Deciding Whether to Buy a Polythene Greenhouse
For the gardener who concentrates on food growing, a polythene greenhouse has one big advantage over a glazed greenhouse — the lower initial cost.
A polyethene house 20 x 10 ft (6 x 3 m), for example, may cost only half as much as a glazed house measuring 10 x 6 ft (3 x 1.8 m). Using polythene, you can therefore put a substantial part of your vegetable plot under cover for a relatively small outlay.
This would suit a gardener with plenty of space, but one with a small plot should consider whether a small greenhouse used principally for raising seedlings might be more practical.
A polythene greenhouse consists of a framework of tubular steel hoops covered by special horticultural polythene. The hoops are driven directly into the soil or into tubes set in the ground.
Foundations are unnecessary and the house can be erected and dismantled quickly. This gives it a mobility that can be exploited in a crop-rotation plan.
The greenhouse can be used, for example, for growing tomatoes on one part of the plot one year and moved to another part where the tomatoes are to be grown the following year.
In late winter and spring, seedlings can be raised in pots or trays on a trestle table or on staging sold by the greenhouse manufacturers.
A polythene greenhouse may have certain disadvantages, however. The chief of these is condensation, producing a high humidity which can cause a number of plant disorders. This problem varies with the make and design, so choose a greenhouse with ventilators on the door and the far gable.
Heating, combined with good ventilation, will also reduce condensation, but the choice of heater is important. Naturalor paraffin heaters only exacerbate the problem.
An electric heater is better, and the ideal is an electric fan heater which drives warm air to all parts of the greenhouse. In summer, the fan can be switched on without the heating element to keep the air moving.
A second drawback with polythene is that it is not as efficient as glass in keeping out frost. During frosty spells, therefore, shut the ventilators during the afternoon to increase the humidity. Condensation will then act as an insulator and slightly reduce the loss of heat.
A third disadvantage is that the polythene needs replacing at least every two to four years and this regular expense must be balanced against the initial cost of the greenhouse.
Polythene used in most greenhouse kits is treated with an ultra-violet light inhibitor to slow down deterioration caused by sunlight. If you decide to make a greenhouse of your own, ask the supplier for this material.
Wind is also a problem with a polythene greenhouse, not so much for wear and tear as for the loss of heat.
Making the most of your greenhouse
To get maximum value out of your greenhouse, aim to have crops at various stages for at least ten months of the year. The table alongside will help you to decide what to grow and when to make the sowings.
The growing programme can be divided into two periods. During the first, from January to May, make sowings of crops that will continue to maturity in the greenhouse, and of others that will eventually be transplanted in the open.
During the second, overlapping period — from March to October — grow a selection of plants, such as tomatoes, in pots or in the greenhouse border.
During the summer, growing space will be almost doubled if you dismantle the staging and store it elsewhere. Do this in late May, when the last of the tender young plants are moved outdoors.