Gardening earlier with cold frames: This is how it works!

Last updated on October 23rd, 2023 at 08:16 pm

Cold frames are a great way to start gardening even though the weather isn’t really cooperating yet. Under the protection of glass, fleece or plastic panels, early seeds are nice and cozy and warm. This way you create a transition between winter and spring and can start gardening and harvesting sooner!

What is a cold frame and how does it work?

The name says it all: cold frames are designed to allow beds to be used as early as possible, i.e. already in late winter. Covers reduce the risk of frost, provide wind protection and raise soil temperatures. While outdoors it is not warm enough for many seeds and young plants until April or even May, cold frames catch the first rays of sunshine of the approaching spring and create a pleasant climate. In principle, they work like a greenhouse, only on a smaller scale: sun gets in, the warm air stays in.

Strictly speaking, cold frames are also at the same time “late beds”, with which the gardening season can be significantly extended from October.

Seeds have a certain germination temperature, which must be at least given, so that they wake up from their “hibernation” and germinate. Depending on the plant, these temperatures can be quite different. Unfortunately, in a cold frame you won’t achieve 25 degrees to germinate bell pepper seeds, for example, but there are many vegetables whose minimum germination temperature is much lower and which also have a short cultivation period – these are the perfect candidates for the cold frame.

However, the cold frame can also be used to preplant vegetables with a longer growing season, which will later move to other beds, for example, various types of cabbage.

Very important for all cold frames: On sunny days, regularly ventilate for a few hours during the day! It can get quite warm and humid under the cover. This is especially important when the plants have germinated and need to be slowed down somewhat in their growth, so that they do not shoot up unstably. But it also applies: Do not forget to water!

The ideal location & timing

Young plants need warmth and lots of light. The cold frame must therefore be placed in a sunny spot. In February and March, trees and many shrubs do not yet have foliage, so places that tend to be in partial shade in the summer can also be considered. Also make sure you have a spot that is sheltered from the wind, so the bed can warm up even better.

All cold frames need a sloped roof to allow rainwater to drain away quickly. This sloping side should face south.

When exactly you can start sowing or planting depends on the climatic conditions where you live and on the weather. If it is still very cold, freezing or even snowing, you should be patient with the start of the cold frame. Are you already using your cold frame and suddenly frosty nights are forecast? Then you can, depending on the design, additionally cover the cover with fleece, jute bags or mats of all kinds to store the heat.

Monthly data are therefore always guidelines for orientation. In most cases, the first sowing can start at the end of February or beginning of March.

What plants can grow in a cold frame?

Radishes, kohlrabi, early lettuce, arugula, carrots, but also herbs such as dill or cress can be sown first in the cold frame.

In the course of March or when it gets a little warmer, young plants that have been grown indoors can be planted in the bed, for example, various types of lettuce or early cabbage. However, these can also be sown a little later directly in the cold frame if you don’t preplant them indoors.

Depending on the design, cucumbers or bushy tomatoes can also be sown from April or planted later, which can remain permanently in the cold frame. With taller “roofs” on the beds, there is enough room to accommodate plants throughout the season.

Later in the year, you can then use the bed as a “late bed” and grow winter vegetables in it, for example, lamb’s lettuce, spinach, various types of cabbage and leeks.

By the way, you can start very early in so-called manure beds. These are cold frames with natural underfloor heating. For this purpose, the soil is dug deeper and a thick layer of manure is spread under the actual planting soil. The hole should be about 40-50 cm deep, with a 20-30 cm layer of manure and 20 cm of soil on top.

Manure generates heat – that’s why manure piles often steam. This is of course great on the one hand, but on the other hand, dung and possibly compost soil also mean a lot of nutrients. Therefore, such a warm manure bed is more recommended for heavy-duty vegetable plants such as tomatoes, zucchini or pumpkins, which have a longer cultivation period. If they grow larger, they can either be planted out – depending on the design of the bed – or the bed top is removed.

Growing in manure beds is more challenging than cold frames and is more for gardening professionals.

5 ideas for a practical cold frame

There are many different designs for cold frames. They all have the following basic functions in common:

• A cover ensures that sunlight reaches the soil. This warms up and the heat remains “trapped” under the cover.
• Covers can be easily opened to ventilate the bed during the day, especially after germination.
• Covers are beveled or rounded to allow rainwater to drain away.
• The covers can also be removed completely or opened very wide during the day later when it gets really nice and spring-like warm and sunny.
• Depending on the model, the cold frame can also be used as a normal bed without a cover from around mid-May.

