Where to Position a Greenhouse
Site Selection and Preparation
Some climatic factors in greenhouse siting
Careful consideration of terrain and climate is important in siting the commercial greenhouse if it is to be economically viable in face of competition from more favoured countries. The non-professional greenhouse gardener, though he may have no choice but to stay where he is, ought at least to be aware of basic climatic principles and their significance.
The nearer a country is to the equator the nearer it is to the sun; the farther one journeys away from the equator, broadly speaking, the colder it gets.
Effect of large areas of water
Water, like glass, has reflective properties and large bulks of it add to the total amount of illumination available.
The thermal warmth of large areas of water is also of great significance, as heat is stored in the water and radiated back to the air. The influence of the Gulf Stream or Mid-Atlantic Drift on the whole western and southern coast of Britain is considerable in the raising of air temperatures and the reduction of winter frost.
The prevailing wind for the whole of Britain is a south-westerly, coming from warm areas, and this, combined with the Gulf Stream, makes the whole western part of the country milder and wetter than the east.
Areas near large bulks of water and areas of high rainfall have high humidity; so do densely planted areas due to transpiration of water through the leaves of the trees. A high moisture content in the air not only affects the growth of plants but also the incidence of disease.
Many gardeners erect a greenhouse and expect it to be (in terms of horticultural capabilities) the exact image of any other greenhouse. Such is not the case. An accurate assessment of weather patterns is an invaluable exercise which should under no circumstances be neglected, the paramount issues most certainly being light intensities and exposure.
While a dwelling house could conceivably be designed around a well-sited greenhouse or conservatory, it is more usual to design a dwelling house primarily for the living facilities it offers, and to relegate the greenhouse or conservatory to a position of less importance. There is, however, much to be said for incorporating the greenhouse or conservatory as a special feature in the home, both for aesthetic and practical reasons. It offers a pleasant adjunct to the home and is also an economical way of greenhouse gardening since the heating can be linked to the domestic system.
These considerations, however, do not affect the fundamental importance of siting the structure where it will receive good light, though obviously if a conservatory is to be used mainly for leisure and pleasure one will wish to avoid having the temperature raised to an unacceptable level by the midday sun and will prefer a west orientation enjoying the evening sun.
Good light is highly desirable for many horticultural activities, especially the propagation and growing of light-loving plants during the winter months. It is therefore elementary to choose a situation for the greenhouse which is as ideal as possible for light reception, especially during the winter months. A site which lies in brilliant sunshine most of the day from March to October may be in real gloom for most of the winter when light is at a premium. Check with a sighting angle. This will indicate whether buildings or dense trees are likely to be a real problem.
Is shelter necessary?
Questions of light and shelter are obviously closely related. A greenhouse can be erected in a garden closely surrounded by tall hedges or fences and enjoy the very best of shelter yet the poorest of light as a result.
It is in some locations impossible to garden effectively or have a greenhouse without shelter.
In many gardens shelter is in fact of no real consequence, as the whole general area does not have an exposure problem. There is no better natural guide to this than the ‘shaping’ of trees in the neighbourhood — better even than the advice of local gardeners, though this should not be discounted.
Wind can play many unpredictable tricks by bouncing off building and hills or creating areas of extreme turbulence on the lee side of walls and fences.
Providing shelter is usually more concerned with the garden as a whole than specifically for a greenhouse. The same general rules apply for both.
Solid wind barriers are now frowned upon. They do not ‘stop’ the wind at all. All they do is deflect it upwards, to descend later with some force, sometimes creating a considerable degree of turbulence immediately behind the barrier. All the evidence of recent research shows clearly that media about 50% permeable are ideal for slowing the speed of the winds, thus avoiding both deflection and turbulence.
Windbreaks are effective for a distance of approximately ten to twelve times the height of the barrier itself, which means that in practical terms a hedge 2.4m (8ft) tall will provide shelter for 24-30m (80 — 100ft). The area of maximum shelter is in the region of five to six times the barrier height, or 12-15m (40-50ft) in the case of a hedge 2.4m (8ft) high, a distance which raises no shade problems. A tall hedge of densely planted poplars, on the other hand, would provide excellent protection for a large area but could pose considerable shade problems in summer.
