Greenhouse Management and Accessories
Correct ventilation, watering and temperature control are necessary for success with greenhouse crops.
Fungus diseases flourish in a warm, stagnant atmosphere. At least two roof ventilators, as well as one or more at a lower level, are needed to ensure an adequate air flow in hot weather. If necessary, the door can be propped open, or an opening hatch fitted in the lower half of a wooden door. An electric fan heater, with the heating element switched off, is also an efficient method of keeping the air flowing.
One is a vent opener which is sensitive to temperature changes and automatically opens or closes the ventilator. The other is an electric extractor fan which is thermostatically controlled to provide a continuous gentle movement of air and also to reduce condensation.
These fans are easy to install and cheap to run but, of course, they need a mains supply.
A regular supply of water is essential at all stages of plant growth, but watering by hand is not always possible.
There are two methods of automatic watering which not only save a good deal of work, but also result in better plants.
The first, capillary irrigation, works on the principle that if a pot plant rests on a bed of wet sand the compost in the pot will soak up sufficient water to keep the plant healthy.
A number of capillary systems are available. In one, mains water is taken from a plastic tank fitted with a ball-valve. The water runs into a trough on one side of the staging and is drawn by a glass-fibre wick into the sand tray. In another system pots are placed on a highly absorbent mat, which again takes water from a mains-supplied trough.
Capillary watering is suitable only for seedlings and for plants small enough to be grown on the greenhouse staging.
An entirely different system — suitable for plants of all sizes — is trickle irrigation, in which a pipe is laid near the plants and drip nozzles release a steady supply of water. Liquid fertiliser can be added so that plants are watered and fed at the same time.
A gardener faces a challenge in trying to keep a greenhouse warm enough for seedlings to thrive in spring, and cool enough to prevent plants from getting scorched in a summer heatwave.
The biggest problem with artificial heating is the cost. Happily, vegetables and fruits do not need such high temperatures as ornamental plants, and worthwhile crops can be grown even in an unheated greenhouse. If you are trying to economise, wait until February or early March before making your first sowings.
The only time when fairly high temperatures are needed is when sowing seeds of some tender vegetables. But even then there is no need to heat the whole house. Either use an electric propagator for raising the seedlings, or put the greenhouse heater under the staging and erect a polythene canopy over the top to trap the warmth.
A saving on heating bills can also be made by insulating the greenhouse in winter and early spring with a light-gauge polythene lining. Kits, including fixing clips or suckers, are available from horticultural suppliers for various sizes of greenhouses. The linings are claimed to save 20% on heating costs.
If you decide to warm the whole greenhouse — and you can then enjoy a variety of crops while shop prices are very high — the choice of heaters is wide:
Designed to burn mains, this relatively new type of heater has accurate temperature controls and costs less to run than most other forms of heating. But the initial outlay for the heater and for having it installed may be high, especially if the greenhouse is some distance from the house.
Gas heaters can be adapted for bottled gas, but running costs are higher than with mains gas.
Portable paraffin heaters give reasonable frost protection, but they have limitations as the sole source of heating. Water vapour produced during combustion creates excessive condensation unless some ventilation is provided all the time.
Running costs are lower than forbut higher than mains gas. Only one or two types of paraffin heaters have automatic temperature control.
This is the dearest form of heating, but no other system is as versatile. When a supply has been connected to the greenhouse, equipment other than heaters can be used — such as a propagator, lighting and a fan extractor. Employ a qualified electrician for the.
There are two main forms of electrical heaters: tubular and fan. Tubular heaters are fixed to the sides or floor of the greenhouse, and take up little room. Fan heaters move the air more vigorously, as well as raising the temperature. With all forms of ‘leafing it is useful to hang a maximum-minimum thermometer in the greenhouse, to inform you how the system is behaving when you are not there.
Manufacturers supply staging — slatted benching — to fit their houses. Alternatively, make it yourself using 50 x 50 mm. Timber for the legs, and 50 x 25 mm Timber for the top framework. Nail 50 x 12 mm battens to the frame to form the slatted bench.
Cross-bracing (50 x 25 mm) between the legs and upper frame will keep the structure steady, provided the legs are placed on bricks or stone slabs.
A greenhouse provides ideal conditions for plant growth, but these can be just as encouraging to pests and diseases.
Inspect plants daily during the growing season and apply the appropriate spray if the trouble is easily diagnosed.
Both pests and diseases can be controlled by fumigation, a process known as ‘smoking’. Pesticidal smoke bombs control all insect pests, while fungicidal bombs prevent fungus diseases such as botrytis and mildew.
The advantage of these bombs is that the cloud of microscopic particles they emit penetrates to every part of a plant and to every crevice in the greenhouse.
Most smokes are harmless to food crops, but read the instructions carefully because some are lethal to plants such as melons and cucumbers.
Choose calm, warm weather, but do not fumigate in bright sunshine. Close the ventilators, block any gaps, and light the first bomb at the far end of the greenhouse. Working backwards towards the door, light any additional bombs as quickly as possible. Close the door firmly and lock it or put a notice on it.
The next day, open the door wide, wait for a few minutes, then open the ventilators.
In late autumn, when growth has stopped, wash down the interior with disinfectant.
Before sowing in late winter, clean the glass inside and out to let in as much light as possible.
Ideally, shading should be on the outside of the roof, an inch or two above the glass. Slatted wooden blinds that can be rolled up and down are the most efficient, but they are expensive.
Less costly alternatives are adjustable green plastic blinds inside the house, sheets of green polythene for clipping to the inside bars of the roof, and fine-mesh plastic netting that also allows air to circulate.
At one time gardeners sprayed the glass with a mixture of lime-wash and milk. Unfortunately, this left plants in the gloom during a cloudy spell. This disadvantage has been overcome by a spray now available from horticultural suppliers which turns opaque in bright sunlight but becomes more transparent on dull days.