Nature’s got it COVERED
It can cause havoc, but a blanket of snow isn’t all bad news for your garden, says Monty Don
AS I write this, high up in our converted hop kiln, the snow is swirling around the windows and fragmenting the garden below into a thousand soft white shards and spangles.
Snow is lovely but can cause great damage. It can break branches and crumple seemingly robust structures. Some years ago I put up an expensive fruit cage, and left the light netting on after the last of the fruit had been cleared in autumn. An inch of snow that then froze onto the nylon mesh was enough to buckle the aluminium frame irreparably.
But snow does very little harm to most garden plants and, in fact, acts as an insulating blanket against icy winds. As it thaws it also provides moisture, and so is an important source of water for the spring growth of many plants.
By and large it only snows in this country at about or just below freezing, which is not a disastrously cold temperature for hardy plants. If the snow is more than a few inches thick it will insulate the ground — and all the plants sheltering beneath its blanket — from any further drop in temperature.
HARDY plants can withstand very cold snaps, down to -15°C or so, and can survive weeks of temperatures around -5°C. Half-hardy plants such as penstemons, salvias and many camellias do not, as a rule, tolerate temperatures below -5°C but can cope with the odd touch of frost; tender plants such as basil or zinnias will not survive below 5°C.
This temperature, 5°C, when averaged out across a full 24 hours, is the point at which most plants start to grow. They will grow with increasing vigour and speed as the temperature rises — provided they have enough water — up to 25°C or so. Of course, as the temperature rises and the plant gets bigger, its demand for water increases greatly. This is where the effect of snow stretches into summer, as plants draw upon the reserves of meltwater in the soil.
Winter cold is healthy only because plants prepare for it. Our long autumns and springs wean plants into dormancy and then growth. This is why a sudden frost in autumn or, especially, spring can have disastrous effects. A plant that has withstood a month of sub-zero temperatures can have half its growth killed by a sudden frost in May. The new growth is simply not expecting it.
Most temperate garden plants have developed effective means to counter cold. Deciduous trees and shrubs drop their leaves and stop almost all root growth.
Herbaceous plants will happily survive frozen ground because they’ve shut down all growth and gone into hibernation.
Annuals die as plants but leave seeds that will survive the cold and grow in spring.
In fact, the greatest damage done by cold to a garden is from the wind.
Plants suffer from the wind-chill factor as much as humans, and wind will also dry plants in cold weather as effectively as a hair dryer. If this is combined with frozen soil, it will kill the plant.
Thus, a still night with a frost of -5°C will do little harm to any hardy plant. But with a wind of just 15mph the air temperature drops to -12°C and quite a few plants will be damaged if not killed. Unless your garden is stocked only with plants adapted to life on the prairie or the steppes, it should have hedges and windbreaks, protecting everything growing in their shelter.