I’ve been Shattered

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As a good student of wine, I can tell you what shatter, aka coulure is. I can even quote (probably verbatim) the information on it from the CSW Study Guide—here goes!

“A malady known as coulure (“shatter” in English) can cause poor fruit set, with many flowers failing to become fully developed berries.”

Yep. That’s it—and that was about the extent of my knowledge until one night when I suddenly wanted to know more. So, a few textbooks, websites, and phone calls later, here is what I now know about coulure.

For starters, the word is pronounced “coo-LYUR.”

Put simply, coulure occurs when a large number of a vine’s grape blossoms stay closed, or for some other reason they fail to become fertilized. These failed flowers never get the chance to develop into grape berries but rather shrivel and fall off the vine.

In French, weather-induced coulure is referred to as coulure climatique, and the main culprit behind coulure is indeed the weather.

• The uncooperative weather—when too cold, rainy, or wet (shatter is a bigger threat in areas with marginal climates)—slows down the plant’s rate of photosynthesis, which leads to carbohydrate deficiencies in the vine. This deficiency causes the vine to conserve its resources, and the energy that would go into ripening flowers and fruit is diverted elsewhere. The blossoms eventually shrivel and drop off, along with the small stems that attached the blossoms to the vine.
• Excessively high temperatures during flowering and fruit set can cause coulure—again as a result of the shut down of photosynthesis—as the necessary enzymes lose their shape and functionality. In a prolonged heat event, coulure may be caused via excessive shoot-and-leaf growth as well as cellular respiration, either of which can lead to carbohydrate deficiencies.

Photo by Hvczech (Public Domain/via Wikimedia Commons)

Besides the weather, coulure can be caused by overly-fertile vineyard soils and excessive use of high-nitrogen fertilizers, both of which can lead to excessively vigorous vine growth that can drain a plant’s carbohydrate reserves. Another possible cause is overly vigorous pruning, which can fail to provide enough potential for the growth of amount of leaf grown necessary for photosynthesis. In addition, certain grapes, such as Grenache, Malbec, Merlot, and Muscat Ottonel are particularly prone to coulure.

Coulure is not always preventable, but some good practices to follow include:

• Taking care not to over-prune and encouraging the vines to develop enough leaf coverage to provide for adequate photosynthesis.
• Tip-trimming (snipping the tips of developing shoots) near the end of the flowering period can help prevent carbohydrate deficiencies.
• In some places, performing winter pruning late in the season may reduce the risk of coulure by delaying bud break which in turn increases the probably for warmer weather at flowering.

Hopefully, all this chitter-chatter has cleared up a bit about shatter (sha ooobie shatter)!

References/for more information:

• Robinson, Jancis and Julia Harding: The Oxford Companion to Wine, 4th Edition. Oxford, 2015: The Oxford University Press.
• Skelton, Stephen. Viticulture: An Introduction to Commercial Grape Growing for Wine Production. Published by the author, 2009.
• http://www.winewisdom.com/articles/techie/coulure-and-millerandage/
• https://sciencing.com/effect-temperature-rate-photosynthesis-19595.html

The Bubbly Professor is “Miss Jane” Nickles of Austin, Texas… missjane@prodigy.net