Garden Cloches: Glass or Plastic Tunnels

Glass or plastic tunnels placed over crops or other plants to keep out frost and cold winds will speed up growth and extend the growing season.

Gardeners discovered long ago that a plant could be forced into early growth by protecting it with a glass cover. A Frenchman — whose name has escaped the history books — evolved a practical and efficient shape for protecting and forcing individual plants. This was a bell-shaped glass dome and it became known as a cloche, the French for bell.

The name has persisted to the present day, but cloches are no longer bell-shaped and they are more often used to protect whole rows of plants instead of individual ones. The final change from the original concept has come about with glass — because of its relatively high cost — being largely replaced with plastic.

Since most vegetables are grown in straight rows, a number of cloches set in a line, with the ends closed to exclude draughts, are ideal for producing early crops and for protecting tender plants. Generally, sowing and planting are possible two weeks earlier than in the open ground.

This earlier start — resulting in earlier harvesting — makes it possible for more than one crop to be grown on the same ground in a single growing season.

As an example of a cropping programme, lettuces sown in mid autumn may be cut in mid spring, to be followed by early-fruiting dwarf tomatoes. These, in turn, may be succeeded by spring cabbages planted in the open. The cloches, meanwhile, are transferred to another part of the garden to restart the cycle.

This mobility enables a gardener to use a cropping technique whereby cloches are moved from one row to another as the season progresses.

Early carrots, for example, may be given a good start by being covered in early and mid spring. They then go on to mature in the open and the cloches are moved in the middle of late spring to enable newly planted tomatoes to become established. Three or four weeks later the cloches are again moved, this time to protect tender, heat-loving plants — such as aubergines, melons, ridge cucumbers and sweet peppers — for the rest of the summer.

Cloche types

A variety of shapes and sizes of cloches is available from most garden centres and from specialist manufacturers. They may be made from several materials. Alternatively, you can construct your own custom-designed cloches.

Obviously, costs are related to the size of the cloche, so decide what plants you intend growing before buying them. If you plant or grow only early lettuces, for instance, a cloche 30cm (lft) wide and 23cm (9in) high is adequate. For growing melons to the fruiting stage, however, you will need cloches about 45cm (1-1/2ft) wide and 30cm (1ft) high.

Each type of material used for cloches has advantages and disadvantages so choose carefully to suit your particular needs.

Glass cloches have proved their worth for many years, but the cost of glass and delivery has made them an expensive investment.

Many garden centres now carry only a restricted stock, while firms that sell by mail order advertise only the wire supports — you have to buy the glass from a local glazier. If you want glass, order horticultural quality which is cheaper than window glass.

Inevitable breakages also add to costs, but glass has some advantages over other forms of cladding. When kept clean, glass cloches let in the maximum amount of sunlight, and on cold, clear nights they retain heat better than polythene. Adjustable locking devices make for good ventilation and on exposed, windy sites their weight makes them more secure than lighter materials

There are two main types of glass cloches — tents and barns. A tent cloche consists of two 60 x 30cm (2 x 1ft) sheets of glass fixed at the top by a galvanized-iron clip to form a pitched roof or tent. This type is useful for raising seedlings or for single rows of low-growing crops such as lettuces, carrots, beetroots and strawberries.

Barn cloches generally have four panes of glass, two forming sides and two forming the roof. A low barn cloche is 60cm (2ft) long and 30cm (lft) high. With a width of about 60cm (2ft), it is possible to grow a central row of lettuces and outer rows of carrots or beetroots, for instance. The lettuces will be harvested first, leaving the others space to mature.

A high barn cloche is as wide and long as a low barn cloche, but the height is about 49cm (19in). A row of high barn cloches is useful for getting plants such as dwarf tomatoes, sweet peppers and aubergines to a fairly advanced stage before protection is no longer necessary in summer.

Solid plastic cloches are cheaper than those made from glass but, initially, are more expensive than polythene sheeting types. Solid plastic lasts longer than glass, which is easily broken, or sheet polythene, which must be replaced every few years. They trap less heat than glass cloches.

