On a permaculture plot, the gardener is always looking for ways to maximize yield. There are many ways of trying to achieve this, including interplanting different species to fit more plants into a space, and choosing native species which are adapted to the local climate, so are more likely to thrive and produce an abundant crop.
One of the key factors in determining what will grow and how well it will grow in a site is the climate. In the early stages of making a permaculture design, analysis of the climate – from patterns of rainfall and drought, to winds, temperature and frost – is central to deciding what to plant where.
However, a permaculture gardener can also use a variety of techniques to improve or lessen the impact of climate and weather conditions, often prolonging the growing season for fruiting crops, thus maximizing yield. Techniques include using plants and manmade structures to influence microclimates, starting seeds indoors or in a greenhouse so they have a ‘head start’ when they are planted out in the garden, and mulching to prevent the soil temperature falling too far. Another method of prolonging the growing season is to use a cold frame.
Essentially, a cold frame acts like a miniature greenhouse. It has four walls that enclose plants, and a transparent lid that allows sunlight to enter. The structure serves to protect the plants from the worst of the weather – such as snow, frost and wind – and traps heat from the sun, making the temperature inside the cold frame significantly higher than the surrounding air. This can serve to extend the growing season by as much as a month either end, meaning you can get more crops. In some more temperate climates, a cold frame may enable you to grow plants throughout the winter that would not normally survive the cooler temperatures. You can either establish a permanent cold frame, or make one that you can move and store during the warmer weather. You can purchase prefabricated cold frames, but constructing one yourself is very easy and can be done with recycled and repurposed materials.
The ideal position for a cold frame is a south facing location that receives full sun. By facing south, the frame will get the maximum amount of sunshine during the day. It is also advisable to site the cold frame adjacent to a building or other large structure to protect it from strong winds, which will cause the temperature to lower.
While you can purchase a cold frame completed readymade, or indeed buy the constituent parts new and construct it yourself, the greenest and most cost-effective method is to source discarded materials and work with those. The main thing you are looking for is an old window with the glass intact. This will form the lid of the cold frame. You want to avoid frames that have been treated with lead-based paint. If you cannot find a window frame that still has the glass attached, or you are in a climate where the glass is unlikely to withstand the extremes of weather (for instance, in areas that get heavy snowfall, the glass may not hold up against the weight of the snow) you could salvage some plastic or fibreglass and stretch that across the frame. If you can’t find even the frame, you can easily construct a lid from some old lengths of wood and cover with the plastic. Besides the lid, you will also need something with which to make the rectangular frame that encloses the plants and supports the lid. This is usually wooden boards but could be old bricks or breezeblocks. If using lumber, try to source untreated wood so you are not introducing any chemicals to the site.
You could in theory, make a cold frame as large as you like, depending on the suitability of the site and the size of the materials you have sourced, but a standard size is a rectangle that is 3 feet wide and six feet long. This allows the gardener to reach all parts of the bed inside to harvest the crops, and is likely to work in conjunction with a recycled window (or two, depending on the size).
You want the cold frame to be higher at the back than at the front, so the lid is sloping. This has two advantages: firstly, it allows more sunlight to reach the plants inside, and secondly it means that rain and snow will drain off the cold frame. So if you are using boards to make the frame, cut the front one so it is shallower than the back, and cut the side boards at an angle. If using bricks or blocks, simply add an extra layer to the back wall.
Construct a frame from the boards or the other material you are using. Lay the lid over the frame. If you wish, you can add hinges to the back of the lid to make lifting it easier, but you can simply lay it on top and remove it completely when you want to access the plants inside. If you wish, you can add insulation to the insides of the frame walls.
If using concrete blocks, consider painting them (with a non lead-based paint) black to absorb more heat. Piling soil, leaves, woodchips or straw around the outside of the frame will help to insulate it. You can also up the temperature inside the cold frame by adding fresh manure under a layer of soil, or simply placing a bowl of water inside. Water absorbs more heat than the ground and releases it more slowly, so will add heat during the night. Be aware that even though the idea behind the cold frame is to trap heat, on very sunny days (even those during winter) it can get so hot inside that the plants become damaged, so check the cold frame regularly and, if necessary, open the lid to vent some hot air, or provide some shade to protect it from the highest temperatures.