Shatter explained: A perfect flower, but not a perfect bloom

By Levi Glenn

Every year there is a one- to two-week period when the vineyard smells wonderful. That's when you know bloom has arrived. It's not the unbelievably effusive smell of an orange grove in full blossom. Or the sweetly intoxicating blue bush lupine, a beautiful native wildflower we see each spring. The scent of a vineyard in full bloom is a bit more understated, elusive even. It's got a sweet floral note underscored by a deeper earthy character. The smell is fleeting, as is bloom. At least in most years.

A Mourvedre cluster in full bloom

Simply put, bloom is the window of time in which each individual flower pollinates itself. Grapevines have what is referred to as a perfect flower. Many crops require both male and female plants to produce fruit. A male flower's pollen is moved by wind (and often aided by honeybees) to help find its way to a female flower. Perfect flowers -- including grapes -- can self-pollinate. That is, unless something goes wrong. 

In our area, weather during bloom is typically optimal for even fruit set: warm, dry, not too much wind. 2015 has been a bit different. After a historically warm first four months and a correspondingly early emergence from dormancy, May was unseasonably cool. We had quite a bit of wind, and the fog produced by the onshore flow seemed relentless. There were even a couple light rain events. Wind can blow the pollen away, and rain or fog can make the flower cap stick. Both result in an unfertilized flower. With optimal conditions, bloom can be as fast as a week. This year, we have seen some blocks take close to a month to complete flowering. When a flower doesn't turn into a berry for whatever reason, we call that shatter (or coulure in french). When this is widespread over a vineyard, crop loss can be severe. The two examples below are the two ends of the spectrum.

 
A fully-fertilized Grenache cluster

A Grenache cluster with lots of shatter

Unfertilized Grenache berries

In addition to the Grenache -- which is known as a shatter-prone variety -- we have seen some shatter in Syrah.  But it's not even across the entire vineyard.  Grenache from warmer blocks that flowered first, during warm weather in late April, set quite well. Grenache from cooler, lower-lying parts of the vineyard that didn't get around to flowering until May show more shatter.  The Mourvedre and Roussanne that are finishing in our warm weather now don't show any signs of shatter. 

The conditions during bloom can dictate crop levels not only for this year, but also for next year. The 2016 inflorescence (cluster) is being formed right now whithin the bud located inside this years shoot. Growing conditions this year can affect how many clusters (typically one to three) will be inside next year's buds, and what size they will be. As an example, weather during the 2014 bloom period was ideal, so we saw some shoots with three clusters on them this year.

Mild-to-moderate shatter in a variety like Grenache isn't always a bad thing. This variety tends to produce large, often dense clusters. The berries that are on the interior of the cluster aren't exposed to sunlight and can therefore stay pale in color, producing correspondingly lighter wines. With some shatter, the more open clusters receive more even light exposure, creating darker and more concentrated wines.  Looser clusters also reduce clusters' susceptibility to mildew, to which Grenache can be prone.

And, of course, bloom is just the beginning.  Crop level and quality are affected by the full season's weather conditions, and we adjust what we do in the vineyard depending on what we see.  Blocks with shatter, or fewer buds per shoot, will need less, or even no, thinning to produce top quality fruit.  The more productive blocks give us more options, but are also more work.

Overall, our unusually cool May appears to have reduced the amount of crop in some varieties, but crop levels on average don't look that different from 2013 or 2014. Given that our last two years produced perhaps the highest-quality back-to-back vintages in our history, knowing that crop levels this year are comparable is a good early indicator of quality.  Stay tuned.