Advice for heating your greenhouse

Greenhouse generic
Greenhouse generic

In its heyday, Clumber’s glasshouses grew a wide range of ornamental and edible plants and used lots of coal to heat their boilers, especially to produce crops out of season.

Those days are long gone. As fuel costs continue to rise, a heated greenhouse for many of us is now a luxury. Traditionally we have over-wintered tender plants such as pelargoniums or summer patio plants, which need to be kept frost free, but also in good light. Additionally, we have made early sowings of summer bedding plants which need a long growing period before they are ready to flower in June. If we try and garden sustainably, which is good for the planet, it can also be good for our pockets, given the high price of fuel.

Moulds and rots can be very damaging over the winter. To reduce this problem you need to ventilate, which obviously will let warm air out.

For this reason an electric heater would be my first choice. A maximum-minimum thermometer is a good investment. It will prove its use all year round, helping to ensure that plants aren’t too cold during the winter and guiding you when to open the vents during the summer so plants don’t over heat.

If you do choose to heat your greenhouse there are obvious ways to insulate it and keep heating costs down. Firstly, check that none of the panes has slipped or are broken and cracked. A suitable all weather tape can be used to seal any cracks. Bubble films are now widely used for greenhouse insulation; these types are UV stabilised, so they don’t degrade quickly in sunlight and should last for several seasons. This creates a layer of warmer air between the film and the glass. The downside is that any insulating layer will reduce light transmission, which will affect the growth of the plants, but a 10% reduction in light is more than made up for by the savings in heat. Inside the greenhouse, horticultural fleece can be used to cover plants during cold spells and fleece, bubble film or sacking can be placed around the containers of plants in pots to protect their rootball.

Not all tender perennials need light over the winter. Dormant dahlia tubers, begonias and gladioli corms will store over winter in a frost-free shed or garage. If heating an entire glasshouse is beyond your budget, heating a propagator may not be. These can be used to raise seed of pelargoniums, lobelias and begonias in the early part of the season.

The ideal temperature depends on what you are growing. A minimum of 3 to 5C will protect most tender plants, but you will need to take care if the weather forecasters forecast plummeting overnight temperatures or a really cold spell; a minimum of around 7 C is a safer option.

Jobs for the Month – February

Place the tubers of early potato varieties in seed trays in good light in a frost-free place to encourage the tubers to produce short shoots before they are planted.

Begin pruning bush and tree forms of apples and pears. Hybrid tea, floribunda, shrub and climbing roses can also be pruned, provided conditions are not too frosty.

Check stored tubers and corms, such as dahlias, gladioli, cannas and begonias for signs of rotting.