The beginning of this stage in the annual vegetation cycle of the grapevine is about six to eight weeks after budbreak. The exact time depends on the year-specific temperature development, the temperature totals reached at the respective location as well as the vine variety-specific growth under changing weather. Constantly warm temperatures with a good water supply accelerate the speed of shoot growth and thus the formation of the inflorescence (lat. inflorescence, depending on the country and region also flower, umbel, dotzen or geschein) on the third to seventh shoot node. Depending on the zonal climate, microclimate and solar radiation, flowering takes place in the northern hemisphere from mid-May to late July (in Central Europe, according to old winegrowers' rules, it starts on 24 June = St. John's Day, plus or minus eight days), in the southern hemisphere from November to mid-December. In contrast to many other flowering plants, the individual flowers of the vine are small, greenish-yellow and rather inconspicuous due to the absence of petals and show flowers. The individual grape berries form from the individual flowers when the fruit ripens.
Beginning of flowering
In the pre-flowering stage, the longitudinal axis of the inflorescence first stretches, then the lateral branches with the still closely packed, closed individual flowers spread out. With increasing development, the individual flowers detach themselves from the inflorescence, swell and change colour to greenish-yellow shortly before the actual start of flowering. Flowering starts when the first petals (perianthium), which have grown together to form a flower cap, separate from the flower base in dry, warm weather. When humidity is low, the flower capsules are shed by means of an ingenious jumping mechanism so that the honey-scented stigmas are exposed and the stamens can spread. At this point, late frost would have an extremely negative effect.
The individual flowers of the cultivated vine used in viticulture are hermaphroditic or bisexual in most grape varieties. This means that they contain both the stamens with the male gametes in the pollen grains (= pollen) and the ovary with the female ova and the stigma to be pollinated (see also in detail under Flower Bud). The stage of full flowering is reached when about half of the flower capsules have been shed. However, the individual flowers on the clusters never blossom at the same time, but in a somewhat delayed sequence, just as the individual clusters of the vine, depending on exposure (sunlight) and position on the fruit shoot, do not begin to blossom at the same time, but with a time delay.
Flowering duration & flowering rate
The total flowering period is normally about eight days, but it can also last up to three weeks in bad, wet and cold weather. With increasing global warming or climate change, there is a trend towards earlier flowering dates and shorter flowering times. The number and size of the clusters, as well as the number of individual flowers, were already established a year earlier during the formation of the winter bud (see under initiation). On a fruit shoot there are usually one to three (up to a maximum of five) inflorescences in the form of a so-called panicle (actually, therefore, the term raceme is not correct for the inflorescence of the vine). After flowering, pollination and fertilisation, the ovary develops into a berry and the cluster into a grape. On average, each grape has about 150 berries. However, it depends on the success of...