The directional placement of the shelter in relation to the greenhouse will depend, as we have said, on the prevailing wind arid aspect.
Roots of trees or hedges situated adjacent to a greenhouse can be a problem particularly where the greenhouse border is used for crops, taking away moisture and nutrients and also disturbing foundations, poplars being notorious in this respect.
Water shed from the roof of a greenhouse is best collected in a gutter and conveyed by a downpipe to a drain. Withof newer design and smaller size, rainwater is frequently allowed to run off into the , but only if natural is sufficiently good, otherwise a rubble, tile or plastic drain must be run along the outside of the greenhouse to collect this rainwater and prevent seepage into the greenhouse. Drains should always have an effective outlet.
Site levelling and soil conditions
Since the physical and nutritional characteristics of soil and the whole question of soil analysis will be examined in detail later, it is sufficient at this stage merely to draw attention to the importance of having a good soil, of even depth and good drainage, when growing plants in the greenhouse border. This matter is often airily dismissed as being of little significance in the small amateur greenhouse, yet in fact even a moderately sized structure 5.4 x 3.6m (18 x 12ft) will require 20m2 (24 sq yd) of soil 30 cm (1ft) deep or, to put it another way, 6m3 (8cu yd) of soil — not the sort of soil bulk readily come across or physically easy to handle. For a benched greenhouse or for container or bed systems of culture, the importance of good soil diminishes. The popularity of growbags today is a typical instance where soil quality in borders is of no concern, although drainage is still important.
Every attempt should be made to preserve the top layer of fertile soil and to replace it after levelling. Failure to do this will result in an intermingling of poor quality subsoil and top soil, with decidedly deleterious effects on any border grown crops. Soil in its natural state has a porous structure due to the activity of micro-organisms, earthworms, and the action of plant roots. Subject this to pressure with heavy mechanical levelling equipment and much of the natural structure is destroyed by the breaking down of porous crumb formation and the compression of air and moisture channels.
Levelling methods currently used are as follows:
- Cut and fill. After stripping off the top soil, the difference in level is made up by splitting the difference between the lower and higher levels by movement of subsoil, avoiding compaction. The top soil is replaced carefully. A banking is left and a retaining wall with a catchment drain may be necessary.
- Levelling to lowest point. In this method the top soil is first removed and then the sub-soil is excavated to allow a level to be made at the lowest point. This will undoubtedly result in a much steeper banking than with the ‘cut and fill’ method, and this can give rise to proportionately greater drainage problems. Alternatively a retaining wall with a drain behind it may be necessary.
- Levelling to highest point. In this method, soil or ballast is used to bring up the level to the highest point. The advantages of this system are roughly counterbalanced by its disadvantages. While it avoids disturbing the existing soil structure it does, on the other hand, leave the greenhouse sitting proud of the natural levels and this can lead to over-effective drainage.
With all the above methods, a suitable period of time must be allowed for subsidence before the building of the greenhouse can commence. Alternatively, deeper foundations which allow contact with existing soil levels can be made, though this of course adds to the erection cost.
The disturbance of soil structure is of less importance where a slab floor is to be laid down and the greenhouse permanently benched, or where growbags or similar growing systems are to be employed. There should, however, still be adequate provision for drainage, and sufficient time for subsidence should be allowed before building.
Little account is usually taken of weeds when building a greenhouse, yet they can be a serious nuisance. There are several highly obnoxious weeds such as Polygonum cuspidatum (Japanese bamboo), Equisetum (horsetail), and Agropyron repens (couch grass) — to mention but a few — which are very difficult to control. If the site is infested with weeds of this character, mere physical levelling or the building of foundations will do nothing to control them. The presence of bad weeds in the periphery of a greenhouse will always be a hazard and a nuisance.
Many weed-controlling chemicals are extremely potent, but when used out of doors they are in time leached out by the rain; this will not be the case in a greenhouse so obviously it would be a much better practice to carry out a general weed-control programme some time before building the greenhouse, treating a sufficiently wide area to prevent the weeds from rapidly recolonizing the greenhouse site. It is wise to consult official literature on the subject, strictly following instructions issued with the chemicals.
