Many gardeners greet frost with a heavy heart. It’s a sign that winter has finally taken hold of our gardens and that any tender plants left outside will have succumbed to its icy grip. While we can protect prized specimens with sheets of fleece, mulches of straw or organic matter, cloches and heated greenhouses, without such shelter or insulation, a heavy frost spells disaster for all sorts of plants – from tender perennials to newly established plants, exotics, succulents and those valiant little half-hardy annuals that are still in bloom.
Frost can strike at any time from autumn until late spring. It normally forms on still, clear, cold nights, when the cool air makes water vapour condense and form droplets as dew. When the temperature falls below 0C (32F) the dew freezes into ice crystals.
There are five main types of frost:
● Air frost occurs when the air temperature falls below freezing point at least 1m (3ft 3in) above the ground
● Ground frost occurs when the surface of the ground, objects or trees, has fallen below freezing point
● Grass frost can occur when grass freezes but manmade concrete or Tarmac surfaces don’t, because they can hold onto warmth
● Hoar frost is a particularly feathery type of frost. Here the ice crystals form when the ground or surface temperature reaches freezing point before dew begins to form. Fog tends to prevent the formation of hoar frost, because it reduces surface cooling
● White frost is more globular than feathery. This occurs when dew forms first, then subsequently freezes.
Frost causes damage because plant cells contain water. As temperatures drop, so the water freezes into ice crystals that can rupture cell walls and contents, and stop plant proteins from working. Symptoms include stems collapsing and foliage becoming scorched, browned or blackened. Sometimes plants can die.
Not every frost is hard enough to kill plants, however. Conditions such as a morning fog can slow down the thawing process, giving plants a chance to thaw out slowly, resulting in less damage. Hardier species contain a type of anti-freeze made from complex sugars and amino acids, which can lower the freezing point of their cell contents, while many shrubs and trees rely on bark to act as an insulation layer.
What’s more, those plants growing in sheltered positions, out of frost pockets, next to the house or shaded from morning sunshine can often escape without any damage at all.
The good news is that a frost-damaged plant isn’t necessarily a write off. After the risk of frost has passed in late spring, the plant may start to show signs of life again. Trim off scorched growth back to an undamaged bud and the plant should respond by producing new shoots.