The English Garden – September 2019

SEPTEMBER 2019 THE ENGLISH GARDEN 87

Clockwise from top left

Wipe away any

greenhouse shading

applied over summer;

pelargoniums should be

cut back before over-

wintering; bubble-wrap

insulation helps retain

heat; space plants out

along the bench for

good air circulation.

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, although your greenhouse will need

mains electricity. When buying and installing a new

greenhouse, always consider whether electricity can

be installed with it – it’s as valuable for lighting as it

is for heating and is rarely regretted. The advantage

electricity has over fuels such as gas or paran, is

that it’s clean and doesn’t emit water, soot or carbon

monoxide, all of which can adversely aect plants.

Gas heaters are an alternative but can be bulky and

heavy, while paran heaters are usually small and

lightweight and, as a result, easy to knock over and a

potential fire hazard that you may prefer not to risk.

With heat, and a minimum temperature of 7-10°C,

plants like pelargoniums will continue to grow

over winter, albeit more slowly due to the reduced

light levels. If they are growing, watering will be

necessary, but be guided by the plant and never over-

water: let the compost dry out between waterings.

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, having dry roots could just save the

life of a half-hardy plant, although if the freeze is

prolonged, they probably won’t get away with it.

A cold, damp atmosphere inside can 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.

What to grow in winter

Get ahead and look forward to the best sweet peas

by sowing in mid to late October in a cold, unheated

greenhouse. Sow single seeds into individual

that means heating. All forms of greenhouse heating
cost money, so carefully evaluate whether it will
be worth it: a prized collection of rare orchids may
warrant it; a few bedding plants will be cheaper to
buy again next year. Many gardeners prefer to take
a chance, overwintering their plants in an unheated
greenhouse and taking a que será será attitude
towards any losses.
If you are going to heat your greenhouse, keep bills
as low as possible by insulating the greenhouse with
bubble wrap. In a larger greenhouse, use a bubble
wrap screen to portion o the smallest possible area
where heat is required rather than needlessly heating
the entire house. Greenhouse supplier Hartley
IMAGES Botanic has a helpful calculator on its website for

GAP/FRIEDRICH STRAUSS/JONATHAN BUCKLEY/ROBERT MABIC/FIONA LEA