Frost can affect many plants, and is particularly damaging to tender new growth and blossom in the spring. The risks of frost damage can be reduced by taking some simple steps to protect the plants in your garden.
Sometimes frost damage is apparent almost immediately following freezing. However, this is not always the case and with some plants, particularly woody ones, the damage may take several months to appear. Look out for the following signs;
- Tender young growth may be damaged by spring frosts, causing scorching and pale brown patches to appear between the leaf veins. This tends to be on the exposed and top edges of the plant e.g. acer and carpenteria
- Hard frost in winter can cause the leaves of hardy evergreen plants to be scorched and turn brown, and may eventually lead to the death of the plant, e.g. bay and pittosporum
- The foliage of tender perennials e.g. dahlia and canna may be blackened by the first frost of autumn. Stems usually collapse
- Spring frosts can damage blossom and young fruits. This may cause a corky layer to form at the flower end of the fruit i.e. apple and damage to blossom may lead to few or no fruits forming
- As a result of late spring frosts summer bedding plants and tender vegetables, such as potatoes and tomatoes, may suffer from leaf scorch, browning and even total plant death
- Prolonged periods of frost may cause spotting on the leaves of some shrubs such as photinia and garrya
- The foliage of certain plants exhibiting early symptoms of frost damage appears water-soaked and dark-green, turning black in time
Causes of frost damage
Ground frost occurs when the temperature of the ground falls below freezing point (0ºC/32ºF) and air frost occurs when the temperature of the air falls below freezing point.
Plant cells can be damaged or even destroyed by frost. Repeated freezing and thawing, or very rapid thawing can be particularly damaging to plants.
Once the temperature has fallen below freezing, a strong wind can make a frost more damaging. Cold winds remove moisture from evergreen foliage more quickly than it can be replenished by the roots; this can cause leaf browning particularly at the tips and margins.
Tender plants survive the winter better when they are planted in a sheltered sunny position. This is because new wood is ripened by the sun accumulating more carbohydrates during the growing season, making it more frost resistant.
Newly planted, young plants can be more susceptible to frost damage than fully established specimens.
Cold air naturally flows downwards on sloping ground, collecting at the lowest point or against a barrier, this is known as a ‘frost pocket’.
Treatment of damage
As most gardeners will testify, it is easy to be caught out by frost. And sometimes frost damage is simply unavoidable. When damaged has occurred, what should be done?
- If no more frost is expected, prune out damaged growth, cutting to an undamaged sideshoot or bud
- After pruning, apply a top dressing of a general-purpose fertiliser, such as Growmore at the manufacturer’s recommended rate, to encourage strong re-growth
- If a fence or hedge is causing a ‘frost pocket’ consider creating a gap, or remove some of the lower growth to improve cold air drainage
- Frost may lift newly-planted shrubs out of the ground, so check and re-firm the ground around them
- In gardens exposed to cold winds, consider creating more shelter by planting a shelterbelt
- Even though the foliage of dahlias and cannas has been blackened by frost the roots are alive and can still be protected or lifted and stored
Important: Do not automatically give up on a plant that has been frost damaged. Many plants can be surprisingly resilient and may well rejuvenate from dormant buds at or below soil level. This takes time so recovery may not be seen until early summer. If the plant is of high value or it is not essential to fill the gap, consider leaving the damaged plant in the ground until mid-summer. If no re-growth has appeared by then, replace the plant.