Frost damage

How do plants protect themselves from frost damage?

Plants can survive frosts by several mechanisms:

1. Sometimes bark can insulate the living water-conductive tissues in the same way that water pipes are lagged to prevent water freezing within cells
2. Some plants accumulate materials, certain sugars and amino acids for example, that act as anti-freeze lowering the freezing point of cell contents – shortening autumn days induce this
3. A more effective mechanism is the ability of some plants to allow their cell contents to 'superfreeze' where the cell contents remain liquid even though below freezing point. To do this plants have to experience several days of cold weather before the freeze and this explains why even hardy plants can be damaged by a sudden autumn frost
4. In very severe climates such as within the arctic circle native trees remove water from their cells, tolerating the dehydration of the cell contents, and place the water between the cells where it can freeze without causing damage. This works where the weather provides a prolonged slow chilling

Symptoms

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’.

Prevention of damage

There are a number of ways to keep your plants safe during cold weather;

• Choose plants that are reliably hardy and suited to your growing conditions. The RHS Plant Selector can help you choose hardy plants
• Select planting positions carefully to avoid ‘frost pockets’
• Slightly tender plants should be grown in a warm sunny spot, e.g. against a south-facing wall, which will provide some extra warmth and winter protection
• Cover plants with a double layer of horticultural fleece or other suitable protection when frost is forecast
• Mulch the root area of evergreens, conifers, tender shrubs and tender perennials with a thick layer of organic matter to prevent the ground becoming frozen
• Move container-grown plants to a sheltered part of the garden in cold weather and provide some extra protection by wrapping the pot in bubble wrap
• Leave the previous seasons’ growth on more tender plants until spring, for example penstemon, as this provides valuable frost protection during the winter
• Tender plants can be lifted or moved to a more sheltered position or greenhouse. If this is not practical then protect them by wrapping examples include bananas and tree ferns
• Lift tender perennials such as dahlias, cannas, pelargoniums and fuchsias before the first frosts
• Protect fruit and strawberries from frost by packing with bracken or straw
• Avoid applying nitrogen-rich fertilisers late in the season as they stimulate soft, sappy growth which is especially vulnerable to frost damage
• Plants exposed to early morning sun may thaw too rapidly after a frost, causing damage to flowers and young growth. Camellia and magnolia flowers in particular can be ruined by a single frost
• Plant tender bedding plants out after the danger of frost has passed; this is generally late May in the south of England and June elsewhere. Always harden off plants before planting outside
• Ensure tender plants are overwintered safely in the greenhouse by providing adequate heating or insulation
  

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.