Important Conditions for Gardening Under Cover

Gardening Under Cover


The value of any cover is much greater if it can be kept frost free. On the smallest scale, you can get seedtray-sized electric propagators to sit on windowsills. These are very convenient for germinating and rooting, but they are so small the plants cannot grow uncongested for long. Then there is nowhere to put them until the weather outdoors has warmed up. Thus we may want to warm a larger area. This may range from simply adding insulation to a coldframe at night and popping in a couple of cans full of hot water each evening to keep it frost free, to introducing a full-scale automatic system with hot-air blowers. The more heat this provides, the earlier you can start cropping and the greater the range of plants you can grow. However, it is expensive to install and run heat. A paraffin stove is the cheapest option, but it causes damp and fumes, although I use one in the spring for extra heat for a week or four, when planting up my polytunnel with tomatoes and so on.

Important Conditions for Gardening Under Cover - greenhouse heaters With electricity available, a fan heater is most useful, especially if it has a thermostat. This is the cheapest way to heat because the capital cost is so low and most heat is needed at night, which can be off-peak cheaper electricity. Gas can be as cheap to run, but is more costly to install and any leaks or fumes may damage the plants. Oil is very similar to gas. Solid fuel heating is also cheap, but a lot of physical effort unless very modern equipment is installed.

Soil-warming cables are very inexpensive to run, often using no more power than a bright light bulb. They heat the soil underneath plants and keep the roots warm which is more important for many plants than warm top growth. They can be used to make propagators, to heat coldframes or to warm an area of floor inside a greenhouse for more tender plants — this can even be in an indoor coldframe.

Passive heating is the ideal. You need much better insulated glazing and a massive back wall that will absorb the energy from the sun during the day and then release it at night as the temperature drops. This system has been used almost unaided to keep homes warm, so why not a greenhouse?


A better coldframe or a bigger propagator

Falling between the coldframe and the greenhouse, I find the best solution is a dead deep freezer. This is best stood in a greenhouse, but will work against a sunny wall or even sunk in the ground. Basically it is a well-insulated box with a lid, which can be fitted with a ventilated glass or plastic cover for daytime use, and shut at night to keep the warmth in. A false floor is needed to bring the plants up near the light. If this is covered in sand, a soil-warming cable can be run in it to keep it warmer still for use as a giant propagator. It makes a superb coldframe and I find it cheaper to run the warming cable only at night on a time switch, rather than have the temperature controlled by a soil thermostat which corrodes away faster than the cables.

Without an electrical heat source, extra warmth can be supplied by putting in bricks warmed on a radiator indoors, or by adding bottles full of hot water. Once the weather is warm enough for moving the plants into a polytunnel or greenhouse, the old freezer can be used for growing melons or even watermelons. If you are really handy, you can cut a panel out of the front of the dead freezer and replace it with glass making the box even better as a coldframe or propagator.

Hotbeds are not often made nowadays. It’s not a shortage of fresh, strawy manure because that can be had from most riding stables. I suspect it is that they are seen as ineffective, certainly I thought so. However, when I first saw how hot a big compost heap got I soon learned to put a thick layer of soil on top and cover it with a coldframe. Without doubt this is a very simple and easy way to grow cucumbers and melons of a very good quality. Now on a smaller, but still effective, scale I make little hotbeds for these crops, inside my polytunnel.

Hotbed mixtures are just compost heaps. Traditionally, strawy dung is preferred, but I find grass clippings work well, too. I use stacks of big tyres to hold the mixture of three-quarters grass clippings, one eighth soil and one eighth straw. As the heat in the tyres drops, they can be swapped for new ones, transferring the top one or two over on to the new stack. The topmost tyre holds the compost and the plant, which is usually sown in situ under an extra small cloche.


You cannot over ventilate if there is sufficient heat. The greater the throughput of air, then the healthier the plants as they consume the carbon dioxide and fresh dry air usually reduces mildews and moulds. However, never suddenly chill the plants by completely opening up too soon, or worse too late when they are half cooked.

To prevent over heating under cover, automatic vents are indispensable. Always ensure enough area can be opened to allow for those few days when you actually have a heat wave. Automatic fans are another good investment. Just keeping the air moving under cover improves conditions and reduces incidences of under and over heating. A small electric fan can even be solar powered — just to move air when the conditions are hot! It is hard to ventilate sufficiently with safety on cold spring days because the vents have to be kept shut. Since there is then little change of air, it is worth fortifying the atmosphere under cover with bottles of fermenting wine or beer which give off carbon dioxide replacing that used up by the plants as they photosynthesise.

Water control

Under cover, in pots or borders, and outdoors against dry walls and in sheltered corners, air moisture is very important. Too much encourages moulds and botrytis, and too little promotes mildew and spider mite. In general, during the growing season it is better to err on the side of too moist, so spray the walls, plants and floor with a fine jet in the morning so it can dry before nightfall. It will save time to arrange an overhead sprinkling system for plants under cover.

