Plan the floor space first. You might prefer a solid floor with fixed staging round the sides, or ground-level growing space, perhaps in raised beds and with removable staging that can be moved outside in summer or collapsed for storing. Growing bags can be arranged on the soil or solid floors and on staging to house plants temporarily. Shelves on the sides and in the roof can extend the growing and display space, while brackets will support hanging baskets and strings or wires for training climbing plants.
POLYTUNNEL ALTERNATIVE
Cheaper and easier to erect than a greenhouse, a polytunnel can offer most of the advantages of a greenhouse, although the cladding of heavy-duty plastic sheeting is less heat-retentive and condensation can be a problem. Depending on its quality, the plastic sheet will need replacing every 3–5 years. Make sure there is a door at each end of the tunnel for efficient ventilation.
Fitting automatic vent openers that can be adjusted to open at a certain temperature can relieve you from worry and prevent injury to plants. An overhead reservoir can be used to supply water to capillary matting on the staging or to drip tubes positioned in containers to alleviate watering chores. Blinds are expensive, but shade netting is available for installing in summer as an alternative to applying shade paint.
TEMPORARY FRAMES
You may be reluctant to sacrifice good growing space to a structure that might be needed for only part of the year, perhaps for spring frost protection. Lightweight collapsible frames are available that can be dismantled when they are not needed. Alternatively, you could make your own from a variety of materials. For example, a large bottomless box with its top replaced by a sheet of clear plastic can cover several seed trays (on very cold nights simply spread an old blanket or sheets of bubble polythene over the top). An enclosure of straw bales covered with old car windscreens or double glazing panels makes a snug frame, and the straw can be used afterwards for mulching or as a carbon ingredient in a compost heap (see pages 116–17). Professional growers often arrange empty crates and boxes to make the frame walls, draping black polythene over the walls and floor like a pond liner, and then sheeting over the top with thick polythene (polytunnel grade).
Cold frames Although they are sometimes regarded as simple greenhouse accessories, cold frames are versatile, sometimes portable, infinitely adjustable and often efficient substitutes for a greenhouse. They are also less expensive than a greenhouse, and provide more space and greater adjustability than cloches. Easily constructed at home, they can be adapted to span a narrow bed or to fit on top of a compost heap to make a hot bed (see page 53).
At its simplest the standard frame is a four-sided box structure with a lid that slopes to shed water and opens to admit air. It is accessed from the top, via a lid that is glazed with plastic or glass, and that can be lifted or removed to adjust ventilation. The sides may be glazed or solid. Fixed frames can have a soil floor for planting, or a solid base of slabs or gravel spread over a weed-proof membrane, which is useful for housing pots and trays. A portable frame can be moved directly over a growing crop for protection until established, after which it is moved elsewhere in the same way as cloches.
SITING A COLD FRAME You usually have little choice over the position of a greenhouse on your allotment – it may be decided for you by accessibility or site rules – but a permanent frame can go almost anywhere. The traditional position is against one side of the greenhouse to avoid carrying plants far and to share some of the stored warmth (some sophisticated frames have adjustable rear panels to allow heat transfer from the greenhouse). Installing a frame on each side of the greenhouse should provide all the space you will need for protection and hardening off.
There will probably be a lot of plant movement to and from the frame, so site a freestanding version in a convenient position: placing it at the end of a nursery bed would keep all plant-raising activities together, or you might prefer a corner of a main vegetable bed to save time when planting out. Make sure the frame is easily accessible all round, and ideally not too far from your water supply. If possible, provide shelter from prevailing winds and avoid shade from overhanging trees. A frame is normally aligned so that its sloping lid receives maximum sunlight and heat, but a lightly shaded frame can be equally useful in summer to avoid scorching sensitive plants.
Covers Protective sheets such as transparent plastic film and woven horticultural fleece can be used to cover crops and exclude a couple of degrees of frost. Horticultural fleece is light and permeable, and may be left in place over the lifetime of a crop for warmth or protection from pests or diseases, floating higher as the plants grow.
Cloches Glass sheets (discarded window panes, for example) are joined with special clips (see page 146) or a home-made arrangement of clothes pegs, string or wire to make tents for covering rows or individual plants. Traditional lantern and commercial barn or tent cloches are also available. Plastic cloches and continuous mini-tunnels of film supported by wire hoops can cover a large area. Use cloches early and late in the year to add several weeks to the growing season.
SEE ALSO ▸ The greenhouse year pages 170–4 The cold frame year page 175
USING A COLD FRAME
▸ Spread a layer of gravel over a weed-proof membrane if you intend to use the frame for containers or for trays and plugs of seedlings (below).
▸ Fit a hinged lid with casement stays or notch a strip of wood to make a support for adjusting ventilation. Hinge the lid and prop it open to ventilate the cold frame during the day (bottom), then close it at night to keep in the heat.
▸ When not in use, prevent wooden frames from rotting by lifting them clear of the ground with a block at each corner.
▸ Stand pots and trays on a layer of gravel over a woven plastic membrane to suppress weeds and deter slugs and snails.
▸ Treat a soil-based frame like an extra vegetable bed: water, manure, mulch and rotate crops as you would in the open ground.
Fruit cages If you can disperse fruit around the plot, it is possible to harvest good crops from unprotected plants without significant losses to birds or squirrels. But smaller plantings, especially of attractive fruit such as redcurrants, raspberries, strawberries and blueberries, may be stripped before they even show colour, and some kind of protection could be vital.
An individual bush can be enclosed with netting draped like a tent over 3–4 flexible canes arched to meet at the top, where they are tied. Protect a row of raspberries or cordon redcurrants by erecting a post at each end, with several timber cross-pieces, like a telegraph pole: attach wire to these, stretched from one end of the row to the other, and arrange curtains of netting over the wires and clear of the fruit.
Gathering vulnerable fruit together in a cage is a more permanent solution. Various ready-made cages are available to buy, or you can build your own from strong bamboo canes, coppiced hazel poles or metal pipes. Erect uprights 1.8–2.1m (6–7ft) high for clear headroom, space them about 1.8m (6ft) apart, and join their tops with cross bars to support the roof. Clad the sides with 1–2cm (1/2;–3/4in) mesh plastic or wire netting (but note that squirrels easily chew through plastic), and the roof with 2cm (3/4in) plastic netting.
RECYCLING SCAFFOLDING
Discarded scaffolding poles and their unions are a valuable resource for a host of structures on the plot. Use them, for example, to build fruit cages and low frames round brassica beds for netting against birds in winter; use them as row supports for runner beans or sweet peas and trained fruit like raspberries or tree-fruit cordons and espaliers; also for arches over paths, planted with squash,