1. cold frame with cover

A square frame with a hinged or removable “lid” is the classic among cold frames. This is usually done by digging a precisely fitting piece of turf, into which the frame is then inserted. However, it can also be placed at ground level on an existing bed. In the photo you can see that it was placed on a kind of mound bed made of straw and loamy soil.

You can buy ready-made cold frames, but building them yourself can be fun.

Such a frame is usually made of wood, but there is also the possibility of building a border of stones. With stones it is a little more difficult to create the two side walls with a slight slope.

Suitable covers are old window panes or homemade wooden frames with inserts, for example, from twin-wall sheets or Plexiglas. Wooden frames can also be covered with tear-resistant film, which can also be used for greenhouse construction.

The cover can either be attached to the frame with hinges or placed loosely. If the cover is very light, there is a risk that it will be caught by a gust of wind, so you should weigh it down in such a case, for example with a large stone.

If you have purchased a cold frame or built one yourself, you can look for the best location in your garden and, if necessary, take a spade to hand. The dimensions for the hole are given by the box.

Dig out the appropriate rectangular area, you may have to cut out sod and clean the soil well from weeds and roots. The depth of the hole depends on how high the edges of your box are. You should not set the box too deep and not too shallow, ideal is for example 10 cm in the front and 20 – 30 cm in the back – depending on how big the box is and how much the slope of the cover. The box is then backfilled with the excavated soil. It is best to mix it with proper compost soil. If you have very loamy soil, a little sand won’t hurt.

If you want to use the box only above ground, there is no need for digging. Instead, prepare the bed and make sure the surface is as level as possible. Since you don’t have dug-up soil with this option, fill the box with compost or a mixture of compost and potting soil. The layer of loose soil should be at least 30 cm high. Ground-level beds like this can be edged all around with leaves or other mulch for extra insulation.

A great variation is a cold frame with a peaked roof. If you build it yourself, you can determine the height and make it high enough to grow well protected cucumbers under it in the summer, for example. Thus, you almost have a real greenhouse in the garden.

2: Cold frame attachment for raised bed

This idea takes advantage of the fact that the soil in raised beds warms up faster in the spring than in ground-level beds – perfect for early sowing.

Depending on what kind of raised bed it is, you can buy ready-made tops. Our raised bed used to be an apple crate, so we built the frame ourselves. It is a mobile wooden construction with a removable lid covered with a sturdy foil. But again, it would be possible to use other roof shapes and materials.

3: Mini greenhouse to assemble

A cold frame with a pointed roof is also available in a mobile and space-saving form: Similar to a balcony greenhouse, a framework of metal struts is put together, over which a stable foil is then put like a tent. Practical are the zippers for ventilation, the size and the possibility to easily store the house again when it is no longer needed.

Such a mini-greenhouse can also be placed over freshly planted seedlings later in the year, for example, to protect them at night from still fresh temperatures or slugs.

4: Foil or fleece tunnel

Such tunnels come in many different sizes. Sometimes they can be so high that you can comfortably stand in them. At a low height, they make practical cold frames. They cover a large area and are less expensive than ready-made cold frames.

The tunnels are constructed by inserting metal arches into the ground. These are then covered with foil or fleece. It is very important that the material is thick and strong enough not to blow away or tear. The material is rolled up for airing, i.e. it is moved around a lot and must therefore be able to withstand a lot. In addition, thicker material also insulates better – which makes the seedlings and young plants happy.

5: Greenhouse

A greenhouse is a greenhouse and not a cold frame! Or is it? Well, those who don’t plant their greenhouse with tomatoes until mid-May might be of this opinion. However, it’s a great shame for all that space if you don’t use your greenhouse much earlier in the year. However, due to their size and the amount of air that must first be heated by the sun, greenhouses tend to be cooler in early spring than small, shallow cold frames. However, greenhouses are ideal for stocking with lettuces grown indoors. By the time the tomatoes, peppers or chilies move in, the green heads will be long ready for harvest.

You have plenty of choice when it comes to models. There are small, inexpensive models, even for the balcony or terrace. For the garden, the price limit is upwards, but for less money you can sometimes get hold of used greenhouses nearby. If you want to invest a lot of time and energy, you can also build a greenhouse from old window panes. For weight reasons, the roof can be covered with foil. Of course, it is also possible to cover the house with foil all around.