June 21, 2019 | by Lisa Mattson

During the month of June, grapevine berries have formed and are beginning to grow. This critical time in the development of the vine is called the grape fruit set. The below photo gallery shows fruit set images at various stages.

Fruit set happens at slightly different times for white grapes and red grapes. The temperature of the wine region also plays a factor. For Jordan, chardonnay grapes grown in the cooler Russian River Valley tend to flower and set fruit in mid-May, around the same time that our merlot grapes flower in the warmer Alexander Valley. Cabernet sauvignon is a later-ripening grape variety and typically doesn’t flower until 2-4 weeks after early-ripening grapes.

The flowering of grapevines in the spring determines the number of berries that form and their size. Without consistent, moderate weather during flowering — also known as bloom — grape flowers cannot turn into berries and have what farmers call a great “set.” What winemakers and grape growers hope Mother Nature will deliver every May is moderately warm days with very little wind, no rain and no heat spikes. In this case, she grants their wish for both early- and late-ripening white and red grapes. But in inclement years during flowering, chardonnay, merlot and other early-ripening varieties don’t have a great fruit set. When May weather is a mix of cool days, rain showers and even heat waves, the bloom of grapevines will be uneven –some flowers won’t even pollinate– leading to fewer berries per cluster of fruit. But this transition from flowering to fruit set determines quantity, not quality. With a few chardonnay clusters in these grape fruit set photos, you’ll also see examples of what we call “hens and chicks,” where the they grow at different sizes due to inconsistent flowering times. You’ll also notice that a handful of the grape flowers that didn’t turn into berries are visible in the bottom-left photo.

Once the fruit sets, it goes through rapid cell development, expanding in size quickly. Before the end of July, red wine grapes will begin to change color, the next step in the life cycle of the grapevine called veraison. Geek out on fruit set in the vineyard with our other wine 101 blog.

Fruit Set Grape Images: Vineyard Photography