Greenhouses, Frames and Cloches
A greenhouse helps to lengthen the gardening year, turning late winter into spring and coaxing summer to linger into autumn.
For this reason alone, a greenhouse is worth having. For a gardener keen on growing food, it is also a sound investment. The initial cost can be returned in a few years by the savings on plants raised from seed, and by harvesting vegetables when they are at their most expensive in the shops.
As a greenhouse is a long-term investment, give some thought to the type and size best suited to your needs and pocket.
How big a greenhouse?
If you want to grow vegetables, such as early lettuces, tomatoes, cucumbers, sweet peppers and aubergines, as well as raising seedlings, a glazed greenhouse measuring about 10 x 8 ft (3 x 2.5 m) is ideal.
If you cannot afford this, either buy a similar-size house made of polythene or purchase a smaller glazed house and use it mainly for raising early plants and for growing a summer crop of tomatoes.
When deciding on the size, consider the cost of heating if you plan to grow plants much earlier than their normal season. The larger the area of glass, the more expensive the house will be to heat.
Types of greenhouses
Although greenhouses are made in many shapes and sizes, there are four main types:
Free-standing buildings with a span roof and vertical side walls — or walls set at only a slight angle — are the most popular type of garden greenhouse. Some have timber cladding up to the staging, or are designed to be built on low brick walls, but for food growing buy one with the glass extending to the ground on at least one side. This will enable you to plant tomatoes, melons or cucumbers directly in the greenhouse border.
Large panes and angled side walls admit the maximum amount of light. Houses of this type are ideal for growing plants directly in the soil because the glass extends to the ground.
Temporary staging can be put up in late winter and spring for trays of seedlings, and removed in summer to make space for growing plants in the ground.
As its name implies, this type of greenhouse is erected against a wall of a building, preferably facing south where it will get most sunlight. It is warmer than a free-standing house, and it is sometimes possible to heat it by an extension of the household central heating.
The approval of the local authority is needed if the greenhouse is to be erected against the wall of a house.
Although it is of unconventional appearance, a house of this shape has many advantages. Light is let in evenly and little floor space is wasted because the gardener, working in the centre, can reach all the plants without having to walk up a path.
The flow of air is controlled by ventilators at ground level and an adjustable dome at the top.
Wood or metal?
The choice of cedar wood or an aluminium-framed greenhouse is again a personal one. Cedar blends more happily into the garden background, and needs to be treated with a wood preservative only every two or three years.
Wooden greenhouses are generally delivered in prefabricated sections. The two sides, two gable ends and two roof sections are easily bolted together, but the glazing may take a day or two to complete.
Here the aluminium greenhouses have an advantage, because the panes can be clipped into place in a few hours.
Another advantage of the aluminium type is that it needs no maintenance.
MINIATURE LEAN-TO GREENHOUSES
The combination of rising costs and small gardens has created the need for a really small greenhouse. The mini-greenhouse is basically a garden frame, turned on end, to form a small lean-to against a house or garage wall.
Sliding doors give access to shelves that can be placed at any height. The gardener works from outside the house — a disadvantage in bad weather.
Though of limited value for growing food crops, the mini greenhouse would nevertheless suit a gardener with restricted space, or a fiat-dweller who could erect it on a sunny patio. It is ideal for raising seedlings, and the wall against which it is sited will help to retain day-time warmth for dispersal during the night.
Siting a greenhouse
Choose a position where the greenhouse catches the most sunlight, especially in late winter and early spring when seedlings are beginning to grow.
Ideally, the greenhouse should be as near the house as possible to make it more convenient to layand . Wherever the site, lay a path leading to the door of the greenhouse.
Opinion is divided on whether a north-south or east-west alignment is preferable, but it does not matter a great deal. In a small garden the site itself will probably dictate which way it must go, but try to position the door on a side away from icy winds.
If the site is near a fence or wall, allow space all round for maintenance and glazing.
LAYING FOUNDATIONS FOR A GREENHOUSE
A greenhouse of any type needs a firm base for support and anchorage.
A base of old railway sleepers, laid on firm soil, is sufficient for the smallest sizes, but the best foundation is a course of bricks laid on a concrete strip.
Exact measurements are given in the manufacturer’s instructions. When making the site, check that the corners are perfect right-angles by measuring diagonally between them. Provided facing sides are of equal length, the diagonal measurements should be identical.
For the concrete foundations dig out a trench 6-8in (150-200mm) wide and 4in (100 mm) deep. Place a spirit level on a long, straight batten to check that the concrete is level. Place in. (12 mm) diameter rag-bolts — two or three on each side and end — in positions where they will correspond with the base frame of the greenhouse.
Lay the brick course a week later, if necessary cutting bricks to ensure that the mortar joints correspond with the rag-bolt positions. When erecting the house, drill holes for the bolts in the base frame and secure with nuts. Place a strip of damp-proof course or roofing felt between the bricks and a timber frame.