General Greenhouse maintenance FAQs

What are the basic essentials for environmental control?

Maintain the temperature range required by the type of plants grown and do not mix plant types with widely differing needs. Good ventilation is vital in winter, whenever weather permits, and a low humidity level should be maintained throughout the cold months. In summer, shade the greenhouse when necessary, and remember to ventilate it to avoid scorching temperatures.

Avoid erratic watering at any time, especially in summer, and keep conditions generally on the dry side in winter when temperatures are low. In summer, damp-down frequently to raise the humidity. Prompt attention to any pest or disease is vital. Make sure that the plants do not lack essential nutrients by using recommended quantities of the appropriate balanced fertilisers; but always avoid over-feeding as well as over-watering. As a guide to watering always aim at moist roots, as distinct from absolutely dry or waterlogged compost.

How critical is humidity?

It has a considerable effect on the well-being of plants. In summer, when temperatures are high and plants are making active growth, a moist atmosphere reduces the rate at which plants lose moisture (and therefore the frequency with which they have to be watered), and they grow better and more vigorously as a result. In winter, when outside temperatures are low, the presence of excessively moist air in the greenhouse—especially if it is stagnant—is generally harmful: it encourages moulds and mildews, which attack the plants and can become a nuisance; hence the importance of winter ventilation. So watering must be kept to the minimum—and do not carry out damping-down .

What is the best type of shading?

For greatest cooling efficiency the shading should intercept the sun’s rays before they pass through the glass. This means either exterior blinds or a shading paint applied to the outside of the glass. Blinds should preferably be slatted to allow for degrees of shadow. They must be of substantial construction, or they will be liable to catch the wind and blow off. These requirements and the possibility that the equipment may have to be custom-made to fit specific areas, means that blinds are rather expensive. Most people prefer a shading paint. This must be coloured white—not green, as is so often seen. A recent development has been the electrostatic type of shading fluid which can be diluted from an easy-mix concentrate to give almost any degree of shading required. It can be applied by brush or spray, and it will not wash off with the rain; yet it can be instantly removed by friction with a dry duster at any time. But note that it is difficult to remove completely from certain plastics.

What is meant by the term ‘damping-down’?

This involves thoroughly sprinkling the greenhouse floor and staging with water, and, if necessary and recommended, spraying the plants with a mist of water from time to time if they are high-humidity lovers. The purpose is to elevate humidity, but it may also help to keep down temperatures in summer if done in combination with judicious ventilation. This is because water absorbs heat from its surroundings in the process of evaporation. Damping-down is rarely, if ever, done in winter, but may occasionally be needed then if tropical plants from hot and humid climates are being grown.

How can I prevent my greenhouse from overheating in summer?

Many beginners at greenhouse gardening fail through letting excessive temperatures develop.

Ventilation combined with damping-down is only part of the answer; if the vents are wide open, a sudden gust of wind may blow over or damage the plants. Special care must be given to satisfactory shading. Indeed, only very rarely is a summer so bad that your greenhouse will not need to be shaded at some time or other. On the other hand, the shading must not be such that it casts a perpetual gloom: it must protect the plants from scorching rays and reduce the amount of heat trapped by the greenhouse, but at the same time allow adequate light for the plants to remain sturdy and keep a good colour. In inadequate light, plant growth will be poor, weak, spindly, and pale.

Should I use mains water for my plants or is rain water better?

Rainwater that has been collected from roofs and stored in open butts can be a ‘soup’ of insect pests, diseases, organisms, weed seeds, and algae slime—and as such is likely to be a menace to all plants grown in the greenhouse. It is senseless to use such water if you are using carefully prepared, reasonably sterile composts. If your mains water is very hard or limy it is possible that some plants that are lime haters may not grow so well if it is used. If necessary, then, clean rainwater can be collected in clean containers and stored in closed vessels especially for use with these plants. There are now available, however, special fertilisers in a form that can be absorbed by plants despite alkaline conditions resulting from soil or water. These are known as ‘chelated’ preparations. Plants known to be lime hating, such as azaleas, many primulas, citrus, and many ericas, should be potted in a special ericaceous (lime-free) potting compost available from good garden shops.

Can you give me a basic guide to feeding greenhouse plants?

Crude animal manures must never be used: they would introduce all manner of pests and diseases. All properly prepared composts contain enough nutrients in the right proportions to take the plants to an advanced stage. Further feeding can be done using a proprietary balanced feed—that is, the right proportions of nitrogen, phosphorous, and potassium (N, P, and K), and trace elements such as iron, manganese, and other minerals. Soluble feeds that can be dissolved in water and applied when watering are the most efficient, but slow-dissolving feeding tablets that are pressed into the compost within the pot are also useful. An important time to feed is when flower buds are just beginning to form. Do not feed dormant plants, and try to regulate the degree of feeding according to the vigour and growth of the species or variety. Over-feeding, like over-watering, is harmful; and do not feed plants when they are dry at the roots. The special proprietary foliar feeds, applied to the leaves by spraying, are also effective on greenhouse plants.

Is pest control more difficult in the greenhouse?

No—it is considerably easier. There are now a number of modern pesticides that control almost all the common pests without the necessity of having a ‘medicine chest’ of chemicals. Most of these work by systemic action and remain effective for a long period on plants protected from the weather. Fumigants (in the form of aerosol sprays or ‘firework’-like canisters) are also an efficient way to control many greenhouse pests, as well as moulds and mildews.

Remember that pesticides and fungicides are poisons. Always follow carefully the manufacturers’ instructions for their use, wash thoroughly any equipment after use, and keep the containers out of reach of children and pets.

What is sterilisation and how should it be carried out?

In the greenhouse this means the destruction of all living things that could jeopardise the culture of the plants. It must, of course, be done when the greenhouse is empty. The safest method of sterilising the home greenhouse is to use one of the proprietary preparations based on cresol and to follow the instructions on the label exactly. Usually the greenhouse has to be left empty for some weeks after treatment, so planning of the operation, especially with regard to plant accommodation, is necessary. If possible the greenhouse interior structure and floor should be sterilised at the beginning of each annual growing period; early February is a good time. Seed and potting composts are also ‘sterilised’ and are in this condition when bought as proprietary brands or as approved John Innes types, so always seal bags of unused composts.

Can you give me any hints to help me ensure that my plants survive the winter?

See that their minimum temperature requirement is maintained at all times, and be very cautious when watering, especially on the colder days: plants on the dry side are more likely to survive the cold. Plants not making any active growth or in the dormant state should not be watered or fed. Ventilate when the outside temperature allows it, so as to get the air moving freely; this discourages mould or mildew.

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