Gardening – Greenhouse growing

Lots of people grow vegetables, happy to potter along growing crops with the seasons. Fanatical gardeners though 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. You can gain an advantage of at least a month at the start of the season and more than a month at the end of the season by using an unheated greenhouse, or more if you can heat it (even though the cost of heating is probably not cost-effective). So instead of tomatoes from December to April, you may be eating tomatoes from November to June.

You can make a greenhouse from glass, polythene or twin-wall polycarbonate sheeting. The latter option is increasingly preferred by home gardeners as it is lightweight, reasonably cost-effective and safe. A double-glazed greenhouse is much more efficient at holding heat than single layers, which is where the twin-wall polycarbonate excels.

Glass has the best light transmission and lasts the longest, but is costly, heavy (needing more structural support) and carries an inherent danger of injury. 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 people making renovations; connecting these together makes a lovely retro looking greenhouse. Polythene plastic film is the lightest and cheapest option, but it doesn’t have the permanence of the other two options and can be easily destroyed in a storm. Although it heats up well during the day, it does lose heat faster than glass or twinwall at night.

One critical area when choosing a design for your greenhouse 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. 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 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.

As we move into winter, make sure there are no gaps in the cladding, such as broken panes, badly fitting doors or windows that don’t close fully. Then look at the possibility of adding an extra layer of cladding. This is the same principle as double-glazing – the aim is to create a layer of air between the two layers. I’ve found the easiest way to achieve this is to run wires the length of the greenhouse at the peak, at the gables and near the floor. A sheet of clear polythene can then be hung over the wires, much like a tent. Sealing the joins with greenhouse tape adds to the insulation quality. The heat retention of this extra layer is nearly double that of a single layer cladding and well worth the effort.

Another useful tip is to cover the southern wall of the greenhouse with a solid material such as plywood or corflute. The light gain from the southern wall is minimal, but the heat loss can be quite high when a southerly wind is blowing, so this wall is better covered for increased insulation. A side-benefit of this snug environment is the pleasant working conditions for sun-deprived gardeners; much better than working outside in winter!