Viticulture 101: Grape bloom, flowering and fruit set in the vineyard

Like wine? Want to better understand the winemaking process? Most winemakers will tell you that great wines start in the vineyard. So here’s your opportunity to learn more about how wine grapes are cultivated. In our Viticulture 101 series, we will explain the basics of the different growth stages, just as many of Oregon’s vineyards are experiencing them. Follow along as we walk you through the 2016 growing season.

Following our last article on bud break, next comes bloom (also known as flowering) and fruit set. This important phase in the grape growing season will determine the number of berries in each grape cluster. With this information, winemakers can anticipate fruit loads and begin to plan for harvest.

Flowering or bloom generally occurs about a month to six weeks after bud break. The flower clusters are called inflorescence and emit a lovely fragrance that envelops the entire vineyard in sweet aromas. Although honeybees do visit the vineyard, they are not essential to the grape pollinating process since cultivated grapevines are hermaphroditic, possessing both female ovaries and male stamens. Just by the nature of the plant, nearly 50 percent of the flowers in a given grape inflorescence are likely not to set fruit.

During bloom, the delicate grape flowers are very vulnerable to damage from wind, rain or an unexpected late frost. So with the odds already stacked against them, vineyard managers do everything they can to ensure an even and abundant fruit set occurs. In some grape growing regions, such as the Napa Valley, giant fans are used to circulate the air in the vineyard, in order to keep the overnight temperatures above freezing, thereby protecting the flowering vines.

Stress to the grape vines, including lack of water, can contribute to inflorescence necrosis, where the entire flower cluster is damaged. As you can imagine, losing entire flower clusters can be detrimental and is avoided if at all possible. Loss of the flowers within the inflorescence is called flower necrosis, and will result in grape clusters that have varied berry sizes, often referred to as hens and chicks. Hens and chicks describe both big and small berries growing in a single grape cluster. Flower necrosis isn’t always such a bad outcome and can actually contribute to better cluster ripening and reduce the possibility of mold since air can flow between the berries.

Carbon and nitrogen levels in the vineyard soil can also affect fruit set. For example, planting nitrogen-rich cover crops in a vineyard that is deficient will balance the levels and promote better fruit set than in those vineyards where nothing is done.

About 10-14 days after full bloom, fruit set should be well established and the pollinated flowers have begun to form small green berries that will eventually grow into individual grapes. At this time vineyard managers can begin to estimate the percentage of each grape cluster that will grow into fruit. While significant weather events can still affect the season, fruit set is the first indicator of how abundant (or not) harvest may be.

The Willamette Valley is excited about fruit set and you can find photos all over social media. It truly is the 2016 vintage in the making! Visit again as we continue our Viticulture 101 series and discuss canopy management and why it’s so important for ripe grapes.

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