Outside, these are wasteful as much of the water just evaporates and chills the soil. Sprinklers are inexpensive and will soon repay their cost, but for only a little more you can have seep hoses and trickle feeders which use water more efficiently by applying it directly to the roots. In a greenhouse with a larger investment a microchip-operated system can be installed that will control accurately the soil and air moisture levels and the heating and ventilation. Less expensive systems can be run with timers or simple moisture sensors. In a polytunnel, the air conditions tend to err on the too moist side and more heat or ventilation is required.


Although all plants need light not all need full sunlight and many need partial shade or they may get leaf burn. In very hot weather plants may cook without adequate ventilation, or even glass shading or blinds. The problem is worst when a period of cold dull weather is suddenly replaced by clear blue skies and non-stop sun. The plants get too much too soon, and effectively get sunburnt just like us.

Generally, though, more often the problem is of insufficient light, made worse in small gardens by overhanging trees and nearby buildings. Pruning may allow more light in and dark walls can be lightened with white paint. In winter, all glass and plastic should be kept scrupulously clean to prevent further light loss. Electric light can supplement weak winter sun; it is especially useful for early sowings and valuable plants.

Ordinary incandescent bulbs are counter-productive as they give the wrong spectrum. Instead, use special fluorescent tubes and discharge lamps made for the purpose, though these are expensive. I now believe it is better to add extra layers of insulation to retain heat, and add extra artificial light to compensate the young plants at the start of the year. It is much cheaper to provide lighting than it is to heat the structure that gives brighter conditions by doing without the extra insulation.

Pest and disease control under cover

While a cloche isolates a plant to an extent it is still in the soil and only temporarily covered. Growing under walk-in cover is more artificial and requires more intervention as the natural systems cannot control pests and diseases unaided. The old approach of sterilise and poison everything in sight with chemical fumigants does not appeal to the organic gardener who does not want a ‘sterile’ environment — though in extremes a greenhouse or tunnel can be cleansed thoroughly with a high pressure steam jet.

The natural alternative is to encourage predators for pest control. Supply water in saucers and film canisters, nests made of straw filled pots, sections of hollow-stemmed plants and rolled up cardboard tied in dry nooks and crannies. Groundcover plants can be included under staging and rock piles in shady corners for beetles, spiders, frogs and toads. I hang strings just to give my spiders good frames for their webs which soon fill them all in. Companion plants can bring in and feed more predators and pollinators, such as ladybirds, hoverflies and lacewings. I grow French marigolds in the greenhouse or tunnel especially by the door where you brush against them as these keep whitefly out, but attract bees and hoverflies in. I also find sweet tobacco and wild tobacco (Nicotiana sylvestris) beneficial as their sticky stems trap many little insects like thrips. Most effectively bought-in predators can be introduced to control specific pests, then refrain from even organic pesticides and the yellow sticky pest traps sold commercially or the predators may suffer.

Soil care under cover

In the greenhouse or polytunnel, as everywhere else, it is preferable to grow plants to fruition directly in the soil rather than in containers as it saves considerably on watering and compost. The greater root run in the soil gives plants the most stable conditions and the best chance of finding nutrients for themselves. Even with a rotation and moving the plants around each year and adding copious amounts of garden compost and lime the yields may theoretically start to drop after five years or so as the soil becomes worked out. (I have never found this, but I suppose it is possible.) The answer is simply to dig out the topsoil and replace it with compost and fresh soil dug from a clean part of the garden. This is hard work, but does not need doing more often than one year in five and is much less work than the watering and carrying in and out the compost for containers or ‘bags’ over the same period of time.

For the paths I use old carpet and cardboard to cover bare soil because this has several advantages. It prevents weeds and moisture loss, it stops splashing of soil onto crops, it is more pleasant to work on and rolling the strips back allows me to pick off slugs and other pests hiding there.

Hardening off tender plants the ideal way

I start off all my tender plants in a heated old freezer-propagator and move the hardiest out first into a coldframe in the greenhouse, one section of which is heated. As they get bigger and tougher they move to the unheated section of the coldframe, and then out of the frame and into the greenhouse proper. Some of them are eventually hardened off and go outside. As each type of plant moves through this sequence, it leaves space for others so that I can raise many dozens of plants with very little total space or much electricity. The general order is indoor tomatoes and cucumbers are started first, soon joined by peppers and aubergines.

As the first of these move on, outdoor tomatoes, sweet corn and indoor melons are sown, then the other cucurbits. Sweet corn, ridge cucumbers and courgettes move from the warmth then the squashes, leaving the space for indoor cucumbers, melons, watermelons and okra, the last of which stay all summer because they need the extra heat continuously.

06. January 2011 by admin
Categories: Featured, Greenhouse Gardening | Tags: , | Comments Off on Important Conditions for Gardening Under Cover


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