If you live in a cool climate, why not become a season-cheating gardener and get the jump on your neighbours by employing cold frames in your vegie patch? These devices are easily made, often out of recycled materials. They trap the heat from the winter sun and provide a cosy micro-climate for the plants. Inside the cold frame it can be spring, while outside it is still winter. For example, an early crop of beans can be started in early spring, followed by sweet corn when the weather has warmed slightly and finally tomatoes somewhat later.
Other small scale devices include large plastic bottles with the bottom cut off, old car or house windows suspended on bricks or sheets of corrugated plastic – anything to accumulate extra heat around the plants. Bear in mind that many of these devices will accumulate more heat than is good for the plants, at least some of the time. The solution is to provide adequate ventilation during the hottest part of the day, closing them up as the day cools to retain heat overnight. Polythene tunnels may only last one or two seasons as the ultraviolet light from the sun will breakdown the polythene. You can purchase specialist greenhouse plastic film (for about three times the price) which is guaranteed to last at least five years. recycled plastic bottles as mini glasshouses
To create a more even soil temperature between day and night, place rocks or cans (painted black) filled with water, inside the tunnel. The rocks or water filled can will release heat during the night. Water containers are best, as they retain twice as much heat as rocks, per unit of volume.
Remember to check moisture levels often as rainfall may not reach plants under cold frames.