7 Ways to Use a Cold Frame

Cold frame gardening is a great way to extend your growing season or get an early start on the gardening year. A cold frame is a simple structure that utilizes solar energy and insulation to create a microclimate within your garden. These frames are easy to construct and can be made from materials like wood, glass, or plastic. With a cold frame in your garden, you can enjoy fresh herbs and vegetables all year round!

This post will explain all you need to know about cold frame gardening and our top 7 ways to use a cold frame in your garden. I hope you will see that cold frames offer so much more to us keen gardeners than just as a way to acclimate crops to outdoor conditions. 

What are cold frames?

A cold frame is an enclosure with a transparent roof built low to the ground. These structures are primarily used to protect plants from harsh weather conditions by trapping solar energy and providing insulation to create a more hospitable environment during the spring, fall, and winter. Additionally, they safeguard against garden pests, frost, and strong winds.

Like a greenhouse, or even a car on a hot day, cold frames capture and concentrate sunlight to help warm the soil and air. They are, however, much smaller in size, making them an ideal choice for anyone with limited space in the garden. Both inexpensive and portable, it’s easy to see why cold frames are a worthwhile investment for your garden.

The 7 ways to use a cold frame

1. Frost protection

If you have plants that are sensitive to frost, you can move them into a cold frame to protect them. A cold frame will be several degrees warmer than the outside temperature, which can help to shield your plants from a light frost.

When the weather is particularly harsh, it is essential to insulate your cold frame further to protect your crops. Additional warmth can be generated with a burlap bag filled with leaves, covering the frame with layers of newspapers, or even wrapping the frame with bubble wrap that you’ve received from past deliveries. 

A common rule of thumb when it comes to growing plants under protection, for example a cold frame or hoop house, is that each covered layer extends your growing season by about 4 weeks in the spring and in autumn. This means that growing undercover gives you an extra two months of growing potential versus just growing outside.

For this reason, having undercover space is extremely valuable especially if you wish to reduce your food bills or are aiming towards self-sufficiency.

2. Warming the soil

Cold frames are primarily used for helping your tender plants acclimate to their new outdoor environment. To increase their chance of survival, it is essential to warm the soil to temperatures that are more suited to growing crops. You can do this by placing a lightweight cold frame over the area where you plan to plant your crops several weeks before planting. When you are ready to plant, the soil will be several degrees warmer, creating a more suitable environment for direct-sown seeds to germinate or for young plants to adjust to the outdoors.

One great way to utilise full compost bins for growing heat-loving crops like squash is to plant squash on top of your compost pile and then place a coldframe over to keep in the heat and allow your squash to continue producing into autumn in a warmer environment than outside. 

Image: A cold frame placed over a compost bin with squash growing underneath

3. Starting seedlings

One of the best ways to use a cold frame is for starting seeds. You can start your seeds earlier in the season and then transfer them into your garden when the weather warms up. Just be sure to vent the cold frame on warm days to prevent the seedlings from getting too hot. Usually cold frames have a swivel stand to open the frame and allow for easy ventilation, however any piece of scrap wood will work. A huge advantage of starting seedlings in a cold frame is that it also protects them from any lingering storms or hailstones that can severely damage tender seedlings.

Plants that do well in cold frames

Cold frames are typically used to grow cool weather crops like spinach, kale, radish, winter lettuces, arugula, chard, broccoli, Brussel sprouts, onions, garlic, and scallions

For late winter and early spring you can place cold frames over a raised bed and plant some new potatoes underneath. This will allow you to harvest delicious spuds around three weeks earlier than if they were just planted in the ground without protection.

4. Extra propagation space

Cold frames can be an excellent solution for creating extra propagation space when you run out of room in your greenhouse. If this happens, you will likely not have enough seeds to justify another greenhouse (and you may not even have the space in your garden). Since cold frames and greenhouses are quite similar, you can use cold frames for starting seedlings when there is no more room left inside your greenhouse. This is something I often do in summer when I remove some shelving space in the polytunnel to make space for growing extra high-value crops like chilies

Image: Cold frames make fantastic early porpagation space allowing you to start seedlings 3-4 weeks earlier than you would for sowing outside. 

