Selecting your structure
When selecting a greenhouse you are looking for a structure which will maximise the amount of light transmitted into the greenhouse and which will be best suited to the plants you wish to grow; eg well insulated for tender plants, good headroom for tall plants (and tall gardeners!) or low maintenance if you’re short on time.
There are four main choices you will need to make when purchasing your structure:
Type
There are many different types of structure you can consider. Some of the principle ones are:
Traditional
The traditional style of greenhouses is the same basic shape as most sheds, except that the walls and ceiling are see-through (although the lower portion or the wall may be brick or wood). They are spacious structures with the straight side walls providing good headroom and a vertical surface which is useful, for example, to attach tomato plant supports to. They are a deservedly popular good ‘all rounder’.
Dutch light
Similar to traditional style greenhouses, Dutch light glasshouses are designed to let in more light so the sides slope outwards. They are excellent for low growing plants, though can be a bit awkward for taller plants.
Three-quarter span
These are generally traditional in style, but set against a wall, so the wall forms one of the long sides of the structure. This gives the extra warming effect of the wall while retaining quite a lot of working space (certainly more than a lean to). It needs to be set against a sunny wall, however this means that additional shading is required in the summer.
Lean-to
Where space is limited, a lean to greenhouse is a good solution. They often resemble conservatories in design, only with more focus on letting in light. However, the smaller space and use of a wall as one side of the structure will mean that it gets very hot in summer, so an east or west facing wall is best as it doesn’t receive full sun all day. The advantage of this is that heat will be retained overnight due to the brick wall, so less heating will be needed.
Mansard
Also called ‘curvilinear’ greenhouses, a mansard has slanted sides, like a Dutch light greenhouse, but they are designed to give an overall curved appearance. By creating additional angles the structure maximises the light transmitted and is useful for plants which require as much light as possible, particularly in the winter months.
Dome shaped
As well as being visually appealing, dome shaped greenhouses are more stable as the curved sides offer less wind resistance and the many angles of the glass provides excellent light transmission. Headroom can be an issue around the edges and you may need to spend more money in order to purchase bespoke staging rather than off the shelf products.
Polygonal
Another attractive design, polygonal (often octagonal) greenhouses are constructed on the same principle as traditional greenhouses (with straight sides and a slanting roof) and therefore have good headroom and good light transmission. However it may be costly to replace broken panes.
Alpine house
These specialist constructions are used to house alpine plants which are hardy and require bright, well ventilated conditions. Generally constructed in the same basic shape as a traditional greenhouse, the length of the sides have louvre vents and all the roof panes open as separate windows. The windows and vents are only closed in the coldest weather.
Mini
These are small structures, often the size of a large wardrobe, which are placed against a wall or fence. They are a good, low cost choice if space is limited and/or you only need to put a few plants in it. Ventilation may be an issue and often they suffer from sudden changes in temperature, so shading in the summer is particularly important.
Polytunnel
Polytunnels are a low cost solution to provide a large amount of space. These tunnel like structures are covered with transparent plastic which will need replacing every few years. You have less control over the growing environment as they are unheated and ventilation may be an issue (without creating a wind tunnel), but they do provide a large growing area very cheaply. In the summer the plastic sheeting can be replaced with netting to provide additional light and ventilation. Generally they are used for growing in the soil rather than on staging. They are sometimes called ‘quonsets’ after the prefabricated huts which are a similar shape.
Doors
With the exception of polytunnels, there are two basic types of doors for glasshouse structures: sliding and hinged. Sliding doors are often considered easier as it is simple to leave them open at varying degrees to provide ventilation. However, they are more drafty than hinged doors which should close with a better fit.
Size
As a general rule you should always buy the largest size structure which you can fit in and afford. The space rarely goes to waste and if you purchase one that’s too small you will find yourself running out of space even if you didn’t expect to. Plus it’s easier to control the environmental conditions in a larger structure; there are fewer draughts and you don’t get the sudden temperature fluctuations of a smaller greenhouse. If you’re not sure you’re going to use it enough to justify the expense, why not start off with a mini greenhouse for a season before deciding to commit to a full size one?
For anything except a mini the smallest size practical is 2m wide by 2.5m long. If you’re intending to grow in the borders within the structure then you need to ensure that each border has at least a 1m depth, as well as the 2.5m length. If you need to access the structure in a wheelchair or with a wheelbarrow, then the central path will also need to be at least 2m wide to allow room.
Frame material
Wood
Often the preferred material for an attractive, traditional look, wooden frames retain warmth well but do have some significant downsides. They are often costly to purchase and heavy to construct. The thickness of the wood will reduce the amount of light entering the greenhouse. Always choose a hardwood which has been pressure treated, though even these will need treating with a preservative every couple of years.
Aluminium
This is a low maintenance, lightweight choice of frame which is usually very slim, maximising the light which is transmitted into the greenhouse. They do not retain heat as well as wood does, but this does not tend to be a significant disadvantage.
Steel
Steel has many of the same advantages as almumium, being lightweight and slim, and is usually a cheaper material. However, it will need painting regularly to prevent rust.
Plastic
Plastic frames are generally only found on mini greenhouses as they are not strong enough to support glass or larger structures. Plastic is thin and lightweight, making the greenhouse easily moveable and providing good light transmission.
Glazing material
Glass
As a general rule, glass is the best material for glazing.
Horticultural glass is generally used for greenhouses as it is thinner at around 3mm thick (therefore offering less insulation) and cheaper than normal glass. Glass transmits up to 91% of the light which hits it, making it the best material for light transmission. It is also the best insulator of heat. Smoked glass is available for hot climates and shading paints work well with glass for seasonal shelter.
Glass is simple to maintain as it cleans easily, doesn’t discolour and is long lasting. However, it is easily cracked or broken, heavy to handle and can be a hazard around young children or pets.
Polycarbonate
This clear and rigid material is lightweight and has less UV light degradation than plastic. It transmits up to 87% of light, which makes it a good choice if you have safety or handling concerns with glass. However it can be quite expensive and scratches more easily than glass.
Rigid acrylic/plastic
This can be single or double skinned sheets of rigid material. The light transmission is good (better with a single layer) as is heat insulation (better with a double layer). Ensure that the material is UV stabilised to reduce the degradation over time.
Fibre glass
This is a lightweight material which is easy to install. It is usually supplied in flat or corrugated sheets. However, over time it will deteriorate and become cloudy.
Plastic film (eg polythene)
This is generally only found on polytunnels and mini greenhouses. It is a cheap option which can be rolled up to provide additional ventilation and provides good light transmission when new (around 85%). It should be considered a temporary cladding as it degrades quickly (compared to the other materials) and will need to be replaced quite frequently. It is also susceptible to tearing and cannot be cleaned easily.
