Warm up your greenhouse

imageAs you prepare to jump-start your garden, use tricks that protect your plants and save fuel at the same time, says Bunny Guinness Burning furniture to keep your greenhouse warm could be seen as rather extreme. But when last year’s big freeze coincided with an oil crisis, specialist herb grower Jekka McVicar had to sacrifice all her house and oven heating over Christmas to keep her precious stock alive in the greenhouse until the oil tanker could get to her in rural South Gloucestershire to refuel. As the greenhouse plants in question were for my M & G Chelsea garden, I was exceptionally grateful.

The prolonged icy winters over the past three years have meant that keeping a greenhouse even frost-free has been quite a cost. Now, just as we want to jump-start the garden and get lots of early plants going, it makes sense to use canny tricks to save on fuel.

For those putting in a greenhouse, an ideal spot is against a south wall where it can bask in the radiated heat at night. The east-west alignment will maximise the amount of sunshine, too. If, like me, you’re stuck with an old greenhouse on a north-facing wall, don’t despair. Paint the outside wall white to reflect extra light and it will benefit from radiated heat. Mine grows much better tomatoes and cucumbers than you would think. My watercress loves it too!

The old-style pit greenhouses which are sunken about 70cm to a metre below ground level apparently really do save on heating. A canny builder client of mine built himself one and I copied him for my second greenhouse. The speed at which you can excavate the area with a mini digger and the fact that you can get away with concrete blocks (as opposed to bricks/stone) for the surrounding walls as they are not seen externally, probably means the difference in construction costs is fairly negligible.

Even if you are not sinking it, building lowish brick or stone sides conserves heat. Timber framework as opposed to metal reduces heat loss.

If you are looking to reduce running costs on an existing greenhouse, it makes sense to divide it up and only to heat the area you have to. Much of the heat is lost in the apex and many commercial growers pull over an insulated cover of some sort with a silvered underside to span the building at the eves during the night. You could consider putting a twin wall polycarbonate sheet across (light and easy to fix) and leave it in situ for colder periods.

In Germany, where growers are very energy conscious, many of their greenhouses are made entirely from twin-wall polycarbonate, although it has poorer light transmission – its heat loss coefficient (U value) is 0.6. (glass is 1.1 and 50mm polystyrene board is 0.1.) In a pit greenhouse you can fix the polycarbonate on top of the retaining walls for lower-growing plants. Filling all draughts with putty or draught-proofing strips is a must, and even sheltering the structure with a fence or hedge helps protect it from the prevailing wind. At night and in cold periods, adding extra duvets over plants in the form of thick fleece or bubble wrap will help save a few more degrees.

Trying to get some extra free heat can more than just tip the balance. Water barrels painted black, earth or stone floors will heat up by day and radiate heat at night. Making a compost bin, as they do in the polytunnel at the Centre for Alternative Technology (cat.org.uk), can help too.

Chloe Ward, display gardener from CAT, said they made rat-proof wire cages 1.5m (5ft) x 1.8m (6ft) x 1.2m (4ft) and put about 800mm (2.6ft) of straw in the base. They then add 100mm (4in) layers of alternating green waste and straw, making sure to add 200mm (8in) of new material each week to keep it going. They start filling in January and as they take about three weeks to heat up, are ideal for generating gentle heat to help vulnerable seedlings in early spring. You could make a similar system using a large builder’s bag: free heat and then compost.

Jack Strawbridge, an eco engineer and author of Practical Self-Sufficiency, says he does not have time to do that. Instead he made a heat sump in his greenhouse, detailed in his book. He dug a hole with a volume of one cubic metre and put in a series of plumbing pipes to allow the air to circulate and then filled it with broken, recycled glass. A fan (a small, solar-charged one from a computer) sucks air from the apex and blows the warm air through the glass during the day. The glass is heated up during the day and the fan reverses the process at night, giving heat out. They have had frosts of 21F (-6C) outside and inside it is always frost free. They have salads all winter and aubergines in February, but there is also a south-facing rear wall which no doubt helps too.

After last winter, I am biting the bullet and putting electricity and heating into my greenhouses. Having two greenhouses sounds flash, but both are home-made using recycled windows. The most efficient heaters are thermostatically controlled propane ones, though these give off water as well as extra CO2, which can exacerbate fungal problems. Additionally, the CO2 output is too much for orchids and citrus. The latter will quickly defoliate.

Alfred Brusius of Green Ideas (biogreen.de) sells propane, paraffin and electric heaters and he reckons the running costs of their propane heater (Black Forest, £199) is about 15 per cent cheaper than his most efficient electric one (Phoenix, £199 from amazon.co.uk). The latter is most definitely the most popular. It is highly efficient to run as it has a precise thermostat that can be set so it comes on just at 32F (0C). A lot of the cheaper ones will frequently heat when unnecessary and not heat when necessary. It was highly recommended to me by my Gardeners’ Question Time colleague Anne Swithinbank (whose husband trialled different ones). Even though Alfred’s paraffin heaters are of a new generation, they still emit messy carbon and a lot of moisture. As they have no thermostat they are the most expensive to run — around 50 per cent more than propane.

This weekend I am sowing peas, beans, cosmos, lettuce, tomatoes, aubergines, leeks, cleomes, beetroot, carrots and more. If we get a late frosty snap, I want to sleep soundly in my bed, knowing that they are snug too.

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