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 around may when average daily temperatures stay between 15–20 °C (59–68 °F). 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.
Each flower has the potential to form a single grape berry. Frost and wind are concerns at this stage of development. When there is danger of frost, vintners and growers take steps to protect the tender young shoots, including the use of large fans to circulate the cold air; sprinkling the vines with water to coat them in a blanket of protective ice; and use of heaters to warm the air temperature in the vineyard.
The stage of fruit set follows flowering almost immediately, when the fertilized flower begins to develop a seed and grape berry to protect the seed. In the Northern Hemisphere, this normally takes place in May and in the Southern Hemisphere in November.This stage is very critical for wine production since it determines the potential crop yield. Not every flower on the vine gets fertilized, with the unfertilized flowers eventually falling off the vine. The percentage of fertilized flowers averages around 30 but can get as high as 60 or be much lower. Climate and the health of the vine play an important role with low humidity, high temperatures and water stress having the potential of severely reducing the amount flowers that get fertilized.