Bud Break

The biology

A grapevine bud is a small, compressed package containing three pre-formed shoots, leaves and a tiny cluster of flowers. The bud was laid down in the previous summer, hardened off through the winter, and is waiting for a combination of warm soil and warm air to tell it that the new season has begun. Once temperatures consistently exceed about 10 degrees Celsius for several days, the bud swells, the protective scales fall away and the shoot begins to grow, often at a centimetre a day in good conditions.

The earliest buds to break are the primary ones, and these carry the best fruit. If a primary bud is killed by frost, the vine pushes a secondary bud, which is less fruitful and less fruitful at the right place on the cane. Tertiary buds carry little or no fruit at all.

Why the date matters

Earlier bud break means a longer growing season, which usually means riper fruit. But it also means a higher risk of frost damage. A bud break in mid-April followed by a hard frost in late April can wipe out a significant part of the crop. UK growers therefore track bud break closely, log it in their vineyard records, and use it to plan frost protection and to think about which clones to plant.

Late-budding varieties, such as Müller-Thurgau, Seyval Blanc and Pinot Meunier, are popular on frost-prone sites because they tend to push after the worst of the spring frosts have passed.

Bud break across the UK

Bud break dates vary by region. In the warmest sites in Sussex, Kent and Hampshire, Chardonnay and Pinot Noir typically break bud in the second or third week of April. In cooler sites in East Anglia, the South West, the Midlands, Wales and Yorkshire, bud break is usually a week or two later, often in early May. Climate change has been gradually shifting these dates earlier, with some growers now reporting bud break in the first week of April in the most sheltered South East sites.

Bud break is also a major logistical moment. Spraying for downy mildew usually starts shortly after bud break because the young leaves are the most vulnerable. Mowing, cover-cropping and soil work all need to be timed around the new growth. Many UK growers hold a vineyard walk at bud break to assess uniformity and count live buds per vine.

Frost protection at bud break

Spring frost is the single biggest weather risk to a UK vineyard. A frost of minus 2 degrees Celsius at bud break can destroy 30 to 80 percent of the primary buds on a given block. Protection methods include wind machines that pull warmer air down into the vineyard, sprinklers that coat the buds in ice (the freezing process actually releases heat), and, in the most exposed sites, heaters or even helicopters for hire.

Some UK vineyards have invested in the latest frost-protection technology, including heated wires strung along the trellis and so-called frost fans that use a small jet engine to drive warm air downwards. The most protected English vineyards, including Nyetimber's sites in West Sussex and Chapel Down's sites in Kent, run active frost programmes every spring.