Frost can occur when the temperature falls below freezing and may be particularly damaging to plants, especially after they have produced new growth in spring. There are a number of different types of frost:

• Black frost: This is a dry freeze that occurs when there is not enough moisture in the air to produce a visible frost. Shortly after being affected by this frost, the leaves and stems of affected plants turn black and die.
• Ground frost: This is formed from the condensation of water vapour in the air when the temperature of the ground surface falls below freezing. This type of frost is particularly dangerous on calm, clear nights when cold air collects just above the ground. Plants whose woody tissue has not yet hardened are most at risk.
• Hoar frost: Crystals of ice form from the moisture in the air, freezing instantly onto the surface of plants.

Frost Damage

When plants are struck by frosts, ice crystals form within the plant cells; these crystals expand upon thawing which may cause the cells walls to rupture. Tender plants are most at risk, but half-hardy and even fully-hardy plants may be affected. Unexpected, severe spring frosts are especially destructive to gardens, damaging tender new shoots and buds and even splitting the bark of some woody plants.

Frost can also loosen the soil, causing young, newly planted or shallowly rooted plants to rise or 'lift' out of the soil. These plants should be refirmed carefully as soon as the thaw begins as their exposed roots will be vulnerable to damage from low temperatures and drying winds.

When the ground freezes, water is no longer available to the roots of plants. Whilst this may not affect plants whose roots extend below the frost-line, shallow-rooted evergreen plants may become dehydrated as they are unable to regain the moisture lost by transpiration.

The amount of damage caused by a frost depends on its severity and duration. A sustained temperature of -3°C (27°F) for 3 hours may cause considerable injury to plants, whilst the same temperature for 15 minutes may not cause any damage at all.

However, frost can also act as a helping hand in the garden, reducing the numbers of some soil-inhabiting pests. The freeze/thaw action of soil water can also shatter clods of soil into much smaller particles, which can be useful for heavy, clay soils.

Frost Pockets

Frost pockets are places where frost is more likely to form. Cold air is denser and heavier than warm air and tends to sink to the lowest point it can reach, becoming trapped in hollows or at the bottom of valleys and slopes. Here it accumulates, forming a pocket of cold air and damaging any plants in the area. If the cold air cannot escape, its area of potential damage will increase as it is forced back up the slope.

Frost pockets will also form where the cold air is obstructed by a barrier such as a row of closely planted trees, walls, fences or hedges. If a frost pocket affects your garden, try to create an escape route for the cold air by thinning out trees or removing solid obstructions. If this is not possible, make sure that any planting in the vicinity of the frost pocket is fully-hardy, unless you are prepared to provide it with adequate protection.

Protecting Plants

Half hardy plants should be protected either in situ or brought under cover along with tender plants until the risk of spring frost has passed. Keep a close eye on the weather before planting out tender plants such as dahlias, chrysanthemums, tomatoes, runner beans and half-hardy bedding plants or sowing seeds. It is usually best to delay planting for about a week after it seems safe; this will hopefully reduce the chances of a late frost destroying your young or tender plants.