Coulure — Poor Fruit Set in the Vineyard

Coulure derives from the French verb 'couler,' meaning 'to flow' or 'to fall,' describing the way unfertilized flowers drop from the cluster. In English the term 'shatter' is sometimes used. The condition is a viticultural hazard resulting from metabolic reactions that cause a failure of grapes to develop after flowering. When weather is the primary trigger, the French specifically call it coulure climatique. Coulure is distinct from millerandage: in coulure, flowers fail to become berries at all, leaving visible gaps in the cluster; in millerandage, flowers are pollinated but the resulting berries develop without seeds and remain small.

• Pronounced coo-LYUR; universally adopted across winemaking regions as the standard technical term
• Coulure climatique refers specifically to weather-driven coulure, caused by cloudy and wet conditions limiting photosynthesis during flowering
• Key distinction: coulure leaves gaps in the cluster where berries never formed; millerandage produces clusters with mixed berry sizes

Coulure is caused by a carbohydrate deficiency in the plant tissues during flowering. When the vine cannot produce or allocate sufficient carbohydrates, the stems connecting developing berries shrivel and the small grapes, roughly 5 mm (0.2 inches) in diameter, eventually fall off. Two main factors deplete carbohydrate supply: reduced photosynthesis from cold, cloudy, or wet weather during bloom; and excessive shoot vigor, which diverts carbohydrates away from flower development toward vegetative growth. To some degree, flower drop is a natural self-regulating mechanism, but severe coulure events have a material negative impact on yield and producer income.

• Carbohydrate shortfall causes stem shriveling and berry drop, typically beginning shortly after flowering when the deficit becomes critical
• Reduced photosynthesis from poor weather is the most common trigger; excessive shoot growth and nitrogen-rich soils compound the risk
• Some degree of flower drop is normal and healthy; it becomes a hazard when exacerbated by weather and results in well below average fruit set

Grapevines require dry conditions with sufficient sunlight and ambient air temperatures around 15°C (59°F) for pollination to proceed smoothly during the 7-14 day flowering window. Cold, cloudy, rainy, or windy weather during this period disrupts photosynthesis and reduces carbohydrate availability, directly increasing coulure risk. Excessively high out-of-season temperatures can also trigger coulure in some varieties by promoting cellular respiration and shoot growth that competes with berries for carbohydrates. Non-weather factors including overly fertile soils, nitrogen-rich fertilizers, vigorous rootstock, and severe pruning that limits leaf area all increase susceptibility.

• Flowering occurs in May-June in the Northern Hemisphere and November-December in the Southern Hemisphere; ideal conditions are warm, dry, and sunny
• Cold, wet, or windy weather reduces photosynthesis, lowers vine carbohydrate reserves, and prevents proper flower fertilization
• Excessively fertile soils, nitrogen-heavy fertilization, and vigorous rootstocks increase shoot competition for carbohydrates and raise coulure risk

Grape varieties with the highest documented proclivity to coulure are Grenache, Malbec, Merlot, and Muscat Ottonel. Merlot is particularly noted for its susceptibility during cool, wet spring conditions, and its staggered flowering window makes it especially vulnerable to disruption by inclement weather. Growers can choose coulure-resistant clones of Merlot and Malbec, which are now commercially available. Additional mitigation practices include avoiding severe pruning to maintain adequate leaf coverage, tip-trimming developing shoots near the end of the flowering period to reduce carbohydrate competition, and ensuring balanced vine nutrition. Late winter pruning can also reduce risk by delaying bud break and increasing the chance of warmer conditions at flowering.

• Grenache, Malbec, Merlot, and Muscat Ottonel show the highest coulure susceptibility among commercial varieties
• Coulure-resistant clones are commercially available for Merlot and Malbec, offering a direct long-term mitigation option
• Practical vineyard measures include balanced pruning, shoot-tip trimming at end of flowering, avoiding excess nitrogen, and optimizing canopy for photosynthesis

The 2013 Bordeaux vintage provides the most clearly documented modern coulure event: cold, wet, overcast conditions during a very late June flowering caused massive coulure on Merlot across the region, with the CIVB estimating coulure alone reduced the harvest by approximately 800,000 hectoliters. The 2013 harvest was Bordeaux's smallest since 1991. In 2010, cool and wet June weather in Bordeaux caused coulure and millerandage in Merlot, reducing yields, though the summer and autumn were outstanding, resulting in a celebrated vintage of limited quantity. In 2024, Bordeaux again experienced coulure and millerandage affecting Merlot during flowering, with red wine yields down 18-29% against the 20-year average.

• 2013 Bordeaux: widespread coulure on Merlot from late, wet flowering; CIVB cited coulure as causing a loss of approximately 800,000 hl, the smallest harvest since 1991
• 2010 Bordeaux: cool, wet June weather caused coulure and millerandage in Merlot; summer and autumn were ideal, producing a celebrated but lower-yielding vintage
• 2024 Bordeaux: poor fruit set from coulure and millerandage during flowering reduced red wine production 18-29% versus the prior 20-year average

Coulure exists within a cluster of related fruit-set disorders. Millerandage is a distinct phenomenon: the flowers are pollinated, but the resulting berries develop without seeds and remain small. Both conditions are often caused by inclement weather during the flowering and fruit-set period and both reduce yields, but their effects on wine quality differ. Millerandage may reduce yield while producing more concentrated, higher skin-to-juice ratio fruit, and in some New World regions it is viewed positively for certain varieties such as Pinot Noir. Shot berries, the small seedless berries that can accompany millerandage, ripen faster and can add concentration to the remaining fruit. Coulure, by contrast, leaves no fruit at all in affected positions on the cluster.

• Millerandage: pollinated but seedless, undersized berries form alongside normal ones; can improve skin-to-juice ratios and concentration in varieties like Pinot Noir
• Shot berries (hens and chicks): the small seedless berries associated with millerandage, which ripen earlier and can add phenolic concentration
• Coulure and millerandage frequently occur together in the same vintage and vineyard, as both are driven by poor weather during flowering