How, why and when to use cold frames in the garden

Cold frames are an excellent way to extend the growing season. These easy-to-build structures can be placed on a windowsill, balcony or patio and provide a great place for young plants to grow during winter.

To most people new to gardening, cold frame gardening is seen as an easy way to grow vegetables like tomatoes, lettuce and cucumbers all year round.

A properly built, well-maintained cold frame can be an excellent place for growing plants all year round with little effort or expense on your part. It also helps keep dormant plants healthy by protecting them from harsh winds that dry leaves.

A cold frame is also ideal for housing tender plants that would not survive outdoors in colder climates. In this article, we will discuss how, why and when to use cold frames in your garden.

cold frames main

Table of Contents

What is a cold frame?

A cold frame is usually a roofless, transparent-sided box. This box is constructed with sturdy materials that will withstand the outdoor elements when the frame is outside. The bottom of the cold frame needs to be covered with insulation. Make sure you have drainage holes to keep leaves and water from accumulating. Cold frames are an excellent way to extend the growing season during the winter months too.

Cold frames are by far the easiest and cheapest way to maintain a garden in winter. They’re simply small, stand-alone structures that can be put on top of beds or directly on the cold ground during the frosty months for one purpose: To protect your plants from harsh temperatures. A greenhouse is an expensive option when it comes to maintenance; they can often have their own heating system as well as ventilation equipment so you don’t need any other special care!

cold frame gardening

A glance at a cold frame

Top – A light-permeable cover such as a glass, plexiglass or greenhouse plastic is used for the top of a cold frame

Sides – The sides of the cold frame are made of any material that will support the cover.

Bottom – With few exceptions, a bottom is not necessary for your cold frame. Most people just use soil, but you can create a base for your cold frame if you want to.

Materials

The Lid – All you need is an old window or windows, if you can’t get your hands on this, then use clear glass secured on wood for the top of your box design, an old shower door would be perfect. Use hinges to secure it in place, as well as handles for easier access.

The Frame – To make the box that your lid sits on, cut lengths of pressure-treated lumber so they are as long or slightly longer than the dimensions of your glass (or lids). Make sure that you use one board from a higher spot on the back so there’s an incline and wetter throughs can drain.

For our example, we’re using three boards at the front and four at the rear. Six shortboards make up the sides. One length is cut in half diagonally to give two identical triangular boards – one for each side, to match the angle of slope.

The boards for the cold frame can all be screwed to four posts at varying heights. The pieces will need to fit together in an A-frame shape, with two shorter battens used to prop open the lid during the day.

successful cold frame gardening

Tools

In order to construct a cold frame, you will need wood screws, a drill and a screwdriver.

How to make a cold frame

To build a cold frame, position the corner posts in the ground at each of the four corners. Screw two screws in to attach them and drill holes for any pilot points first. Position a narrow end of one board from zig-zag pattern at each of the lower-level boards.

Now attach the rear boards to your posts on each corner in exactly the same way.

The finished frame is now ready for the lid or lids. Carefully position the lid onto the frame so it is flush with the frame at the back and add your hinges. Longer lids may need several hinges along their length to support the weight.

building a cold frame

Do cold frames really work?

Cold frames are by far the easiest and cheapest way to maintain a garden in winter. They’re simply small, stand-alone structures that can be put on top of beds or directly on the cold ground during the frosty months for one purpose: To protect your plants from harsh temperatures.

A greenhouse is an expensive option when it comes to maintenance; they can often have their own heating system as well as ventilation equipment so you don’t need any other special care!

You won’t need a lot of material, most cold frame plans will only require two short lengths of boards (cut into four), one length of board cut in half diagonally and some screws – this should cost between £15-£20.

plants inside

What is the best type of cold frame?

The best type of cold frame is one that can be moved around, as you need to rotate the crops in order for them to grow properly. When it comes to materials used, I would recommend using polycarbonate sheets – these are shatter-proof and UV resistant so they will last a lot longer than glass or plexiglass.

Can you start seeds in a cold frame?

Cold frames are an excellent alternative to greenhouses for seed-starting. Gardeners can start seeds without stressing their plants with too much time indoors and move them into the garden before transplanting

How do you keep a cold frame warm at night?

At night, during cold weather, you will need to cover the frame with a light blanket. You can also get plastic sheets that are specifically designed for this purpose and they’re usually pretty cheap!

winter crops

At what temperature should I open my cold frame?

A cold frame’s temperature is usually between 10°C-15.5°C. For this reason, they’re ideal for keeping plants warm that might not be otherwise able to survive the winter outside like tomatoes and eggplants! You can open the lid slightly when the temperature reaches 1.6°C to 4.4°C and open it up completely when it reaches 10°C

Do cold frames need to be airtight?

Cold frames can be airtight, but it’s not necessary. An open-topped cold frame is preferable if you want to allow in as much light and warmth during the day while still protecting your plants at night.

Should cold frames be in the sun

It’s best to put your own cold frame in the sunniest spot possible – this will make it easier for you and your plants! If there is not a lot of sunlight where you are, consider adding some light fixtures. It can be beneficial to add these as well if one side is shadier than the other; this way they’re getting equal amounts of warmth.

The only time we would recommend having a shady area within an enclosed glass structure like a greenhouse or garden cold-frame is when propagating new plant material.

warm weather

What can I plant in a cold frame?

There are a lot of plants that can be grown in a temporary cold frame and one thing they all have in common is the need for light! The most popular options you’ll come across will include:

  • Tomatoes
  • Peppers
  • Eggplants
  • Cucumbers
  • Zucchini

It’s also possible to grow things like strawberries, lettuce, spinach or herbs inside it. Note that these last ones don’t require much space so they’re perfect if you only want to use your own cold frame during certain months.

Does a cold frame protect from frost?

A cold frame can protect plants from frost, but it will not provide complete protection. The best way to prevent frost is by using a row cover or cloches – these are designed for this purpose and they’re usually quite cheap!

Cold frames come in all shapes and sizes so if you want something more permanent then you’ll need to construct your own. If not, there’s plenty of pre-made options that are inexpensive and easy to assemble like collapsible models made out of PVC piping.

frost

Can a cold frame get too hot?

A cold frame is not designed to be a greenhouse and so they’re not going to get very hot. Cold frame gardening is perfect for keeping excess moisture out – this means that there’s no need for an expensive automatic watering system or any complicated systems other than just water your plants like you would if they were outside.

Should you insulate a cold frame?

Cold frames are already insulated to a certain degree but it can help to add extra insulation. You might want to consider using a bubble wrap layer as an insulator for your cold frames if you live in colder climates or find that your metal frame doesn’t retain enough heat during the day.

Should I paint my cold frame?

Painting your diy cold frame is a good idea if you want to keep it looking nice for an extended period of time. Painting will also help improve the insulation properties and can be done before or after assembly.

The best paint to use on a newly constructed cold frame? There’s no one type that we recommend over another – just make sure whichever you decide on has high enough heat resistance, which should usually be indicated by its label!

You could try using exterior house paint or even leftover latex wall paint from other projects in your home (although this may require some sanding down).

painting cold frame

Conclusion

We hope this article has been helpful for you! We’ve covered a lot of ground and hopefully, all your questions have been answered. Just remember that if you are looking at using a cold frame as summer comes to an end, make sure to insulate it and continually check the temperature so you know that your plants and vegetables are coping ok. Keeping your plants alive during the colder seasons isn’t an exact science, however, it can be done with a little bit of care.

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Oliver Wright

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