Growing Grapes in Your Backyard

As with all fruits, the grape flowers must bloom first and be pollinated before the berries (fruit) begin to develop. Grape flower clusters begin their differentiation and development in dormant buds during the summer before the year that grapes are actually harvested. Flower differentiation is completed during the fall, by the time the vines lose their leaves. After dormancy and bud burst, (see the next few sections on cultivation techniques) grape shoots begin to grow rapidly. When shoots reach 4 to 6 inches (10 to 15 cm) in length, grape flower clusters can be distinguished, and by 12 inches (30 cm), the flower clusters are well-separated on the growing shoot. The development of flower clusters on the emerging shoots usually starts in mid-March, and the flowers typically begin to bloom in May. Grape flowers have a protective cap, known as a calyptra, which covers the male and female sex organs until bloom. The calyptra cracks open from the bottom and exposes the flower parts to pollination, which must occur before berries can develop. Because of their size and berrylike shape, flower clusters can be confused with developing berries before they bloom (see Figures 15.1–4, linked below).

After pollination, the male flower parts fall off (see Figure 15.5, linked below), and the grape berries begin to grow and develop. Unpollinated flowers fall to the ground during the "shatter" stage. Pollinated flowers begin to form berries that continue to grow in size and increase in sugar content until harvest (see Figure 15.6, linked below). The annual growth cycle of a typical grape vine is shown in Figure 15.7, linked below. Note that the cycle has many facets, with several events, described above, occurring simultaneously.

Figure 15.7, linked below, represents the annual growth and fruiting cycle of a grapevine during the calendar year. For emphasis, the various events for the current and following year are shown separately. Actually, many of these events occur simultaneously in a mature grapevine.

Winter-dormant buds, which are formed the previous growing season, contain the young fruit clusters (called “primordia”) as well as the stem, leaves, and tendrils that will form the first several nodes of the shoot. Each bud actually consists of a primary bud and two secondary buds. Usually only the primary bud grows, but one or both secondary buds may also grow. If the primary bud is cut off or injured, or if the shoot or cane that grows from the primary bud is cut off, either of the two remaining secondary buds, or both, may grow. Secondary buds usually produce shoots that bear fruit.

When new shoots are a few inches long in early spring, flower clusters can be seen near the shoot tips. Clusters are found on about the 3rd to 6th nodes (counting out from the trunk), and each shoot will form up to about two or three clusters. The number of clusters that typically form per shoot can vary; sometimes only one or none may form on a given shoot, and sometimes up to three may form. Sometimes late clusters appear on the shoot well beyond the main clusters; these will ripen much later and are usually fine for eating.

As each shoot grows, secondary lateral shoots often grow from the original shoot, creating a dense canopy. In some cases it may be useful to remove some lateral shoots. Many commercial growers remove  excess main shoots in the spring in order to open up the canopy, but on vigorous vines this often causes more lateral shoots to grow because sunlight is increased.

Illustration of dormant grape bud - Ingels

Annual growth and fruiting cycle of grapevine, Figure 15.7 (pdf) in California Master Gardener Handbook. UC. Buy Publication

Stages of grape flowers, Figure 15.1-6 (pdf) in California Master Gardener Handbook. UC. Buy Publication

Wine Grapevine Structure, and Cluster and Berry Size and Shape (pdf) in Wine Grape Varieties in California. UC. Buy Publication