Frost

Why frost is so damaging

A grapevine bud break in mid to late April in the UK, when the young shoots are 1 to 5 centimetres long. At that stage, the tissues are full of water and highly vulnerable to ice crystal formation. A frost of minus 2 degrees Celsius for just 30 minutes can kill 30 to 80 percent of the primary buds. The vine will push secondary and tertiary buds, but these carry less fruit, and the resulting crop is much smaller.

Late spring frosts are particularly devastating because they come when the vine has already invested energy in the new season's growth. The vine can recover, but the season is set back, and the crop is reduced. A severe frost can reduce yield by 50 percent or more.

Radiation vs advection frosts

UK frosts are usually radiation frosts, where the ground loses heat to a clear night sky and a layer of cold air forms near the surface. The cold air is heavier than warm air and pools in low-lying areas, so vineyards on slopes and hillsides are less at risk than vineyards in valleys. Advection frosts, where a mass of cold air moves in from elsewhere, are less common in the UK but can be more severe.

Frost protection in the UK

UK growers use a range of techniques. The first is site selection: vineyards on slopes, with good air drainage, are less frost-prone than those in low-lying areas. The second is wind machines, large fans that pull warmer air from above down into the vineyard, raising the temperature by 1 to 3 degrees. The third is sprinklers, which coat the buds in ice; the freezing process actually releases latent heat, keeping the buds at 0 degrees Celsius.

Other techniques include heated wires strung along the trellis, oil burners or smudge pots, and even helicopters for hire that fly low over the vineyard to mix the air layers. Each method has its place, and the best UK frost protection programmes combine several.

The cost of frost

Frost protection is expensive. A wind machine costs around 25,000 to 40,000 pounds. A sprinkler system costs more. Helicopter hire, used by some English producers, costs several thousand pounds per night. Smaller growers rely on passive protection, choosing sites carefully and accepting some frost risk.

The economic impact of a single bad frost can be severe. A 50 percent crop loss on a 10-hectare vineyard can mean the difference between a profitable year and a loss. Insurance is available but expensive, and it does not always cover the full cost.

Frost and climate change

Climate change is shifting the UK frost risk. Warmer winters are encouraging earlier bud break, which means the vines are vulnerable to frost for longer. A bud break in early April exposes the vine to a full month of frost risk, compared to a bud break in late April. At the same time, the absolute number of frost days may be decreasing, but the impact of any single frost is greater because the vine is further advanced.

UK growers are responding with later pruning, with the selection of late-budding varieties, and with more active frost protection. The most exposed sites are increasingly being planted to varieties that bud later, such as Pinot Meunier, Seyval Blanc and Müller-Thurgau.