Supersize It
Clare Foggett on how the greater capacity of a full-size greenhouse increases the possibilities for overwintering plants
While coldframes are very useful, if you are a serious grower then there is nothing quite like a fullsize greenhouse. These allow you to overwinter tall plants and offer room for the gardener to enter – albeit with diffculty when the space is crammed with tender treasures!
Compared with using a coldframe, there is less stooping and a greenhouse can give you the opportunity to potter among your protégés when the weather is grim. Furthermore, the greater capacity of a greenhouse means that temperature control is easier since there is usually the option to add electricity and a thermostatically controlled heater.
Even if you don’t heat your greenhouse over winter, just by virtue of being enclosed in glass its interior will be a degree or two warmer than outside. That may not sound like much, but it could be all you need to nurse borderline hardy plants through the colder months – many need only a minimum of 3°C to tick over as long as they are kept dry. And it also enables you to do much more: an unheated greenhouse can be used to prepare winter blooms for the house, start off seeds for next year, propagate plants and grow winter crops. Use heating to boost the temperature and even more possibilities open up.
Winter should start with a clean greenhouse. Remove summer shading and wash the glass until it sparkles – maximising light is essential for healthy plant growth over winter. If the prospect of a bucket of soapy water and hours of elbow grease engenders feelings of dread, greenhouse supplier Hartley Botanic can provide self-cleaning glass, while some suppliers offer a cleaning service. Winter is also the time to have any broken panes of glass replaced and to repair cracks or damaged seals.
As the temperature begins to drop, start bringing plants inside. Thoroughly check pot plants before
putting them in the greenhouse, removing dead leaves, old flower stalks and lurking pests. Check for root damage courtesy of vine weevil grubs and ensure plants are otherwise healthy. There’s little point expending effort and potentially costly heating to cosset plants that aren’t worth it.
To give tender plants the best chance of surviving winter, the temperature really needs to be at least 7°C and ideally 10°C. In the UK that means heating. All forms of heating cost money, so evaluate whether it will be worth it: a prized collection of rare orchids may warrant the expenditure, whereas a few bedding plants will be cheaper to buy again next year. Many gardeners prefer to take a chance, overwintering plants in an unheated greenhouse and taking a que será será attitude to any losses.
If you do heat your greenhouse, keep bills as low as possible by insulating with bubble wrap. In a larger greenhouse, you can use a bubble wrap screen to portion off the smallest possible area where heat is required rather than heating the entire space. Hartley Botanic has a helpful calculator on its website for working out, roughly, how much power will be required to heat a greenhouse depending on its size.
A thermostatically controlled electric fan heater is the best option for heating, although the greenhouse will then need mains electricity. When adding a new greenhouse, always consider whether electricity can be installed at the same time – it’s just as valuable for lighting as it is for heating and is rarely regretted. The advantage electricity has over fuels such as gas or paraffn is that it’s clean and doesn’t emit water, soot or carbon monoxide, all of which can adversely affect plants. Gas heaters can be heavy and bulky, while paraffn heaters are usually small and lightweight and, as a result, all too easy to knock over – a potential fire hazard that you may prefer not to risk.
With a minimum temperature of 7-10°C, plants like pelargoniums will continue to grow over winter, albeit more slowly due to reduced light levels. If they are growing, watering will be necessary, but be guided by the plant and never overwater: let the compost dry out between sessions. In an unheated greenhouse where plants are all but dormant over winter, keep compost on the dry side. Often, if the temperature dips below freezing for a short spell, dry roots could just save a half-hardy plant’s life.
A cold, damp atmosphere inside can also sound the death knell for many plants, so, whenever the weather is sunny, open vents and doors to let fresh air in and allow condensation to evaporate away.