Lars Lund

04 Mar 2024 13:34

Coldframes Raises the Temperature by 10-15 Degrees

By Lars Lund

Rain, wind, and cold are not always appreciated by either humans or plants, and it is often said that nothing can be done about the weather. But that's not entirely true. A greenhouse or a garden frame is the solution.

Light enables plants to undergo photosynthesis, and the more light a plant receives, the more beneficial it is for its growth. Low temperatures can kill plants that haven't adapted to withstand frost or simply cold weather. Wind can dry out leaves or small shoots, as moisture evaporates easily, and the wind can topple plants that haven't established themselves. Too much water makes the soil too wet, causing plants not adapted to waterlogging to die. That's why plants that reproduce by seeds in nature produce many flowers and seeds. A large proportion of these seeds never develop into new plants precisely because of the weather, or because the small shoots have been eaten by snails and other pests before they've progressed far in their life cycle.

Windowsill Gardening

To protect the plants, we often start them off on the windowsill. While this is fine, it almost always requires artificial light as well. You can choose a different path or supplement your windowsill gardening with a manure bench. Small shoots or larger plants like tomatoes, grown on the windowsill, always need hardening off before they're planted outside or in the greenhouse. Here, too, the garden frame is a huge asset. In a garden frame, tomatoes can acclimatize to a little wind, slightly fluctuating temperatures, and stronger light than inside on the windowsill. You can start growing all your vegetables or summer flowers earlier, allowing your plants more time to develop. This is especially advantageous for plants with a long development time.

Cold or Hot Bench

Cold Frames have been known since the year 900. In the past, mainly warm garden frames were made. They disappeared for a while but have started to reappear. A warm bench is made by digging a square hole in the ground, preferably half a meter deep. The deeper you go into the ground, the more stable the temperature. In the hole, alternate layers of horse or cow manure with straw and soil are laid. For example, 25 cm of horse manure, 5 cm of soil, 10 cm of manure, and finally 10 cm of soil. Layer upon layer. The top layer ends with soil, which you can sow or prick out in. The manure in the soil undergoes a natural process, turning into compost and soil, generating heat. In a sense, the roots are provided with nature's own heating blanket. To retain the heat in the box, there's a glass cover. During the day, it's slightly lifted to create circulation but also to prevent the plants from getting too hot. Plants die at temperatures above 46 degrees Celsius.

A warm bed is good, but a cold bench isn't bad either. In principle, it's a raised bed, a frame, or a box with sides, placed on the ground. Like with the warm bed, a window is placed over the box. The window absorbs and retains the heat, and just like with the warm bed solution, you need to lift the cover as the sun warms up and close it at night to retain the heat.

How to Succeed with Plants

Seeds have developed different strategies for spreading. Some let themselves be carried by streams or by the sea, others let the wind take care of the spread, and others cling to an animal's fur or human clothing to be spread. When we sow, we have the opportunity to sow densely and in the best soil for the seed. Some seeds are large, so we can easily plant them; others are so small that tweezers are required. Finally, some seeds require more warmth than others, while others need it cool. For example, lettuce doesn't germinate at temperatures of 22 degrees Celsius and above, but it's a bit of an exception.

Use good sowing and pricking out soil. It shouldn't be sterile but free of weeds and have a good structure. You should try to create space between the seeds. The recommended spacing can be found on the seed packet. Nevertheless, it can be difficult to achieve the correct spacing, so you may need to thin them out and remove the weakest ones.


How to Use a Garden Frame

Direct Sowing

You can sow directly into your manure bench, as if it were a large sowing tray, and you can let your plants continue to grow here until harvest. A fairly large proportion of seeds just need a soil temperature of 5 degrees Celsius to germinate, which can be achieved under glass as early as February. However, most people choose to start at 8 degrees and use a manure bench as a nursery before later transplanting outdoors when the plants are large enough. This also leaves room for new plants in the manure bench.

Hardening Off Bench

If you start sowing indoors on the windowsill, you'll need to use a sowing tray here. When the shoots grow, you can prick them out, meaning gently lift the plant and move it to a pot. When moving them to a pot, it shouldn't be too small. If a pot's size is doubled, root and shoot growth, on average, increase by 43 percent. It's important to have good drainage in the soil so that the water can drain away. The hardening off then takes place in the manure bench. Move your pots out. Leave the window slightly open and open it further if it gets too hot, and close it at night. The plants are also strengthened by a bit of wind. In greenhouses, fans are used to create wind. The stems become stronger, the plant becomes bushier, and less spindly. The same effect is achieved by salicylic acid, found in aspirin, heart aspirin, or acetylsalicylic acid, which is sprayed onto the leaves. A aspirin in a liter of water is stirred until dissolved and sprayed on with a sprayer.


How Long Should the Plants Be Hardened Off?

When you move pots from the protected environment on the windowsill, let them sit in the garden frame for a week with the lid slightly open. In the second week, remove the lid completely, and then plant them in the outdoor bed during the third week.

A Soil Thermometer is a Must

In general, it's a very good idea to get yourself a soil thermometer. It's a huge help when sowing and to guide you on which of the mentioned methods you prefer.

Karna Maj from Practical Ecology has made a sowing overview that tells you the time you can normally sow outdoors. The overview gives you two tools. One is that you can measure the temperature in your manure bench with a soil thermometer and get started when the temperature is right. The other tool is that you can see when you can plant your plants outdoors or, for cucumbers, tomatoes, and eggplants, plant them in the greenhouse.