The coming-of-age of a truly passionate gardener is the day their greenhouse is constructed in the garden. Lots of people are happy to potter along growing crops with the seasons, but fanatical gardeners like to push the boundaries, growing crops that would otherwise struggle to grow in their environment or to get a jump start on the season. Of course, a not inconsequential side-benefit is being able to sit in near-tropical warmth and enjoy your plants while the outside world is chilly and wet.
You can gain an advantage of at least a month at the start of the season by using an unheated greenhouse. Another month or more can be tacked on to the end of the season for cold sensitive crops like beans and tomatoes. So, instead of tomatoes from December to April, you may be eating tomatoes from November to June. A capable gardener, with good disease control and the right varieties, can even have tomatoes all year round, although woe betide if you take your eye off the crop for more than a few days!
Each style of house, type of framing and covering material has its advantages and disadvantages. You can make a greenhouse from glass, polythene or twinwall polycarbonate sheeting (this latter option is increasingly preferred by home gardeners). For those on a tight budget, old wooden windows with their sashes still attached can often be picked up for free or very cheap from house demolitions or renovations. I’ve seen some very stylish and effective greenhouses made this way.
Glass has the best light transmission but can be a safety issue particularly with children. Plants can also scorch more easily under glass on sunny days. If buying new, glass is also the most expensive option, but does last the longest. Polythene has less light transmission than glass, but also less chance of scorching. It is the least durable of the covering materials but also by far the least expensive. Twin-wall polycarbonate has the best insulation properties and although it doesn’t have the light transmission quality of glass, its high diffusion rating means less shading is needed in summer to avoid scorching. It is midway between the other two for cost, but has good longevity if a quality, UV protected brand is used.
A double-glazed greenhouse is much more efficient at holding heat than single layers, which is where the twin-wall polycarbonate excels. This effect can also be achieved by using two separate skins of polythene, two layers of glass or even a layer of polythene inside the glass. If possible, get a larger greenhouse rather than a smaller one, as the more air volume in a greenhouse, the more stable the environment; and this is better for your plants.
One critical aspect of greenhouse design is sufficient ventilation. Poor ventilation dramatically increases the amount of leaf diseases and stresses the plants by letting temperatures climb too high. A good rule of thumb for working out the amount of windows and vents you need is that if you take the dimensions of all of the openings at their widest point, they should add up to at least 30% of the floor space. Most summer greenhouse vegetables can take up to 32°C without too much stress so this amount of ventilation should be enough to prevent your greenhouse getting too hot until the middle of summer, when some shading might be needed.
If you are a truly fanatical gardener, you could also look at heating your greenhouse. My tropical greenhouse is currently running five heatpads and, as a completely addicted gardener, I even monitor these via wi-fi while on holiday. Well, that’s assuming the power and internet haven’t gone down in yet another storm!