Annual Growth Cycle of the Grapevine
Budburst and Shoot Growth
In the spring when the daily temperatures are about 50 degrees F (10°C) the previously dormant buds begin to grow producing shoots. This event is commonly referred to as budburst or bud break (See Figure 1.2). Buds are located between the vine’s stem and the petiole (leaf stem). Budburst largely depends on the number of buds left at winter pruning. If too few buds are retained, yields may be reduced to below what the vine would otherwise have the capacity to ripen. Shoot growth may be excessively vigorous because of a lack of competition with the fruit and other shoots, and in some varieties (such as Pinot Noir) this may increase the number of suckers or nonfruitful shoots arising from older wood.
As shoots grow during the growing season, buds form in the axil of the leaf (at the base of the petiole). The single bud that develops in this area is described in botanical terms as an axillary bud or more commonly referred to as the lateral bud.
Lateral buds may differentiate into shoots during the year they are produced, but more commonly they may remain dormant. Shoots developing from lateral buds are termed lateral (summer) shoots.
The dormant bud initiates the year prior to its growth as a shoot. During the current season, the dormant bud undergoes considerable development forming the cluster primordia. Although the dormant bud looks like a simple structure, it is actually a compound bud that consists of three growing points, sometimes referred to as primary, secondary, and tertiary buds. The dormant bud is of major concern at dormant pruning since it contains cluster primordia (the fruit-producing potential for the next season).
Shoot growth begins with budburst and initially the growth is slow, but soon it enters a phase of rapid growth called the "grand period of growth," which typically continues until just after fruit set. Nearly one-half of shoot growth is attainted by flowering.
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