Annual Growth Cycle of Grapevines - Flowering

Flowering

Depending on temperatures, 40–80 days after bud break the process of flowering begins with small flower clusters appearing on the tips of the young shoots looking like buttons. Flowering occurs when average daily temperatures stay between 15–20 °C (59–68 °F) which in the Northern Hemisphere wine regions is generally around May and for the Southern Hemisphere regions around November. A few weeks after the initial clusters appear, the flowers start to grow in size with individual flowers becoming observable. It is during this stage of flowering that the pollination and fertilization of the grapevine takes place with the resulting product being a grape berry, containing 1-4 seeds.

Most Vitis vinifera grape vines are hermaphroditic, with both male stamens and female ovaries, being able to self-pollinate. At the beginning of the flowering process the only part that is visible is the fused cap of petals known as the calyptra. Shortly after the calyptra is shed, liberating the pollen from the anthers of the stamen. Wind and insects generally play only a small role in aiding pollination, with the process being mostly self-contained within the vine. But cross-pollination between vine species is possible as in the cases of the origins of several grape varieties like Cabernet Sauvignon (a cross pollination of Cabernet Franc and Sauvignon blanc) and Petite Sirah (a cross of Syrah and Peloursin). During the process of fertilization, the pollen fertilizes the ovary which produces seeds as the flower begins the transformation into a grape berry, encapsulating the seed. Detrimental weather (cold, wind & rain) can severely affect the flowering process, causing many flowers not to be fertilized and produce a group. It is during this time when the buds that will become next years crops begin to form.