When referring to wine grapes, what is shatter (coulure)?

Dear Dr. Vinny,

I was just wine tasting in Sonoma, and a person pouring the wine was talking about the weather and mentioned “shatter.” What’s that?

—Nick, Chesterfield, Mo.

Dear Nick,

Let me back up a bit and first explain how grape bunches are formed. Baby grapes start as tiny flowers that grow in clusters on a new grapevine shoot. Eventually the flowers lose their petals and pollinate themselves to form small, hard green berries, a process called “fruit set.” Those tiny berries will mature into grapes.

Fruit set is a delicate process, and many things can interfere. Grape shatter—called coulure in France—happens when a cluster never fully develops. The vine’s flowers either don’t get properly pollinated, so they never turn into berries, or the newly formed berries drop off soon after setting. When growers talk about “poor fruit set,” this is what they mean.

You’ll typically see shatter discussed during spring bloom. Rain, wind, sudden temperature swings—too hot or too cold—or even rough handling in the vineyard can all interfere with the process. Recent storms have some California producers concerned about shatter, which could affect both the quantity of grapes grown (yields) and their quality. Sparse, loose clusters might ripen unevenly, and shatter can create conditions that make the grapes susceptible to fungal diseases and rot.

Shatter can affect any grape, but some varieties—like Grenache, Malbec, Merlot and certain clones of Pinot Noir—are more susceptible.

—Dr. Vinny