This is the second stage in the annual vegetative cycle of the vine after flowering (see also BBCH code). The fertilised female ovaries of the flowers develop into berries. In the northern hemisphere, this occurs in the months June to July, in the southern hemisphere from December to January. The amount of berries depends, among other things, on the grape variety. As a rule, about one third (e.g. with Chasselas and Riesling) to a maximum of two thirds (e.g. with Silvaner) of the individual flowers are fertilized. One berry contains up to five (rarely six) grape seeds, which can be used as seeds for breeding new grape varieties by sowing and selection.
Unfavourable environmental conditions such as frost (late frost), diseases and pests or virus infections may result in poor fruit set. Then the unfertilized or insufficiently self-pollinated seed plants no longer mature into vine seeds. The unfertilized flowers trickle (French: coulure), which means that they simply fall off after flowering or wither to small, virgin, seedless berries(small berries, French: millerandage).
In the berry development phase I (cell division phase) the first phase of berry thickness growth takes place. Depending on the variety and vintage, it lasts for a period of 5 to 6 weeks. The condition of the fully grown but still hard and acidic berries, which cover the stem framework, is called grape closure (right picture). In the berry development phase II (suspension phase = standstill) there is a stop in growth purely externally. In this phase the build-up or reduction of the acids to a certain level begins. The duration of this phase is very much dependent on the variety, ranging from a few days to three or even four weeks in the case of Riesling. The next stage of development after these two phases is berry development phase III; see Véraison (start of ripening). A list of grape variety-specific keywords is included under Grapevine.