How to protect your garden plants from frost damage
Wrap those tender plants up warm, writes Gerry Daly
PERIODS of exceptional cold occur every decade or so and the attendant low temperatures can be devastating to plants. Most of the native plants have adapted over about 10,000 years to these unusual periods of cold, but even some natives can suffer. Ivy can be burned back by a late frost affecting its early shoots. Gorse, which might appear tough as nails, has been badly damaged, even killed, by severe frost.
But most of the plants affected by severe cold are not natives. Instead, they come from areas of warm climate including the Mediterranean region, California, South Africa, Chile, New Zealand and Australia. These are plants hardy enough to be grown outdoors year-round. In most years, they are not affected by frost, or only superficially, and recover in spring.
The level of frost varies across the country. In most years, in places around the coastal strip, the temperature rarely drops below minus 5C, and these plants of borderline hardiness can manage quite well. In inland areas, they are likely to be affected much more frequently to the point where some plants cannot be grown at all.
The list of plants that can be affected includes phormium, tree fern, camellia, leptospermum, escallonia, griselinia, ceanothus, hebe, acacia, pittosporum, fuchsia, fatsia, bottlebrush, osteospermum, euryops and many others, some of the most popular garden plants. After the last big freeze in the winters at either end of 2010, many people lost plants and resolved not to grow them again, but these plants proved too tempting, and have been widely re-planted.
However, it remains to be seen just how damaging this bout of cold weather turns out to be. The most severe damage comes as a result of a prolonged period of several days below freezing. Many plants can withstand a couple of days but then frost penetrates the soil and the bark of trees, killing the growth layer known as the cambium. After the frost, trees might seem fine but the bark splits and lifts off.
It is frost and low temperatures that kill plants, snow can actually protect plants by acting as a form of insulation, keeping frost away from the base of the trunk of trees and preventing the frost penetrating the soil. Snow itself can cause problems because its weight breaks or opens out branches on evergreens. A heavy layer of snow can break glasshouse panes and split polytunnel covers. Where there are heavy accumulations of snow, it should be knocked off with a brush.
Not much can be done to protect susceptible shrubs and trees as they are too large, but some plants are small enough to cover with bubblewrap, old blankets or carpet. Tree ferns are notoriously susceptible to frost damage and can have the central growing point packed with any form of loose material, such as straw or bracken, and wrapped around. Straw and dry bracken are not available to most people, but horticultural fleece, newspaper, plastic sheeting and cardboard might be.
The plants mentioned are of borderline hardiness and survive most winters, at least in coastal areas, but more tender plants can be killed even in a greenhouse that hasn’t got a frost protection heater. These include fuschia, lantana, tibouchina, mimulus, dahlias, canna and greenhouse geraniums. Cover these plants with any loose covers to hand.
In areas where dahlia, gladiolus, canna, chocolate cosmos and Peruvian lily are in the open ground, cover them with a good layer of garden compost to prevent frost reaching the roots. Plants in pots can be moved under shelter and this will avoid the pots cracking when the frozen compost expands.