Keeping a greenhouse warm in winter ain’t easy. The thin skin of glass works beautifully in summer, ensuring high – sometimes too high – temperatures for tomatoes and peppers. But when our more tender plants need a snug, sheltered place, a greenhouse offers little or no protection against the cold.

In summer, the temperature inside a glasshouse quickly rises as it absorbs enough warmth from a strong sun to compensate for heat loss overnight. But the much weaker winter sun and long hours of darkness doesn’t do this. Any stored heat is quickly lost, especially when a cold, frosty night follows a clear sunny day. During wet, cloudy weather, the greenhouse stays cold during the day and loses a little overnight to a chillier outdoors.

So we need to store enough heat during the day to withstand overnight loss. This depends on the location and size of the greenhouse and whether it can be heated overnight.

Whatever the size, start by cleaning the greenhouse windows. Many of our greenhouses are fairly small and are in a sunny place some distance from the house. These present the biggest challenge because the low volume of air ensures the warmth will quickly dissipate overnight. The larger the greenhouse the more slowly it cools down.

A greenhouse built against a house is the best option. The wall acts almost like a storage heater, absorbing a little heat during the day and releasing it overnight. Mine is on the south side of the house, with the white wall also reflecting the sun’s rays into the greenhouse during the day. At night only 3 walls emit stored heat.

This lean-to greenhouse has electricity, letting me use soil-warming cables when needed and a thermostatically-controlled heater. It’s programmed to come on when it’s colder than 3C, so I pull the duvet more tightly over me when I hear the fan starting.

An electricity supply is obviously simplest, but this is impossible in many greenhouses and battery heaters aren’t, yet, available. For environmental reasons, I couldn’t recommend paraffin or bottled gas heaters, but some people may choose to use them.

But there are other ways of storing the day’s heat: insulating the greenhouse and arranging pots to ensure minimum heat loss.

Why not make your own storage heaters? Collect plastic bottles, preferably 5 litre, paint black, fill with water and place close to the glass. Black absorbs heat most efficiently, so water warms up during the day and is slowly released overnight. Clearly the more bottles you have, the more effective they’ll be. With enough inside space, you may have room for a water butt, so painted black, it could help a little.

Alternatively, you could collect bubble film from mail order packages and use it to cover the windows, thereby providing some insulation. Since this plastic is rarely collected for recycling, you’d be re-using it rather than consigning it to landfill. But you’d also reduce light levels and the benefits I’ve mentioned above.

This bubble film can also directly insulate plants. Pack pots closely together to reduce heat loss from individual containers and wrap bubble film round the perimeter. Sustainable alternatives include, strips of sheep wool, hessian and straw, whatever you have available. Pots laid on a soil bed lose much less heat than on a concrete base. But even pots on concrete are better insulated than those on staging.

Most importantly, keep pots as far away from the glass as possible. I’ve been known to cram in as many pots as possible, only to find those up against the cold glass were badly frosted.

This has taught me to limit my number of tender plants to the overwintering space available.

Plant of the week

Common Yew, Taxus baccata, has tight evergreen foliage and can be grown as hedging, topiary or a standard tree. Though slow growing, its versatility makes it a valuable garden plant, providing structure, background and the “bones” of a garden.

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