What is a Cold Frame?

 
For budding horticulturalists, a cold frame is a great addition to the garden. Simply put, cold frames are box-like structures that protect seedlings and other young plants from harsh weather conditions.

Typically constructed with a wooden frame and a glass or plastic top, small cold frames are a cost-effective and space-saving solution for growing greens in the winter months.
 
Cold frames work by trapping heat inside their frame. They are usually built low to the ground and have a transparent lid. Unlike some greenhouses, cold frames are not heated externally and rely on sunlight for warmth.

Cold frames are extremely popular among people with smaller gardens/outdoor spaces, and they are simple DIY projects that cost relatively little.
 
In this blog series, we are going to take an in-depth look at cold frames and cold frame glazing. We will cover everything from constructing your very own cold frame to discussing what you can and can’t plant inside a cold frame.
 

What can you Grow in a Cold Frame?

 
Traditionally, cold frames have been used to grow crops during the winter months. Salad greens and leafy vegetables—like spinach, kale, rocket, chard, and a wide variety of different types of lettuces—are the most common goods people grow in a cold frame.

Cold frames are also great for growing a wide range of other root vegetables like carrots and parsnips—perfect for creating a home-grown Sunday lunch!
 
One of the perks of growing salad greens in a cold frame is that you can sow seeds pretty much all year round. For example, you might begin planting in winter so you can harvest in March, and then you can continue to plant more greens in the spring for a second summer harvest.

Make sure any summer seed is prepared well in advance of the year’s first frost, though.
 
If you already get enough of your five-a-day, cold frames can also be used to home a variety of other plants and flowers. Cold frames are often used to harden delicate and young plants during the winter months, preparing them to bloom in the spring and summer when weather conditions are less harsh.

Here’s a quick breakdown of the best things to grow in a cold frame season-by-season:
 

• Spring: This is the perfect time of year to harden young plants in your cold frame. To harden a plant or flower means to acclimatise them to poor weather conditions before they are left to fend for themselves elsewhere in the garden. A small cold frame can act as a mid-stage between transferring your plants from inside the house to the garden.

 

• Summer: Using a cold frame at the beginning of the summer allows you to get a head start on growing some plants that require a lot of heat. As cold frames retain heat well, they will speed up the early germination process. Tomatoes, cucumbers and green beans are the perfect vegetables to begin growing in your cold frame in the early summer before transferring to the garden or a vegetable patch.

 

• Autumn: As the weather begins to cool, you can use your cold frame to prepare for your winter garden beds. Some hardier vegetables, such as broccoli and cabbages, can be grown at this time of the year too.

 

• Winter: During the cold winter months is really when cold frames can demonstrate their full potential. You can use a cold frame to protect alpines from rotting in wet weather. Some overwinter leafy vegetables, such as chard and parsley, can also be grown from November onwards.

How to Keep a Cold Frame Warm at Night 

 
As cold frames are not externally heated and require sunlight to keep warm, they can lose their heat during the long, dark nights of winter—and this can stunt the growth of your tender and young plants. Here are six quick tips to ensure that your cold frame stays warm throughout the night:
 

• Make sure your lid is clear: The easiest way to ensure your cold frame stays warm is to use glass or polycarbonate plastic to create a clear lid. This will let in as much light as possible.

 

• Use manure or compost: Manure and compost retain warmth better than regular soil and release heat when they decompose, ensuring that your cold frame stays temperate.

 

• Make a sunken cold frame: You can use the earth’s natural heat to help keep your cold frame warm! When building or installing a cold frame, sink it into the ground slightly and the residual heat in the soil will help keep your plants toasty.

 

• Use heat-trapping glazing: One of the perks of using polycarbonate glazing over glass for your cold frame is that it traps heat better. With polycarbonate cold frame glazing, you can retain more heat for longer! 4mm thick Twinwall or Multiwall polycarbonate glazing will offer you the best insulating capabilities. 

 

• Create natural insulation: Using dried leaves and other garden products to pack out your cold frame can create a natural form of insulation that helps trap heat inside the frame. Bubble-wrap also works as a great insulator!

 

• Turn to technology: If the temperature drops dramatically, you can always resort to modern technology to save the day! Heated lamps and other garden heaters can be used to keep your cold frame warm.

When Should I Use a Cold Frame?

 
People often think cold frame gardening is reserved for the winter months, but this is not true. Cold frames have a wide range of different uses and are an important part of your gardening arsenal no matter the time of year or weather conditions!
 
Here’s a quick cold frame growing calendar:
 

• Early spring/summer: During the early months of spring, as the temperature slowly begins to creep up, cold frames are used to harden off tender and delicate plants. Cold frames are an ideal halfway house to allow plants to acclimatise to environmental changes when you move them from your greenhouse or home to the garden.  

 

• Late spring/summer: Throughout the summer months, you can use a cold frame to provide extra warmth for tender summer crops, such as tomatoes, peas, and chillies—which can be quite fickle! Cold frames, due to their heat, encourage faster growth, and so quick-maturing plants can also be sowed in the summer, like peppers.

 

• Late summer/autumn: As the temperature begins to cool, cold frames are often used to finish growing summer crops that were planted toward the end of the season. They also provide the perfect environment to mature cuttings and to build your own herb garden! Bulbs and corms can also be hardened in a small cold frame toward the end of summer/start of autumn.

 

• Winter: Cold frames are ideal for shielding alpines from frost and rain during the winter months. Secondly, cold frames can also be used for overwintering produce, such as leafy vegetables and other hardy crops.

 
Some crops and plants can be grown all year round inside a cold frame. For example, a few different varieties of lettuce can mature within a cold frame no matter the temperature outside! Other crops that can be grown no matter weather include cabbages (also known as brassicas), turnips, potatoes, and onions.

What Can I Use Instead of a Cold Frame?

 
There are a few drawbacks to cold frame gardening. Some cold frames are too small and so unable to hold many plants, while others are simply too big and take up too much room. Cold frames also do not get as warm as greenhouses or retain heat as well, and so are not suitable for some plants.
 
With this in mind, here are our top alternatives to use instead of a cold frame:
 

• Growing indoors: Instead of using a cold frame to harden young plants and seedlings, you can always keep your plants inside. Crops such as tomatoes and many different types of berries can be sown and started inside before being moved to your garden's vegetable patch in the summer.

 

• Mini-greenhouse: If you are looking for a cold frame alternative that still saves space, mini-greenhouses are the solution for you! Mini-greenhouses typically look like outdoor cabinets or wardrobes with multiple shelves to store plants and pots on. Nevertheless, humidity can build quickly inside in a mini-greenhouse, so make sure to ventilate it.

 

• Greenhouses: Greenhouses are the most common alternative to cold frame gardening. Bigger in size and better at retaining high levels of heat, greenhouses allow you to grow a wider range of plants and crops no matter the weather or time of year. Greenhouses can also be heated externally and used for garden storage.

 

• Plastic boxes: Many home gardeners are reusing old plastic boxes and recycling boxes as an alternative growing space. Simply punch a few holes in the bottom for drainage and fill with soil, then you have your very own miniature vegetable patch.

 

• Plastic bottles: Our best space-saving gardening hack involves the use of plastic bottles. Cut the tops off and fill with soil and you can create a tiny garden inside an old Coca Cola bottle!

And that about rounds up this blog. If you've decided a cold frame would be ideal for your garden space, you might want to check out our How to Build a Cold Frame for Plants article for some handy tips on how to build your own.

We provide a full range of standard sized cold frame glazing panels and a cut to size service for those more awkward jobs.