One of the simplest ways of obtaining better germination of seeds in the cool weather of spring is to cover the soil with a sheet of clear plastic. This warms the soil a few degrees, often sufficient to allow the seeds the extra heat they need. Some sources say to use black plastic, but clear is more efficient. There is also now a material made from cellulose that is translucent and placed over a growing crop. It accumulates only a little extra heat and does not require the attention to ventilation that plastic tunnels do. It is placed loosely over, say a tomato crop, and the plants push the cloth up as they grow. It is cut where the plants are once the weather has warmed sufficiently and allowed to fall to the ground around the plants.
The most popular method of growing vegetables out of season is to use a greenhouse. Other devices, called cloches, are also used to concentrate the heat of sunlight. These days, they are generally made out of polythene film stretched over some sort of framework. Greenhouses are often highly profitable, but also quite costly to build. There are a few things to bear in mind before indulging in the expense.
Glass, polythene and other plastics transmit different wavelengths of light at varying efficiencies. What this means is that the light reaching the plants has a different colour balance than ordinary, unfiltered sunlight. Research has shown that this results in differing amounts of various nutrients in the produce. Significantly, vitamins are present in noticeably smaller amounts in greenhouse vegetables. Plastic is a worse offender than glass.
Greenhouses are generally used to grow a smaller range of vegetables than the open garden. This is because it is hard to justify the expense of growing low cost vegetables in them. The upshot is that it is difficult to arrange a good, disease preventing rotation. The alternative is to pasteurise the soil with steam, or exchange the soil every few seasons.
A simpler approach is to use polythene tunnels, about 1.2 metres wide, to cover the growing crops in the garden. While these cannot be easily given supplementary heat, as can a conventional greenhouse, they do have several advantages. Their capital cost is much lower and they can be moved from place to place, not just from season to season, but during the season. An early crop of carrots can be forced in July/August, followed by sweet corn when the weather has warmed slightly and finally tomatoes somewhat later. You may choose a different series of crops, but the result is the earlier harvest of several crops rather than only the one. Also, the crops are covered with the device for such a short time, and removed totally before crop maturity, that the negative effects due to changes in light balance, referred to earlier, are lessened.
Other small scale devices for forcing include large plastic bottles with the bottom cut off, old car windows suspended on bricks, anything to accumulate that little 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. Some people cover their polythene tunnels with old sacks on particularly cold nights, especially if frost threatens. Rocks or cans of water inside the tunnel will also hold more heat than the soil. Water holds twice as much heat as rocks, per unit of volume.
Greenhouses are often painted with whitewash to reduce heat gain during the hottest part of the year. Shadecloth makes a reusable alternative that does not wash off in the rain, an important point if you are in an area of high summer rainfall.
The polythene tunnels can be purchased or home made. Many of the commercial types are designed as half circles of metal that are shoved into the ground and the plastic is attached to this frame. In the illustration, you will see the details of a simple, home-made type that I have used. The prototypes were made from pine. This material was too light and they tended to blow away. Heavier timber solved this problem. They are propped up for ventilation, on the side away from the prevailing wind. Polythene becomes brittle in sunlight and when used on greenhouses lasts about five years. If you store your polythene tunnels out of direct sunlight when not in use, you should expect up to twice this lifespan.
The extra heat in a polythene tunnel or greenhouse means that more attention needs to be paid to watering. Air circulation is poorer, so there is a significant increase in the chances of fungal disease. While more water needs to be used than in the open garden, try as far as possible to water with drip irrigation or flood. The increased temperature also means a higher rate of metabolism in the plants. As a consequence, the humus in the soil is used up at a much faster rate. To counter this, use about double the amount of compost that you would outdoors. Some growers have a compost heap inside the greenhouse. The carbon dioxide generated by this means a higher concentration of carbon dioxide in the greenhouse. This results in higher rates of growth. Another source of extra carbon dioxide is a flock of chickens. They can benefit from the extra warmth during the cooler months, as well as improving plant growth.
The closed environment of the greenhouse is an invitation to outbreaks ofpests. The most common is the greenhouse whitefly. This relative of the aphid can be destroyed with liquid (potassium) soap ('Clensel', 'Safers' etc) or sticky traps. The traps consist of cardboard or plastic squares coloured yellow and coated with a non-drying, sticky substance. 'Rentokil' bird repellent works fine. Place the traps no higher than the crop. The tiny parasitic wasps that predate on whitefly are caught with greatest frequency above the height of the crop.
Many vegetables need to be started in a warm place some time before they are planted out into the garden. A simple cold frame makes a good place to raise these seedlings. Even better is to provide supplementary heat. The old fashioned way is to use horse manure in a pit underneath the cold frame as a source of bottom heat. Excavate the soil for a depth of 45-60 cm and replace all but the top 15 cm with the manure. Trample it firmly or the heat will be excessive. The temperature will rise for some time before declining slowly. The cold frame should be patterned along the lines of the drawing and face north. Do not make the frame too deep, or the seedlings will not get enough light. This makes them weak and spindly.
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