5. Hardening off seedlings

When you start seedlings indoors, they become accustomed to a warm and perfect environment. The “hardening off” process gradually introduces seedlings to their permanent location outdoors.

Using a cold frame when the time comes to move your plants outside can help ease the transition. To do this, you’ll want first to place the seedlings into a shaded cold frame, leaving the lid open for a few hours. Next, close the lid and gradually increase the amount of sunlight the plants receive each day as well as how long the lid remains open. Whilst hardening off is a recommended method for many seedlings, I only do it for warm weather crops like squash, corn, and tomatoes. Anything else I will either keep in a vented cold frame for a couple of days before transplanting, or transplant right away. 

6. Double cold weather protection

Severe winter weather and excessive snow can not only disrupt your plants’ growth cycle but also physically weigh them down. One way to avoid this is to place your cold frame inside a polytunnel. This combination will offer double the protection against cold weather and it will also prolong the life of your cold frame. This is because it is in a more protected location away from extreme temperature changes, as well as the winds and rains. The first time I came across a double protection where there was a cold frame inside a hoop house was watching a video many years ago on the One Yard Revolution channel showing just how effective it was to keep plants healthy and productive during the darkest times of the year.

7. Pest protection

If you have a pest problem, another excellent use for cold frames is placing them over crops being eaten to protect them. Doing so provides a simple solution for keeping bugs away while maintaining a completely organic pest-management strategy. For shorter cold frames, which also warm up faster than taller ones, you can grow carrots safely from the risk of carrot root fly if the vented area is covered in a fine mesh. 

Image: Carrot seedlings growing inside a cold frame

I’ve found that growing salads in cold frames massively reduces slug damage overnight provided that the base of the cold frame is blocked so there are no holes for slugs to squeeze past.

BONUS: Dry storage space

When you are not using your cold frame to grow anything, you can use it as a mini shed or storage container. For example, if you’ve harvested many crops in autumn, you can store them in your cold frames temporarily. 

Things to know about cold frames

The best place to situate cold frames is facing south for warmth. Cold frames are typically lightweight, so they are easy to move about to different positions. It is also recommended to have a windbreak to the north or northwest - this can be a building, a bale of hay, or a natural one like a tall row of hedges. When positioning your cold frame, make sure the windbreak does not shade it.

Temperature is crucial when working with cold frames - it should never be too hot or too cold inside of the frame. When necessary, add heat to your cold frame with additional insulation, or ventilate it by opening the lid to prevent the plants from overheating.

Cold frames can be a simple DIY project using materials you likely have lying around at home such as wood off-cuts, windows, and spare screws, or they can be purchased ready-to-use.

Cold frames can have different kinds of insulated covers, from single-layer hoop house plastic to double-walled polycarbonate. Don’t get too caught up with materials though because something is always better than nothing. 

Don’t forget that plants in a cold frame will need more water than plants outside. Firstly they won’t enjoy any rainfall and secondly the higher temperatures can cause greater moisture loss from the soil. When the top 2cm (1inch) of soil or compost is dry, it is a good idea to water.

What size cold frame is best?

Cold frames come in all kinds of shapes and sizes, and whilst there is much debate on what the best size is, it really comes down to what space you have and what tasks you want to prioritize for the cold frame. If you are just hardening off seedlings, a 4x2ft cold frame is perfect as it is easy to reach in to place and take out pots and trays, however if you are looking to grow salads outside all winter then a larger 4x4ft cold frame with light sides will be much better suited. For really small corners you can even have a mini cold frame to make that corner more productive. 

Final Thoughts

Overall, cold frames are a versatile and useful tool for any gardener. Cold frames can be used to start seedlings, propagate plants, harden off seedlings, or even store crops. When positioning a cold frame, be sure to place it in a sunny spot and provide a windbreak to the north or northwest. Temperature is also crucial - too hot or too cold can damage the plants. With proper care, a cold frame can be a valuable addition to any garden!