There are ways to start the gardening year earlier than usual, such as starting seeds indoors. However, if you want to sow them directly in the garden, a cold frame is particularly suitable for home gardeners.
Cold frames are easy to build and can be made from a variety of materials. This makes them an inexpensive alternative to greenhouses for protecting your plants from the cold and frost.
What is a cold frame?
A cold frame is basically a mini greenhouse. Most cold frames are less than 1m high and are, therefore, only suitable for low-growing plants. The main purpose of a cold frame is to successfully grow vegetables in the garden early in the year when frosts are still a threat, whereby extending the growing season. In addition, the bed can be used to pre-grow a wide variety of plants, which can then be transplanted out of the cold frame when the temperatures are warmer. Since cold frames are designed to open upwards, large plants, too, can theoretically be planted in the frame if the lid is left open after the threat of frosts has passed. To work at a comfortable height, there are also cold frames for raised beds.
Essentially, there are two types of cold frames to choose from: hot and cold ones. To make a heated cold frame, known as a hotbed, dig a hole about 0.5m deep, the same width and length as the cold frame. Fill half of the hole with horse manure or other animal dung mixed with straw or wood shavings. Compact this layer well and fill the rest of the hole with good garden soil and some compost. The microbial decomposition of the animal manure creates heat, which naturally warms the hotbed from below. You can read more about this in our article on hotbeds. Hotbeds allow you to grow seedlings that require warmth and plant out cold-tolerant ones a little earlier. This makes the extra effort worthwhile, especially if you want to start the gardening season early. You will find more information on what to grow in a cold frame at the end of this article.
A cold frame is like a hotbed, but without the “manure heater” layer.
How to make a cold frame
There are several ways to build a cold frame yourself. One option is to build a small cold frame tunnel using wood or metal to construct the frame.
It is also possible to build a wooden cold frame using old windows. The best windows are those that were originally designed to open; you do not need to add hinges to a window that already has them. Next, simply fasten four boards together to form the walls of the cold frame. Then, attach the window (i.e. the roof) so that it can be opened upwards. If you do not have any old windows, other materials, such as Plexiglas or regular glass panes, can be used instead. There are almost no limits to your creativity when it comes to building a cold frame. However, bear in mind that the most important aspect of building a cold frame is that it be as watertight as possible to prevent the cold from entering. Secondly, make sure enough sunlight reaches the plants through the glass roof and that the cold frame can be opened for ventilation on occasion. It is best to face the cold frame south.
It should also be noted that different constructions provide varying degrees of cold protection. For example, cold frames with glass panes are better insulated against frost than simpler constructions made with foil.
Materials for making a cold frame:
- (Plexi) glass pane, old windowpane or similar
- Wooden boards or panels for the box and lid, depending on the desired size
- Possibly wooden beams to stabilise thin boards
- Hinges or similar mechanisms for opening
Instructions for a cold frame:
- First, use wooden boards to make a square or rectangular box. It is best to create a slope towards the front, as shown in the picture below. A slanted roof allows water to run off the panes and increases the plants’ exposure to light. In the photo, glass panes were used to form the walls of the frame, but you can also use boards or other panel materials. However, if the panel materials are very thin, use wooden beams to create a frame and stabilise the walls. Do not use pressboard or similar materials for the walls, as they absorb water and are not very weather-resistant. Woods like pine or larch have a good price-performance ratio.
- For the lid, you can simply use old windows or build a wooden frame to which you can attach panes of glass. Make sure the lid is the same size as the box, so it fits snugly with no gaps. The frame in the photo below was glued together, but thicker boards can also be screwed or nailed together. If you are making your own window frame, it is best if the glass sits a little deeper in the frame so that it cannot slip out. Then fix the windowpane to the frame with silicone window sealant or similar adhesive materials.
- The last step is to attach the lid to the box. Hinges are ideal for this. Hinges make it easier to open and close the cold frame. Depending on the weight and size of your lid, you may need more or less hinges; we recommend using at least two hinges per window frame. As the cold frame pictured here is relatively large, three separately opening window frames with two hinges each were used.
Planting the cold frame
Once the cold frame is ready, you can start planting. If you decide on a manure-heated hotbed, you can start planting as early as February. It takes about a week for the microbial activity in the soil to start generating enough energy to warm the plants sufficiently. When selecting plants, it is important to choose plants that are already very cold tolerant and can cope with lower temperatures. Suitable vegetables for hotbeds include lettuce (Lactuca sativa), kohlrabi (Brassica oleracea var. gongylodes), radishes (Raphanus sativus var. sativus), carrots (Daucus carota subsp. sativus) and many more.
If you do not have naturally heated soil, put off planting until the harshest of the night frosts are over and the temperature only drops slightly below 0 °C. Vegetables that tolerate the cold enough to be grown in a cold frame include winter cress (Barbarea vulgaris), spinach (Spinacia oleracea), lamb’s lettuce (Valerianella locusta) and winter purslane (Claytonia perfoliata).
Cold frames can also be used to extend the gardening season. They can be used as late beds for growing vegetables and other plants well into autumn.