This stage of the grapevine 's annual vegetation cycle begins six to eight weeks after budding. The timing depends on the year-specific temperature development, the temperatures reached at the respective location and the typical growth of the variety under changing weather conditions. Constantly warm temperatures with a good water supply accelerate shoot growth and thus the formation of the inflorescence (Latin: inflorescence, also known as the flower, umbel, dome or spur) on the third to seventh shoot node.

1 = open vine flowers after shedding the cap with the sticky stigma secretion at the tip (female), where the pollen/seeds (male) stick and initiate fertilisation; 2 = grape

Hermaphrodite flower

The flower bud of the cultivated grapevine is bisexual. Around 99% of all grapevines are hermaphrodite flowers whose two sexes are united in one organ. The male sexual organs are the stamens or stamina, which consist of the stalk-shaped filament and the yellowish anthers at the tip. The entirety of the stamens of a flower is called the androecium. An anther contains four pollen sacs, where the pollen grains with the male gametes (pollen, pollen) are formed. In the flower, five of these free-standing stamens surround the female reproductive organ in a circle.

The female reproductive organ consists of two fused carpels, which contain two compartments with ovules inside. This is why the grapevine is an angiospermous flowering plant. The organ consists of the ovary with the female ovules and the stigma to be pollinated. This makes fertilisation much easier than with unisexual plants. In 99% of cases, self-pollination (self-fertilisation) occurs so that no external help or influence (such as wind or bees) is necessary.

The picture on the left shows a closed vine flower, the picture in the middle shows the shedding of the caps before pollination and the picture on the right shows the flower bud in full bloom shortly before pollination or fertilisation. 1 = five-lobed corolla (cap), 2 = stigma, 3 = anther, 4 = stamen, 5 = pistil, 6 = ovary, 7 = nectaries (honey glands), 8 = calyx.

Ovary, pistil and stigma

The ovary merges at its tip into the pistil, which ends in the stigma. The disc-shaped stigma is a spongy tissue that secretes sticky secretions (picture above), which cause the pollen to stick during the fertilisation process. The pollen can come from the own flower, a flower of the same vine, a flower of a different vine or a flower of a different vine. There are five nectar glands (honey glands) at the base of the stamens and the ovary, which are fused to form a ring (discus or calyx).

Five weakly developed sepals and five green petals that have grown together to form a cap cover the stamens and ovary in the form of an inverted calyx. This yellow-green perianth (cap, caplet, perianthium) is shed at the beginning of flowering, after which pollination by pollen and subsequent fertilisation can take place.

1 = closed flower buds before the caps are shed; 2 = an open flower bud after the cap has been shed, 3 = a flower in full bloom with about half of the flower buds fertilised

Beginning of flowering

Depending on the zonal climate, microclimate and solar radiation, flowering takes place from mid-May to July in the northern hemisphere (in Central Europe, according to the old winemaker's rule, the start of flowering is on 24 June = St. John's Day, plus or minus eight days), and from November to mid-December in the southern hemisphere. In contrast to many other flowering plants, the flowers of the vine are small, greenish-yellow in colour and rather inconspicuous due to the lack of petals and showy flowers. The individual grape berries form from the individual flowers as the fruit ripens.

Pre-flowering

In the pre-flowering stage, the long axis of the inflorescence stretches, then the lateral branches with the still closely packed, closed individual flowers spread out. With increasing development, the individual flowers detach from the flower cluster, swell and change colour to greenish-yellow shortly before the actual start of flowering. Flowering begins when the first petals (perianthium), which have grown together to form a flower cap, detach from the base of the flower in dry, warm weather. When the humidity is low, the caps are shed by means of a cleverly devised spring mechanism so that the honey-scented stigmas are exposed and the stamens can spread. At this stage, late frost would have an extremely negative effect.

Flowering period & flowering rate

Full flowering is reached when around 50% of the caps have been shed. However, the individual flowers on the shoots never flower at the same time, but in a somewhat delayed...