June 7, 2013 | by Lisa Mattson

Like the yellow snails loitering on the guyot-trained grapevines of Meursault and the vile, “gangster” gastropod Jabba the Hutt of Star Wars, grapevine flowers are hermaphroditic. That is right! Simply put, grape flowers possess both male and female parts. For cultivated grapevines rooted here in Earth’s lithosphere, these hermaphrodites are considered “perfect flowers.” Perfect plants and perfect snails are clearly two different things, however. While I have enjoyed many a garlic-butter-drowned escargot, most of which were classified as “perfect” when washed down with local village Burgundy, the slave-holding, spice-smuggling slug from the deserts of Tatooine falls short in my book.

This blog post digs into the relationship between the flowering, fruit set and development of wine grapes during spring and the beginning of summer, using the 2013 vintage as an example.

Flowering, Fruit Set and Development

With the nearly undisrupted stretch of sunny skies and warm temperatures in the spring of 2013, the flowering stage of our grapevines in the Russian River and Alexander Valley appellations has progressed, well, perfectly. Unscathed by any frost threats, the early bud break commenced the first week of March and set the tone for a beautifully even start to the growing season. Consistently escalating temperatures and daylight hours throughout April and May led into the flowering stage within the expected time frame, but taking its cues from bud break, which began in early March, flowering also commenced three weeks ahead of the typical growth stage calendar in early May. Due to the delicate nature of the flowers, the flowering stage, like bud break, opens up the potential for Mother Nature to swiftly move in and claim a large percentage of the crop through adverse weather. (Perhaps this is her idea of a land usage fee? A hailstorm during this time could be disastrous for grape yields.

Pre-Flowering Grapes: The Calyptra

Flowering parts under the protection of the shell-like petals of the calyptra.

During the period of rapid shoot development discussed in my last “Between the Vines” blog post, the branch system of the cluster, termed the “rachis,” can be observed extending from the lower portion of the new shoots. Although at first it may appear that little baby grapes are forming at the tips of the rachis (kind of like miniature corn), these little green cases are actually the flower parts protected by five, shell-like petals that are fused into a unified enclosed structure called the calyptra. Inside of the calyptra under each of the five petals is a developing stamen—male part—consisting of an elongated filament with two pollen sacs at the end. These five stamens surround the pistil—female part (hmm, quite similar to human behavior)—composed of the ovary, style and stigma. As the pollen at the tips of the stamens and the eggs in the ovary mature nearly simultaneously, the five petals of the calyptra will release from the bottom and curve outward and upward, eventually drying and falling off.

Grape Flowering: Pollen Releases

Shedding of calyptra (brown parts) to expose flowering parts.

This shedding of the calyptra also causes the pollen sacs to rupture and release thousands of pollen grains that will land on the stigma underneath. Usually within two to three days, pollination will reach completion with the fusing of a single sperm with the egg, and the first stage of berry development begins with cell division. Typically, a 30% success rate for the cluster is considered good. If every flower turned into a berry, that would be one problematic, crowded cluster.

Grape Cluster Counting after Fruit Set

First stage of berry development after flowering.

Grape cluster counting to determine yield estimates before thinning clusters has begun in a few of the Merlot blocks and will continue throughout the remainder of the vineyards once kicker canes are removed, leafing has been performed, and of course, when fruit set is complete. Now that the fruit has set in nearly all of our vineyard blocks, however, it looks like we are so far getting a free ride from Mother Nature, but you know what the bumper sticker says about free rides. The adjacent photo illustrates fruit set and the beginning stage of berry development. The little brown dot on each berry is the stylar remnant where the pollen grains first landed. The stamens, as depicted in this photo, have since dried up and fallen off, except for one that is visible in the upper center.

View Fruit Set and Development Photo Gallery

Learn more about Grapevine Flowering