A greenhouse can be a benefit in most gardens, they can extend the growing period, enable you to propagate plants, allow good temperature control (where heating is provided) etc.
Before considering the greenhouse, you need to consider the site.
The site needs to be fairly open to take advantage of natural sunlight - when choosing a greenhouse during summer, remember that the sun will be lower in the sky during the winter, so the greenhouse could be obscured by other structures surrounding the site.
In addition it is essential that the ground is free water draining especially if it is intended to put plants into the ground within the greenhouse.
Air drainage is also important, avoid low areas where cold, damp air can collect and stagnate.
On very exposed sites, considerations may also need to be given to possible wind damage to the greenhouse. The options are basically to find as sheltered area as possible, or to choose a model of greenhouse which is robust.
The main features to look at when looking for a greenhouse are:
If you have used a greenhouse before you will have a general idea of the size of greenhouse you need, or the size may be limited by the space you have to build it.
If you've no idea of the size required, you will need to consider that you intend to use the greenhouse for; growing plants in the soil, grow-bags or pots, propagation of plants from seeds or cuttings on staging, don't forget some storage space for your tools, watering can etc. Lay out the area for each 'activity' on the lawn or patio to get a physical representation which you can measure - don't forget to allow a path to walk along to reach the back of the greenhouse.
The larger the greenhouse, generally the lower cost per square metre, so buy as large a greenhouse you can afford rather than buy a small one with the idea that you can buy another one if the original one becomes too small - but remember that there are sometimes reasons why two greenhouses can be advantageous.
There is an advantage in going for a wider greenhouse rather than a longer one, this means that the walkway will be smaller as a proportion of the floor area and the usable area larger.
As a rule of thumb, once you've decided on the size of greenhouse you want, go for one size larger.
One aspect easily overlooked is headroom, you may be spending many happy hours in your greenhouse so you want to be able to stand upright. A lot of makes of greenhouses offer a base (either included or as an additional option) which can raise the greenhouse up by 150 to 230mm (6 to 9 inches), but beware that those models including a base may in fact not be any taller, the area of glazing may just be less. These bases also have the advantage of providing a fairly rigid base which will prevent aluminium greenhouses twisting without the need for building a special base, their disadvantage is that the base forms a step-over across the door so wheelbarrows cannot pass through. Also, don't forget the height of the doorway, there is not much choice if you are very tall, but it's worth keeping in mind.
There are three types of glazing available in the UK (other countries have different standards), Horticultural glass, Toughened (or Safety) glass and Plastic.
is the cheapest and the traditional glazing. It is perfectly satisfactory but has the disadvantage that when it is broken, it breaks into sharp shards - it should be avoided if small children are likely to play near the greenhouse or the greenhouse is in a very exposed or public area.
is more expensive that horticultural glass but is less likely to be broken and, if it is, breaks into relatively safe pieces.
is usually impossible to break accidentally but has the disadvantage that it cuts down the light which can pass through it.
Some greenhouses models are available with options of which glazing type is fitted, other greenhouses are not.
Half glazed greenhouse are available (with solid panels up to about 600mm (24 inches) all around), these greenhouses are obviously less likely to suffer damage near the ground, but are not practical if ground level planting is required and staging is normally required.
Various shapes are available, but the two most common are the Apex style and the Lean-to.
The Apex style normally has the door in the middle at one end, some large Apex style greenhouses have double doors at one end. On single door models, the door can usually be hung either way round to suit during assembly. The 'path' within Apex style greenhouses is normally under the roof ridge, with growing areas and/or staging on either/both sides. Being free standing, a Apex style greenhouse can be positioned almost anywhere and in the orientation to best suit the site.
Lean-to greenhouses (as discussed here) are not the same as conservatories; they do not normally have access to the interior of a house nor double glazing. A Lean-to greenhouse is built against a wall, normally of the house, and tend to cost less per square metre. Advantage of fitting a lean to greenhouse include:
One disadvantage of Lean-to greenhouses is that the orientation of the wall dictates the orientation of the greenhouse, this may be ideal or not, but there is not much you can do about it.
Traditionally greenhouses were built using timber, however aluminium is now commonly used. Wooden greenhouses do tend to heat up and cool down slower than aluminium ones, some people may also say that they look better, however coloured plastic coated aluminium greenhouse are available so the visual difference is not so great.
Modern aluminium does not suffer from the corrosion experienced on earlier aluminium models and is now effectively maintenance free.
Most timber greenhouses are made from a type of red cedar - these are fairly durable and strong, they do however need some form of protective treatment every couple of years.
Adequate ventilation is essential in any greenhouse to keep the air moving on hot days. As a rule of thumb, the area of opening glazing needs to be about one fifth (20%) of the floor area of the greenhouse. Most 'standard' greenhouses do not come with sufficient vents but most makes offer additional vents as additional options. Two types of ventilation are common:
The door should not be considered as normal ventilation, with cats, birds and pests around the garden, an open door is an open invitation.
Except with very small greenhouses, at least two openings are essential, ideally positioned diagonally to give even ventilation throughout the greenhouse. For flexibility, aim to have one side louver for every two roof vents.
Some models of greenhouse come with guttering, this is useful for collecting rain water in a water butt or barrel for use around the garden. It is well worth the money fitting guttering to other greenhouses (if practical).
If you intend to pot up or need an area to work on within the greenhouse, staging along at least one side is essential. Where staging is only required on one side, position it on the side away from the sun (i.e. on the north side in the UK), this will allow the sunny side to be planted. Staging may be available as an option with the greenhouse you decide to buy but are also available as separate items. Make sure that the stage purchase is robust as there will be times when you will want to rest something heavy on it.
Both slatted and solid staging is available, slatted has the advantage that it does not affect air circulation too much, solid greenhouse staging has the advantage that it is easier to clean and sterilise. When using solid stage, leave a gap of about 150mm (6 inches) between the back/ends of the staging and the greenhouse glazing to allow free air movement.