with glass from a family member.

Where should I place a cold frame?

Cold frames should be positioned south facing, in a sunny sheltered spot to take advantage of the suns warming rays. Placing the frame against the wall of a house, shed, fence, garage, or greenhouse will reduce the effect of chilling winds. If this is not possible, place straw-bales or bags of compost against the north side to provide some extra winter protection.
Sinking the whole frame partial below the soil level will also enable you to help maintain a more consistent environment inside thanks to the surrounding soil absorbing or radiating heat.
Full sun is best, but partial shade - a few hours at the start or end of the day - will not be too much of a problem.
My only tip if you are planning to have one is to place it somewhere where children or pets can't accidentally knock into or drop something on top of it. Picking shattered glass out from seedlings takes a while!
A soil filled cold frame, planted with salad crops with the glass lid propped open
Cold frames can be filled with soil to provide a permanent growing location

The 4 main benefits of using a cold frame:

1) Cold frames for hardening off plants
Hardening off young plants is made a lot faster when all you have to do is prop open a lid instead of carrying trays of seedlings in and out every day.
You do still have to keep an eye on the day's temperature. Ideally, only lifting the lid when the air temperature is above 5C (40F). An automated vent opener is a useful addition; if you cannot pop back to lift the lid, should the weather suddenly change.
Likewise, you may not want to move your young seedlings into a cold frame until the forecast shows relatively stable conditions for the following weeks.
After a couple of weeks getting acclimatised to cooler conditions, a young plant's leaves will have become robust enough to cope with average weather conditions and can be planted out.
2) Cold frames allow you to reheat the soil
If you have a lightweight moveable frame, you can place this over an area you plan to plant up a few weeks before you plant. Warming the soil temperature even a few degrees will help young plug plants get established or direct-sown seeds to germinate. Plus adding a windbreak while the plants are young.
3) With cold frames you can harvest crops through the winter
Nothing is nicer than harvesting fresh crops during the colder season, and there are plenty of edible plants that will grow if provided with some protection. I have put together a collection of varieties which you may like to try. Many of these can be successionally sown for continual crops.
4) Using cold frames as clamps
In colder areas, you can store some winter veg in cold frames for easy access. Veg such as leeks, carrots, parsnips, celeriac, beetroot, and turnips can be layered up in a gritty compost soil mix keeping them relatively dry - but not so dry that they desiccate - and frost-free. A lot quicker than trying to dig the frozen ground.

Things to consider when using cold frames:

1) Mark your diary
Sowing winter veg needs to be done earlier than you expect, some crops will need to be planted in July.
A close up of a frosted aluminium and glass cold frame.
I use a lightweight cold frame to overwinter Echinacea, they can cope with Wiltshire's winter temperatures, just not its rain.
2) Overwintering plants
If you are using your cold frame to house your more tender plants, pre-water the pots and pack them as tightly together as possible. Fill the remaining spaces with straw, leaf mould or compost to help insulate the roots.
3) Try not to over water
These plants are not actively growing, meaning they're not drawing up water from the ground. Excess water around the roots will quickly cause them to rot away.
4) Keep the glass clean
Brush off any leaves or snow that may accumulate on top of the frame. You want as much of the suns heat as possible to reach the plants and soil inside.
Help prevent the build-up of any diseases or mould inside your frame by washing down your frame before using.
5) Paint the inner frame white
Painting the insides of a wooden frame white will help to bounce the sun's rays - and heat - back towards the growing seedlings. Lining the frame with a reflective silver foil also works.
6) Use a minimum/maximum thermometer
Using a thermometer inside your frame will help you keep track of what's going. Shield it from direct sun with a board to give you more accurate readings.
7) Keep an eye on the weather
If freezing temperatures are forecast, you may want to consider placing additional insulating layers across the frame overnight.
If you live in an area where you get regular snowfall, you may also want to leave a layer of snow to act as insulation when freezing weather is forecast, but not so much that the frame breaks from the weight.
During warmer times of the year, ventilation is the key when using a cold frame. Too hot and the plants will stay 'soft', leading to underperformance when you plant them out as well as running the risk of the young plants getting fried from unexpected strong sun rays.

I hope this article has persuaded you to invest in a cold frame. One problem you may have in the future is not having enough of them!