Grapevine blooming, scientifically known as anthesis, is a significant stage in the annual growth cycle of the vine. This brief period shifts the plant’s focus from vegetative growth (leaves and shoots) to reproductive growth. The successful completion of anthesis determines the potential for a harvestable crop later in the season. Understanding this process is central to viticulture because it sets the foundation for the eventual yield and quality of the grapes. Vineyard managers worldwide closely monitor the bloom period.
Seasonal Timing and Environmental Triggers
Grapevines do not follow a fixed calendar date for flowering; the timing is a direct response to accumulated heat following spring bud break. This stage typically occurs in late spring or early summer, usually spanning eight to ten weeks after the first buds swell. Viticulturists track this heat accumulation using a metric called Growing Degree Days (GDD). GDD measures the daily average temperature above a specific baseline, typically \(10^\circ \text{C}\) (\(50^\circ \text{F}\)) for grapevines. The vine must reach a certain GDD threshold before flowering can initiate.
The onset of bloom requires sustained, warm temperatures, generally when the average daily temperature remains between \(15^\circ \text{C}\) and \(20^\circ \text{C}\) (\(59^\circ \text{F}\) and \(68^\circ \text{F}\)) for several days. Warmer regions accumulate GDD more quickly, leading to an earlier flowering window compared to cooler, marginal climates. Once conditions are met, the entire flowering period for a vineyard is usually brief, often lasting only five to seven days. However, cool weather can slow the process, extending the bloom period and increasing the risks associated with this sensitive phase.
The Biological Process of Grapevine Flowering
Grape flowers are almost inconspicuous and do not resemble the large, colorful blooms of many other plants. The flowers develop in tightly packed clusters called inflorescences, which resemble small, green bunches of grapes before they open. Each individual flower is protected by a cap formed by the fusion of the five petals, known as the calyptra.
The actual blooming event, anthesis, begins when the calyptra separates from the base of the flower and is shed. This shedding exposes the reproductive organs: the female pistil in the center and the five male stamens surrounding it. The stamens release pollen, which is then deposited onto the stigma of the pistil, initiating fertilization.
Cultivated grapevines (Vitis vinifera) possess perfect flowers, meaning they contain both male and female parts, making them largely self-pollinating. Unlike many fruit crops that rely heavily on bees, grapevines require minimal assistance from insects or wind for successful fertilization. This self-pollination often occurs before the cap is even fully shed, a process known as cleistogamy. The flower is receptive for only a short window after the calyptra falls, and the speed of pollen tube growth is highly dependent on ambient temperature.
Impact on Fruit Set and Annual Yield
The successful transition from a flower to a tiny berry is called fruit set, and this stage determines the potential size of the year’s harvest. Not every flower is destined to become a grape; typically, only 20 to 50 percent of the flowers on a cluster successfully set fruit. The remaining unfertilized flowers naturally drop off the cluster, a form of self-regulation by the vine.
Adverse weather conditions during bloom pose the greatest threat to a vineyard’s final yield. Cool temperatures, heavy rain, or high humidity can physically prevent the calyptra from shedding, hindering self-pollination. Rain can also dilute the stigmatic fluid, interfering with the germination of the pollen grains. When fertilization is poor, two main problems can occur, both resulting in a reduced crop.
Coulure (Shatter)
Coulure, or “shatter,” is the term for flowers that fail to set fruit and subsequently drop off the cluster. This results in loose, straggly bunches with fewer berries than normal, directly impacting the total quantity of grapes harvested.
Millerandage
The second condition, millerandage, occurs when the flower is poorly fertilized, resulting in a seedless berry that remains attached to the cluster but fails to develop to full size. These small, “shot” berries ripen unevenly and can complicate the winemaking process.