Supplies of water and electricity are essential for the modern greenhouse. The volume and pressure of water required will depend on a number of circumstances best taken up with the appropriate water board. Normally a 2cm (¾ in) supply pipe will suffice for small greenhouses on an amateur scale, although if spraylines and mist irrigation are to be used then a larger pipe is desirable, it being remembered that the smaller the bore of the pipe, the greater the friction, which reduces pressure. Gardeners frequently make temporary connections to a tap at the rear of the dwelling house or garage, using long lengths of small-bore hose, and wonder why they get only a trickle at the greenhouse. This is of course the result of friction.
When permanent pipes are laid into a greenhouse they must be at sufficient depth to avoid freezing. The take-off from a main must be approved by the water board (who will usually make a charge) and invariably must be connected by a qualified plumber.
A compromise arrangement using a semi-permanent supply pipeline underground, but with a detachable connection to a tap, may be favoured and this may avoid the charges and complications of a permanent supply. Note that there should be facilities for draining this pipe in winter, especially if it is of steel. More usually these days pipes are of alkathene, and this tough material is not so badly affected by frost, but it can nevertheless be damaged.
The methods used to distribute the water within the greenhouse can vary considerably, problems of freezing being of importance once again. On the other hand, a deeply laid pipe and a standpipe in a correctly built and well-heated greenhouse will present few problems.
The load necessary for a greenhouse will depend on its size and the range of equipment to be used. If an all-electric greenhouse is planned, the load can be quite heavy. The average small greenhouse of 3 x 2.4m (10 x 8ft) will normally require a 2-1/2kW heater, plus lighting, fan ventilation and possibly soil warming, adding up to a total of 3-4kW. Qualified installation with waterproof fittings is essential.
There should always be ready, unobstructed access to any greenhouse, preferably a hard path of sufficient width to allow the use of a trolley or barrow. Tool stores, potting sheds and supplies of soil should also be in reasonable proximity. There is a lot to be said for a greenhouse being attached to or as near the dwelling-house as possible, for the obvious reason that walking to and fro takes time and can be wet and unpleasant in bad weather.
It is usually wise to seek planning permission before embarking on any work connected with the erection of the greenhouse. For commercial greenhouses under a certain size and over a permitted distance from a main road, planning permission may be completely unnecessary, but it is always advisable to check the situation with the authority or council involved. For amateur greenhouses under a certain size, planning permission may also not be needed, but here again it is better to make sure by approaching the planning body. Procedure differs under different authorities, but in most areas when making planning application it is obligatory to provide:
- A location plan showing the exact siting of the greenhouse in relation to home, roads, and neighbouring buildings on a plan taken from the ordnance map of the area. The siting of the greenhouse can then be drawn in accurately and to scale in red ink, making sure that there is strict adherence to boundary lines (information on this should be requested beforehand from the planning body, and you should find out at the same time whether there are any local building regulations about the type of greenhouse allowed).
- Full details of the greenhouse, its foundations (if any), side and end elevations to scale, details of materials and dimensions of the main structural members. Most greenhouse manufacturers have the necessary plans ready and in a form acceptable to the planning authority. Do-it-yourself enthusiasts could draw plans for themselves.
- Notes on any drainage necessary, especially if this involves linking up to an existing drainage system.
- A completed application form, with the signatures of adjoining neighbours or their landlords or ground superiors certifying that they have no objection to the erection of the greenhouse. Permission may also be necessary from your own landlord or ground superior in the case of lets or feus, unless of course the ground is self-owned. A point of some interest is that where there is a building society involved with either land or house or both, they certainly must be told of your plans, especially if the greenhouse is to be attached to the house.
Regulations for erecting greenhouses, conservatories, porches and home extensions generally have been relaxed in most areas in recent years — but it is always better to check and be safe rather than sorry!
The erection of small plastic greenhouses, frames or cloches requires no planning permission. With conservatories or garden rooms the same general procedure apply as for greenhouses and it is essential to conform with local planning regulations, especially building control.