calyptra, or flower cap, of the vine flower consists of the five petals joined together in the form of an inverted cup. The cap separates as a unit and falls from the grape flower at flowering and exposes the stamens, which produce pollen, and the stigma, which receives pollen. The rate of capfall is slowed by cold and rain; the duration of capfall can stretch from a normal seven to ten days to as long as 15 to 20 days. fruit set is impaired if the caps are retained and so this very small part of the grapevine can affect yield, especially for varieties such as merlot.
