Protected cropping

Going Undercover

Protected cropping refers to a gardening technique that uses specialised structures, such as cloches, greenhouses and polytunnels, to trap the sun’s warmth and shield growing plants from the wind. The microclimate they create gives more flexibility to when and what crops a gardener can grow. For example, over-wintered Oriental leaves, turnips and coriander flourish in this pampered environment, while tropical immigrants like peppers and aubergines find the extra heat invaluable. The protective environment is also ideal for starting transplants, particularly of warm season crops like tomatoes and peppers.

Over-wintered Oriental leaves in a tunnel: just transplanted in October; in January ready for picking

Greenhouse with pepper seedlings

Cropping under cover is not trouble free, and conditions inside can go to dangerous extremes. On sunny, summer days, for example, temperatures in an unventilated structure can be so high they are life-threatening to the resident crop. Most types of protected cropping keep the rainfall out, so drought is a constant threat. Conscientious venting and watering will usually solve the problems, but aren’t always possible when the gardener has to go to work or if the vegetable patch is some distance from home.

Opening a greenhouse vent to prevent it overheating. Tunnel fully vented.

Making the right choice 

Styles of protected cropping vary in their design, cost and effectiveness. With so much choice, there is something to suit every garden, whatever the size.

Fleeces

 Fleeces are a soft, light-weight material that can be draped loosely over small hoops. Usually, however, fleeces are laid directly over a crop like a blanket. They are particularly good for bringing on over-wintered salads and encouraging the growth of early crops like potatoes, lettuces and carrots. Because the crops are completely enclosed, some pests are also kept at bay.

Fleece draped over small hoops to protect a leek crop, and laid directly on a row of cabbages.

To stop them from blowing away, the material must be weighted down by loose soil or fastened with pegs. As the material is composed of many small holes, rainfall easily penetrates to the soil underneath. Unfortunately, a fleece has a short shelf life, though with care they should last at least two or three seasons.

Fleece can be bought from garden centres loose by the metre or as a kit with the hoops.

Garden frames

  Garden frames are exactly what their name says, i.e. they are low-lying square or rectangular structures with movable tops that provide ventilation and access to the inside. Garden centres sell different models, but DIY enthusiasts may want to take on the challenge and build their own.

Garden frames can be bought from garden centres; peppers and tomatoes doing very well in a garden frame.

Making a garden frame is not rocket science, and a range of materials can be used to get one up and running. The sides, for example, can be made from brick, breeze blocks or plywood sheets, while anything that lets in the light, such as recycled shower doors and double-glazed windows, can be used for the tops. Another approach is to stretch a plastic film over a wooden frame, producing a light structure that is easily moved around.

Cloches

  Cloches have a long and honoured history in the garden. Modern versions are often plastic structures that can be easily shifted around the garden. The simplest ones are bell-shaped designs that cover individual plants or pots, while others are reminiscent of miniature tunnels that straddle sections of beds and rows.

Cloches can be simple bell-shaped or like miniature tunnels.

Even clear plastic fizzy drink bottles can be used as individual cloches to protect a plant from frost or to elevate the temperature. To use them simply cut off the bottom of the bottle and drop it over the plant.

A downside to cloches is their tendency to fly away in strong winds. They can, of course, be anchored to the ground, though it might be better to ban them completely from excessively windy sites.

Glass cloches protecting a salad crop in an autumn garden.

Miniature greenhouses

These are small, plastic covered structures that are about 1.5 metres in height. They are cheap to buy, easy to erect and take up very little room in the garden. Their only downside is that the plastic and frames are not very durable, and last only two to three years. However, they fit nicely on patios and are the ideal choice for novices who want to try their hand at protected cropping.

Miniature greenhouse at at garden centre and in use full of chilli peppers.

Walk-in greenhouses and tunnels

 Walk-in greenhouses and tunnels are large structures that usually come with big price tags. They take some effort to erect, but once up will last for years. Greenhouses are made of sheets of glass or stiff polycarbonate plastic supported by wooden or aluminium frames. They are fiddly to erect, though their longevity is more than fair compensation for this shortcoming.

Greenhouses are made of sheets of glass or stiff polycarbonate plastic supported by wooden or aluminium frames.

Some greenhouses can be interesting shapes

Tunnels are less sophisticated than greenhouses, and amount to little more than a row of metal hoops covered with a plastic sheet. This simplicity means that they are easier to erect and, for the area they cover, cheaper to buy.  

Large or small tunnels are little more than a row of metal hoops covered with plastic.

Unlike greenhouse glass, the plastic sheet eventually surrenders to the destructive effects of sunlight, disintegrating after a few years of use. The hoops get hot under the summer sun, causing the plastic to wear out where it touches. Sticking ‘anti-hot spot’ tape – a foam-like material – on the outsides of the hoops insulates the plastic from the hot metal, adding at least an extra year or two to its life.

Sticking hotspot tape on the hoops of tunnels.

Gardeners intimidated by the cost of new tunnels and greenhouses can try buying cheaper second-hand versions. Local newspapers and notice boards often advertise bargains, though the inexperienced should proceed with caution. In the case of tunnels, the hoops have to be checked for rust, while any plastic included in the deal must be inspected for tears, holes, and scratches. Buying second-hand greenhouses requires the same due diligence, such as looking for broken glass, rotting wood and bent aluminium.

Tunnels and greenhouses are high maintenance structures that need some mollycoddling to work. The crops inside must be diligently watered, and algae, the scourge of humid Britain, needs to be washed off regularly. In addition, ventilating through the doors and, in the case of greenhouses, roof vents is compulsory.

Greenhouses should always be kept clean.

Location, location, location

Tunnels and greenhouses cannot go just anywhere, and potential sites must be judged for their ability to accommodate the structures.

A greenhouse at the allotments.

Slope

 The site for a greenhouse should either be level or flat enough to be easily levelled off. A tunnel is another matter, and can be erected on either sloping or level land.

Size of the area

  The site should be big enough to accommodate not only the structure but also a strip of ground running along its four sides. The strip should be wide enough – 1 metre or more – to provide access for repairing, washing and changing the glass or plastic.

Shade

 Greenhouses and tunnels should be kept away from any trees, buildings and fences that might shade the plants inside.

Distance from the house

  Watering and venting will be easier with greenhouses and tunnels located near the house.

Cropping plan

 Though greenhouses and tunnels are ideal for summer crops like tomatoes and peppers, they are just as dependable for producing a wide range of winter vegetables and herbs. As if made for each other, winter and summer plantings can be matched into useful cropping schemes that produce edibles for a good part of the year (see table).

* Vary according to part of country and weather conditions.

** Assumes all crops except carrots and peas are started as transplants.

*** Includes Oriental leaves, parsley, perpetual spinach, lettuces, chicory and endive.

* * * * *

Greenhouses and tunnels at Sea Spring Seeds 

Dorset Naga seed crop at Sea Spring Seeds.

© Michael Michaud