Growing under glass provides a protected environment ideal for raising seedlings, overwintering tender plants, growing crops such as tomatoes, or even cultivating plants that need protection year-round. Greenhouses allow the gardener to extend the growing season, sow plants earlier and provide the ideal place for rooting cuttings.
Ideally, greenhouses should be sited where they can receive uninterrupted sun throughout the day. Provide screening or shelter from cold northerly or easterly winds, which can keep temperatures low in spring and slow the growth of seedlings and young plants.
An east-west orientation will slightly extend light levels during winter. A north-south orientation is suited to summer crops such as tomatoes, with both sides receiving several hours’ sun from the east and the west. With this orientation, the end timbers will reduce the amount of sun reaching the house during the hottest midday period. Decide when you plan to use your greenhouse most, and orient it accordingly if you can.
Aluminium is usually the material of choice for a glasshouse, whether in natural metal or with a painted finish. A coloured frame may fit into the garden better, especially if the structure can be seen from the home. Aluminium needs no upkeep, and the glazing bars are thin, casting little shade.
Wood is an attractive, traditional building material and better-suited to some garden styles. However, it needs periodic upkeep unless you specify more expensive and lower maintenance cedar-wood timber. Wooden frames tend to be bulkier than aluminium and can cast excessive shade inside the greenhouse.
The taller a glasshouse is at the eaves, the better the light transmission and therefore the wider the range of plants that may be grown. The eaves should be at least 1.5m (5ft) tall, and ideally 1.8m (6ft) or more.
The ridge should be at least 60cm (2ft) above the eaves to allow a door that gives easy access, sufficient slope to shed rain and to encourage loss of excess heat.
Glazing to ground level gives the greatest flexibility, but half-walling, using masonry or timber will reduce heat loss.
Domes and other odd-shaped glasshouses can prove more difficult to manage, especially when trying to ventilate efficiently, and they tend to be more expensive than the traditional shape.
It is important to ensure a greenhouse has sufficient ventilation. Roof vents are the most useful, and ideally should be on both sides of the ridge and equivalent to 15-20 percent of the floor area. Side vents are no substitute for roof ventilation, and while louvred vents allow regulation of air flow, they are hard to draught-proof in winter.
Automatic openers that open or close in response to greenhouse temperatures are useful, but slow to respond and need supplementing with manual control, such as opening some windows and the door each morning. Motorised vents activated by sensitive heat sensors are more efficient, but may be too costly for home use.