Sensible Tips, Advice, and Suggestions for Building a Greenhouse
RATES AND LEGALITIES
Before erecting a greenhouse, indeed before buying, it is wise to check whether there would be any objections. Also, it is a matter of courtesy to see neighbors, if it is to be put near a boundary.
If the ground is rented, sometimes any structure erected could become the legal property of the landlord, so that it could not be taken away if you moved. If you are a freeholder, any deed of covenant could restrict building and should be consulted.
In fact there is very rarely any objection to the average small home greenhouse. Rate assessment is also unlikely—unless the structure is to be built on to a dwelling as, for example, a lean-to conservatory.
Greenhouses over 28.3 cu m (1000 cu ft) may be rateable, but buildings that can be regarded as ‘portable’, such as a kit form structure that can be taken down at any time, may not be. Your local authority is usually very helpful and will advise. When a rate is payable it is usually very modest.
Because it can have a dramatic influence on the amount of light reaching the greenhouse, the site may have a corresponding effect on what you will be able to grow. In the home garden the choice of site is often limited, but always look for plenty of light.
As already mentioned, it is easy enough to provide shade artificially w hen wanted, but light as powerful as daylight is a problem. If only a shaded place can be found, there is no need to despair since there are plenty of plants you can grow, but the choice will have to be adjusted to suit conditions.
It is foolish to attempt growing the light lovers, many of which rate high in popularity. (See this for details.*) A site near the dwelling house can be a definite advantage. There is easier access which will encourage more attention during inclement weather, and it is simpler and cheaper to run services like natural gas, water and electricity, which could all prove extremely helpful. It would also be easier to transport solid or liquid fuels.
At one time it was a common practice to hide away the greenhouse at the bottom of the garden. A well-constructed greenhouse need not be an eyesore—why not, when possible, make it a feature? In my own garden I have made two part of the ‘landscape’.
One, for example, used as a display house, is mounted on a raised area reached by stone steps and flanked by ornamental pots. The greenhouse flowers can be seen through the glass from outside and the general effect is quite decorative.
Situations to avoid are large trees in close vicinity that cast shade (particularly evergreens, which may exude gummy substances on to the glass, shed heavy cones or harbor diseases); hollow ground or the foot of slopes (which may get waterlogged or form frost pockets); and very windy and exposed places.
However, in the last case, protection can be given by a suitable windbreak, such as a low wall, fence or hedge, placed at a distance to reduce the wind speed but not cast shade. High velocity, cold wind can cause a serious loss of warmth from the greenhouse.
Choosing a greenhouse in relation to its surroundings has been discussed. Where there are problems I have found white framework seems to blend almost anywhere, whether the home or garden is “olde worlde” or “futuristic modern.”
The ideal orientation for the conventional rectangular greenhouse is, in my opinion, for the roof ridge to run east-west, or as near as possible. You then get maximum sunlight in winter and there is need to shade only the south side in summer.
The south side is useful for such crops as tomatoes grown in low containers if this side can be kept glass to ground. The north side, fitted with staging, will accommodate all the favorite ornamentals.
The word foundation often conjures up visions of laborious concrete mixing and heavy transportation, but for the average home greenhouse this is rarely necessary. However, it is extremely wise to take great care with the foundation.
Movement or subsidence after erection can cause serious problems difficult to correct. Most greenhouse suppliers give full recommendations for the foundation most suited to the design and size—make sure you follow them. Some firms supply ‘plinths’ or ‘curbing’ as a base at little extra cost, and this is well worth having.
More recently introduced are designs to make the laying of foundations very much easier. These use ‘ground anchors’, which are strips of metal with widely flared ends. The framework of the greenhouse can be fully erected on firm flat level ground.
At points along the base small holes are then dug and the ground anchors bolted on to the base so that one anchor, with the flared end at the bottom, is dipping into each hole. The holes are then filled with concrete. Only about one bucket of concrete should be needed for each hole. After the concrete has set, glazing can be carried out and the greenhouse completed.
When a conventional concrete foundation is required, it can usually be made by digging a shallow trench along the outline of the greenhouse base, and pouring in a very fluid mix. This will find its own level, especially if lightly tamped or agitated immediately after pouring. Provided fresh cement is employed, there is no need to fear that a liquid mix will not set.
In all cases, it is vital to use a spirit level for leveling the site, and again frequently during the framework construction. Also check all angles, verticals and horizontals, to get them as accurate as possible. Do not carry out erection on freshly dug or disturbed ground. Leave it to settle as long as possible and firm it well before commencing erection.
A brick base wall should always be given a proper concrete foundation and preferably also a damp course layer. The greenhouse suppliers always provide plans for any brickwork or concrete block construction needing to be done. Some people may have to get a professional builder to do this work, but for a small greenhouse it is often not beyond the capabilities of the DIY enthusiast.
A useful tip is to coat the finished brick or concrete wall with a modern proprietary water repellent both inside and out. This maintains the clean new look of the wall and discourages slime and algae. It also keeps it dry, which means it has maximum heat insulation.
PUTTING UP THE FRAME
Most small greenhouses can be erected single handed, but the help from another person may be helpful and reduce erection time. The instructions given by suppliers often look formidable, and some still persist in using ‘constructional engineering’ terms to describe components.
Fortunately the best firms usually also number each part making identification quite simple. Again the importance of frequent checking for correct angle and levels cannot be over emphasized.
When painting or any form of coating treatment is to be given to metal or timber, it should be done before the glass is put in. Red cedar houses are best given a coat of shellac varnish (knotting) before glazing if putty is to be used. This aids adhesion.