Plastic cloches can be made of corrugated PVC, moulded or twin-walled polypropylene or twin-walled polycarbonate. The manufacturers have taken advantage of the flexibility and lightness of plastic. Some cloches, for example, are 1.8m (6ft) long — a size that would be excessively heavy and cumbersome to handle in glass. Also, the longer the cloches the simpler the task of moving them from crop to crop.

Widths of cloches vary from 30cm (left) for growing seedlings, to 1.2m (4ft), in which two or three rows of vegetables can be grown to maturity. Heights vary from 20cm (8in) for seedlings to 60cm (2ft) for fully grown plants.

Solid plastic cloches are available with straight sides or curved into hooped tunnels. Both types are equally effective, but many straight-sided models have the added refinement of ventilation flaps to reduce condensation and get air flowing. Condensation in solid plastic cloches is not, however, as serious as in various types of polythene cloches.

Plastic equivalents of the old-fashioned bell cloche may be obtained from some suppliers for protecting individual plants —they are ideal for protecting the developing blooms of Christmas roses from harsh winter weather. Polythene cloches are obtainable either as separate units, which are then put end to end, or as a complete tunnel cloche made from a length of polythene sheeting draped over wire hoops.

Polythene is obtainable in various thicknesses. The thinnest – 150 gauge — is the cheapest form of cladding but it will last for only a year or two. Heavier duty polythene — 250-300 gauge — is more expensive but it will last three or four years with care. Polythene treated with an ultraviolet inhibitor to slow down deterioration is also available. This lasts longer, than untreated material of the same gauge.

Condensation is a problem with polythene. It can provide conditions for the spread of disease, so it is important to ventilate the cloches to clear the condensation and to get air circulating.

As polythene is light and unbreakable, it has a big advantage over glass when being moved from one crop to another. Its lightness can be a disadvantage, however, in areas affected by high winds. If gales are forecast, anchor cloches with bricks, or form an inverted `V’ over them with canes driven into the ground. Tunnel cloches are satisfactorily anchored by their method of construction.

The simplest and cheapest of all plastic cloches is the modern floating cloche, which has no support members and consists entirely of a single sheet of perforated polythene anchored across the top of the plants by burying the edges in the soil. A special woven agryl fleece can be used in the same way.

Growing under cloches

If possible, prepare the ground a month before sowing or planting to give the soil time to settle. Dig in one bucketful of well-rotted manure or compost per sq m/yd.

Two weeks before sowing or planting, rake in a dressing of general fertilizer at about 50g (2oz) per sq m/yd. Mark a central row with a string line, leave this in position, and cover the row with the cloches to warm up the soil. Secure the end panels.

Remove the cloches at sowing time — the purpose of leaving the string line is to centre the row where the developing plants will get most headroom.

Sow small seeds about 6mm (1/4in) deeper than in the open ground because the surface dries out during the warming-up period. Do not water until the seeds germinate — enough moisture will filter up from the soil beneath the seeds to begin with. However, water immediately after setting out young plants.

After sowing or planting, scatter slug pellets and replace the cloches in exactly the same position as when the ground was being warmed. Subsequent cultivation is the same as for plants growing in the open. Although the surface may look dry, a few centimetres down it will have the same moisture content as the uncovered soil alongside and water will reach the plants’ roots by capillary action.

If spring days are unusually warm, open up some continuous cloches or slide back the polythene of a tunnel cloche to allow air to circulate. Replace or close the cloches an hour before sunset.

If late spring frosts are forecast, cover the cloches over tender crops with four or five sheets of newspaper in the evening and remove them in the morning.

A week before moving cloches from one row to another, harden off the plants that are about to be left in the open. Leave off some cloches — or slide back the polythene — during the day and replace them in the evening.

When using cloches to cover strawberries, they serve a dual purpose — it will be unnecessary to net the plants against bird attacks and, if put in position in late autumn, will provide an earlier crop than in the open garden.

21. July 2011 by admin
Categories: Cloches, Greenhouse Gardening | Tags: , , | Comments Off on Garden Cloches: Glass or Plastic Tunnels

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