Shatter (Coulure) — Poor Fruit Set at Flowering; Flowers Fail to Develop into Berries; Reduced Yields

Shatter is the English-language term for coulure (pronounced coo-LYUR), a viticultural hazard in which grapevine flowers fail to develop into fruit after flowering. The two words describe the same phenomenon; coulure is the French term and is widely used in English wine literature as well. Coulure occurs when a large number of flowers either remain closed and unpollinated or, after initial pollination, fail to set properly and drop from the cluster. The result is a loose, gap-filled bunch with far fewer berries than normal. A separate but related condition, millerandage, occurs when flowers are pollinated but the resulting berries develop without seeds, remaining small. Coulure and millerandage frequently appear together in the same vintage.

• Coulure derives from the French couler, meaning to flow or drip, describing the way unfertilised flowers drop from the vine
• In severe cases, yields may fall by 50% or more; the percentage of flowers that normally set into berries averages around 30%, so further reductions can be catastrophic
• Millerandage differs from coulure in that the small, seedless shot berries remain attached rather than dropping, producing the characteristic hen-and-chickens cluster pattern
• Both conditions are pre-harvest vineyard problems, not wine faults, though they significantly influence the economics and character of the resulting vintage

The fundamental cause of shatter is a carbohydrate deficiency in vine tissues at the time of flowering. Cold, wet, or cloudy weather during the critical bloom period suppresses photosynthesis, reducing the sugar reserves the vine would otherwise channel into developing berries. When carbohydrate levels drop, the stems connecting small berries to the cluster shrivel and the tiny grapes, at around 5 millimetres in diameter, fall off. Nutritional deficiencies compound this process: boron is essential for pollen tube elongation and reproductive tissue development, and zinc plays a supporting role in fertilisation. Both are most acutely needed during the high-demand window of bloom, when even soils with adequate overall nutrient levels may fail to supply enough. Excessively vigorous vines, heavily nitrogen-fertilised soils, and severe pruning that limits leaf surface area for photosynthesis all increase coulure risk.

• Cool, wet, or cloudy conditions slow photosynthesis, cutting off the carbohydrate supply the vine needs to sustain developing flowers and fruit
• Temperatures below 15°C during flowering slow pollen tube growth too much to permit fertilisation before eggs degenerate within 3 to 4 days of anthesis
• Boron deficiency impairs pollen viability and tube elongation; it is most common on strongly acidic soils (pH 3.5 to 4.5) or sandy, leached soils where boron is easily washed away
• Overly vigorous shoot growth diverts carbohydrates away from the inflorescence, increasing competition during the critical fruit-set window

Not all grape varieties are equally prone to shatter. Grenache, Malbec, Merlot, and Muscat Ottonel are consistently identified as particularly susceptible, a factor that has historically limited the range and viability of some of these varieties in marginal climates. Malbec's sensitivity to coulure is one of the principal reasons it declined significantly in Bordeaux and southwest France. For susceptible varieties, coulure-resistant clones are now commercially available, particularly for Merlot and Malbec, giving growers an important tool for managing risk. At the vineyard level, risk is elevated by poor canopy management, over-use of nitrogen fertilisers, vigorous rootstocks, and early or severe pruning that reduces the leaf area available for photosynthesis during flowering.

• Grenache, Malbec, Merlot, and Muscat Ottonel carry the highest genetic susceptibility to coulure among widely grown varieties
• Coulure-resistant clones of Merlot and Malbec are commercially available and represent one of the most practical long-term mitigation strategies
• Overly fertile soils and high-nitrogen fertilisation promote vigorous vegetative growth that competes with flowers for carbohydrate resources
• Rootstock selection influences vine vigour and therefore indirect susceptibility; highly vigorous rootstocks can exacerbate coulure risk in susceptible varieties

Shatter is visually distinctive and can be diagnosed during or shortly after the flowering window, which in the Northern Hemisphere typically runs from May into June depending on climate and variety. Affected clusters display irregular spacing with gaps where berries should have developed, prominent rachis stems visible between the few attached berries, and a loose, open structure very different from a normally dense cluster. The flowering period itself can last anywhere from a day or two under warm, dry conditions to several weeks under cool, wet weather, so problems may build gradually. In the finished wine, vintages with widespread shatter often show elevated concentration in surviving fruit, since fewer berries compete for the vine's sugar and nutrient supply.

• Loose, gap-filled clusters with a prominent visible rachis are the primary vineyard indicator of significant coulure
• The flowering period ranges from a few days in ideal conditions to a month in cold, wet springs, directly affecting the window of vulnerability
• Surviving berries often show compensatory growth as the vine redirects resources; reduced competition can lead to more concentrated flavours in the resulting wine
• Irregular, open clusters are also more susceptible to fungal disease such as botrytis after the event, adding a secondary management challenge

The 2016 Burgundy vintage illustrates how coulure can combine with other springtime hazards to create a historically difficult year. A catastrophic frost struck the Côte d'Or on the nights of 26 and 27 April, described as the worst frost event in living memory for many growers, followed by persistent wet and cold conditions through May and into June that caused coulure during flowering. Some producers in Chambolle-Musigny, Marsannay, and Chassagne-Montrachet reported crop losses of 65 to 70% or more. The 2021 vintage across France was similarly afflicted, with devastating April frosts hitting roughly 80% of French wine regions; in Bordeaux, producers reported coulure causing an additional 10 to 30% loss of potential production on affected plots, while the overall Bordeaux harvest came in approximately 20% below the 10-year average. In both cases, the surviving fruit, shaped by concentrated vine resources and warm late-season conditions, often produced wines of notable quality despite the hardship.

• In 2016 Burgundy, frost on 26 to 27 April combined with cold, wet flowering conditions to devastate yields; Domaine de la Romanée-Conti used half as many fermentation vats as the following year
• Jancis Robinson reported that producers including Lafarge, Roulot, and Bernard Moreau in Chassagne-Montrachet lost 65 to 70% of their 2016 crop to the combined frost and season-long adversity
• In 2021, coulure was listed alongside frost, mildew, and hail among the catalogue of adversities facing Bordeaux growers; some St-Emilion producers attributed 10 to 30% losses specifically to poor flowering conditions
• The overall 2021 Bordeaux harvest was approximately 20% below the 10-year average, with the Right Bank suffering more severely than the Médoc

Coulure cannot be completely prevented, but growers have several practical tools to reduce its severity. The most fundamental is canopy and vigour management: avoiding excessive nitrogen fertilisation, moderating pruning severity to ensure adequate leaf cover for photosynthesis, and trimming shoot tips near the end of flowering to reduce competition for carbohydrates between new growth and developing berries. For nutritional risk, foliar applications of boron and zinc applied before or during bloom are effective at supplementing the high transient demand during this phenological stage, even in vineyards where soil levels appear adequate. Foliar boron sprays applied the previous autumn have been shown to be particularly effective, as boron is incorporated into dormant buds early. Once shatter has occurred during a flowering event, intervention is not possible and management shifts to optimising the surviving fruit through canopy adjustment and careful harvest timing.

• Trimming shoot tips near the end of the flowering period reduces carbohydrate competition between vegetative growth and the developing inflorescence
• Foliar boron and zinc applications before or during bloom address the high transient nutritional demand at flowering, even when soil levels appear sufficient
• Research shows that autumn foliar boron sprays are more effective than pre-bloom spring sprays, as boron is incorporated into dormant buds ahead of the following season
• Avoiding high-nitrogen fertilisation and selecting less vigorous rootstocks for susceptible varieties reduces the vegetative vigour that competes with flower development