Where glazing tacks are employed, use a type made from galvanized iron, or preferably brass. Ordinary iron nails or tacks eventually rust and stain the glass. Raw (untreated) aluminum framework, and galvanized steel, can be painted with an undercoat and gloss paint if desired.
Aluminum takes and holds paint well. However painting will defeat the advantage of the no-maintenance property, since repainting will become necessary eventually.
The glass is usually cut to size when delivered. If you cannot use it immediately, store it out of the wet and where it cannot get soaked by rain. Wet panes stick together and can be quite difficult to separate. Choose dry weather for glazmg operation, and also avoid windy conditions. It is also best not to handle glass when it is very cold and the hands may become chilled.
Most greenhouse gardeners will find the practice of cutting glass useful at some time or other. Always use a steel wheel cutter obtainable from a builders’ merchant—avoid ‘fancy’ devices. Place the glass, which should be clean, on several sheets of newspaper.
Make sure the surface is absolutely flat. Measurement marks on the glass can be made with a chinagraph pencil. Oil the cutting wheel, and to guide it use a perfectly straight wooden batten held firmly. Also press firmly on the cutter and bring it towards you from the far end of the guide line.
A harsh hissing sound should be heard if the wheel is making a deep scratch on the glass. Avoid going over the same line twice. To break the glass along the scratch mark, turn it over. Then gently but firmly tap along the underneath of the scratch mark with the hammer’ end of the glass cutter.
A crack which follows the scratch mark will form and can be led along by tapping just in front. When the crack has traversed the entire length of the scratch, the glass should fall apart or can be gently pulled. This is a simple, accurate and easy way to divide glass, without risk of accidents.
If a very narrow edge of glass has to e removed, such as is often necessary when a pane is just a tiny bit too large, lever it off with one of the notches to be found at the end of the glass cutter. Lever away from the scratched surface.
VENTILATORS, DOORS AND GUTTERS
Extra care should be taken to see that ventilators and doors fit snugly. There must be no gaps to admit uncontrollable draught. Plenty of ventilators are an advantage. You do not have to have them all open at once. If they are well distributed, you can open them according to wind direction and adjust ventilation to give good air change, but not admit gusts damaging to the greenhouse contents.
I prefer to have roof vents as high up as possible and side vents low. Lightweight warm air passing out at the top will then draw the heavier, cool, fresh air m at the bottom, causing an efficient flow.
The most popular average size home greenhouse is about 2.5 X 3 m (8 x 10 ft) and this size should have at least two ventilators—one roof and one side. Larger houses should have ventilators in proportion. Two ventilators are best placed on opposite sides, but staggered so that air cannot merely pass straight through.
The conventional hinged vent with stay bar is still functional. For a greenhouse side adjacent to a pathway, the louvered vents, which do not open out so far, may be useful, but some tend to admit slight draught.
Sliding doors can be used as adjustable vents and need special care when fitting so that they slide easily and shut tightly. Take even more care in checking angles and levels. Some sliding doors are prone to let in draft, so ensure the design is a good-fitting one when buying.
Many greenhouses now have built-in gutters or guttering is supplied as an extra. A gutter helps to keep the soil around the greenhouse dry and prevent water seeping in. In winter this is a great advantage.
Dry conditions will help to conserve warmth (wet soil being a good heat conductor), and the lower humidity will discourage molds and mildews. Gutter water must not be used for watering plants in the greenhouse; this is important (see this post for details*).
Frames as Accessories
The frame as an important greenhouse adjunct should not be overlooked. Much greenhouse space is often wasted on plants that do not really need the height and could be grown for most of the time in frames.
If frames are employed for propagation and growing-on, more of the greenhouse can be devoted to displaying plants when they are in their decorative stages, or for plants that really need the height. It is also cheaper to heat a frame, and frame space is invaluable for the many plants that have to have a dormant or resting period.
As far as design and construction materials are concerned, much of what has already been outlined for greenhouses also applies. Lites (lights), as the ‘lids’ of frames are called, should for the home gardener be reasonably lightweight and easy to handle. The ‘roller frame’, with its lites set on rails so that they can be slid aside, is becoming more popular.
To help in the growing and temporary accommodation of greenhouse plants, shaded frames are usually more useful. It is therefore common to set them along the north side of an east-west orientated greenhouse.
Sometimes it is possible to arrange that the greenhouse heating system also passes some heat to the frames, but soil warming cables are very effective and convenient where there is electricity. The cables can be used much as described for ‘propagators’ this post*.
For using in conjunction with the greenhouse, the bottom of a frame is best covered with plastic sheeting to prevent the entry of soil pests, then with clean shingle to hold moisture and keep the frame air humid when necessary.
FRAMES IN THE GREENHOUSE
The value of frames inside the greenhouse is often not realized, or overlooked. A frame can form a ‘mini-environment’ for plants with special requirements—thus enabling the range of plant types you can grow under one roof to be greatly varied and extended. It can be used for many forcing jobs; for growing numerous winter vegetables that don’t demand much height; for producing the now popular seeds sprouts*, and for all kinds of propagation.
For these operations, the point of the frame is usually to provide a small area of elevated temperature which, with the internal site, is possible to maintain with little heat waste and therefore at low cost. In some cases, where the warmth demand is very low, the cost can be negligible. If you can only afford to have a frost-free greenhouse, but wish to grow a few tropicals, a warmed frame is the answer.
You could grow anything, provided it’s naturally low growing. The possibilities include many exotic house plants, which you can also propagate, and flowers like the flamboyant cattleyas and other compact orchids. At much lower temperatures you can have a constant source of winter salads like beet, carrot, salad onion, and others (see this article for more info*).