A greenhouse is a great addition to your garden, it can extend your gardening year and help produce high yields. You can start seed sowing early and protect plants from frost at either end of the growing season. You can also propagate your favorite plants, relying on good temperature control which ensures consistent growth during the growing season, and over winter any tender plants.
Whichever model you choose...
It cannot be stressed enough that the creation of a secure and level foundation is the most important first step when erecting a greenhouse, whether using a purchased base or making your own. A good base is also crucial for the long term use and life of your greenhouse. Any unevenness in the foundation will make greenhouse assembly troublesome. It may also cause stresses in the greenhouse frame and, over time, the results could be that glazing cracks or falls out, or that doors and windows get stiff to open and close. Time taken now in getting the base right is well spent.
There are two basic options for greenhouse shape: Free Standing or Lean-To, which is fixed to an existing wall. Free standing greenhouses come in a huge range of sizes to suit every budget. Most are apex style like a miniature house, but there are octagonal or decagonal options too. In contrast, Lean-To greenhouses have to be fixed onto a wall, however its design has the added benefit of protection and warmth from the wall it is built against. The location of the wall will probably dictate the position of this type of greenhouse, but it may also be that water and electricity supplies are nearby and therefore cheap to access.
The final location of your greenhouse is very important. Do a quick survey of your garden and consider the following:
Is your garden in an exposed position? A greenhouse can be vulnerable to strong winds. If so, any hedges or fences could possibly provide shelter from the prevailing wind. It is also a good idea not to position the greenhouse under any trees, as apart from the possible shading effect from leaves in the summer and fallen leaves in the winter, any storm damage to the tree can result in damage to the greenhouse from falling branches etc.
The ideal site needs to offer protection from the wind whilst still allowing exposure to natural sunlight - do remember that the sun will be lower in the sky during the winter, so the greenhouse could be shadowed by large nearby structures or trees. Also try to find a spot that is free water draining, especially if you want to plant directly into the ground inside the greenhouse. It is also worth considering that cold damp air can collect in low spots which will affect the performance of the greenhouse.
Positioning and maintenance
Allow at least 1m around all sides of the greenhouse, as it will be necessary for erection of roof panels, and any cleaning or repair work. Some greenhouses require the roof panels to be slid into place once the rest of the frame is erected, ensuring there is room to do so is vital and prevents complication during installation.
Ideally the greenhouse should be positioned so that its longest side is fully exposed to the sun from the south (but don't put any staging along this side, or the plants will burn). Orientation is more important with long narrow greenhouses; with almost square greenhouses such as the 6' x 8', an east or west orientation makes hardly any difference. Just make sure that the entrance is not north facing as it will let in too much cold air.
If you can see the greenhouse from your house this may affect the shape, materials or glazing type you choose as you will want the most pleasing appearance. What it looks like will affect your choice anyway, but so much more if you see it every time you look out of the window.
To choose the right size, you will need to consider what you intend to use the greenhouse for; growing plants directly in the soil, grow-bags or pots, or using staging for seeds or plant propagation. Also allow for some storage space for your tools, gloves, watering can etc. The surest way to choose the right size is to mock up your planned use with sticks, canes or cardboard boxes on the lawn, that way you can get a real feel for the space and can measure it out. Remember to allow for a path down the middle. If you intend to grow from seed, for example, allow for the fact that when you pot on from your original seed trays, you may be looking at anything from 10 to 100 pots per seed tray. The larger the greenhouse, generally the lower cost per sq metre, and the cost per sq metre of growing therefore, so buy as large a greenhouse you can afford. As a general rule, once you've measured the size you think you'll need, choose one a size larger! It is usually a better use of space to buy a wider greenhouse shape rather than a longer one; it means that the central path uses a smaller proportion of the floor area. Headroom is important as you may be spending hours in your greenhouse so you want to be able to stand upright.
Greenhouse frames are usually made from either wood or aluminium, painted, stained or plain, and the glazing can be either glass or plastic.
Aluminium greenhouses are either plain or painted. Both types have thin bars and so afford very little shading on the plants inside. They heat up and cool down quicker than the wooden type and are generally cheaper to purchase. The quality of this type of greenhouse has improved over the years so that they now no longer require regular maintanance and do not corrode. Plain aluminium frames won't stay shiny; they will eventually oxidise harmlessly to some extent and will look whitish. Powder coated aluminium can come in a variety of colours, the most popular of which are black, cream and green.
Wood is traditional and suits most types of garden. It may be that being able to see the greenhouse from the house will incline you to this choice. The wooden greenhouse is usually made from Cedar and is naturally protected for several years by its own oils. However, it will need a protective treatment every couple of years to keep it in good order. Wooden greenhouses tend to be warmer and also therefore cheaper to heat in the winter. They lend themselves very easily to insulation as materials like bubble wrap can be stapled directly to the wooden frame.
Glass transmits 90% of available light and does not degrade over time. Horticultural glass is traditional, it is thin and breakable but easy to replace and quite cheap. Toughened glass is not easy to break, and if it does, it breaks into small pieces that will not cause safety problems to children or animals, but it costs more than the horticultural glass.
Twin-walled Polycarbonate diffuses and transmits 83% of available light, is not easy to break and retains the heat better. It can, however, become filled with a greenish film after several years that is not easy to clean. Acrylic plastic transmits 85% of available light, but is more brittle than polycarbonate and can sometimes crack during installation.
You will only realise the importance of this once you start to use your greenhouse. Ideally, roof openings on either side and diagonally positioned will work best. Approximately 15-20% of the total greenhouse's floor area should be open-able. This is not including the door, and roof vents are generally preferable to side louvers.