Growing Table Grapes in a Temperate Climate (E2774)
November 11, 2015 - Author: Thomas Zabadal
Table grapes have become a very popular fruit in the United States. Annual per capita consumption of table grapes in the United States has quadrupled over the past quarter century. It now exceeds 7 pounds per person, and grapes rank fourth after bananas, apples and oranges. Improved storage technology and extensive winter importation now make table grapes available year round.
Though the major U.S. table grape production regions are in warm climates such as the San Joaquin Valley of California, many areas with temperate climates have potential for exciting and profitable production of flavorful table grapes. Temperate climates are those that may have a warm growing season but have other temperature-related characteristics that challenge grape production. These may include frost-free growing seasons of 165 days or less, winter minimum temperatures of -5ºF or lower, and growing season heat accumulations of 3,000 growing degree-days or less (base 50ºF). Breeding programs have developed table grape varieties well suited to temperate climates. With the proper varieties and good management, it is possible to grow flavorful, high quality table grapes. This publication guides growers to that goal and complements several other publications in this series: Vineyard Establishment (Zabadal, 1997; Zabadal and Andresen, 1997), Table Grape Varieties for Michigan (Zabadal et al., 1997) and Pest Control in Small Vineyards (Zabadal, 1999). Sources of these publications are listed in Appendix A.
Section I – The Structure of a Grapevine
Whether you are a backyard viticulturist with no grape growing experience or a veteran grape grower, all efforts to grow table grapes must begin with an understanding of the aboveground structure of a grapevine.
Shoots, Laterals, Trunks, Arms and Cordons
The structure of a grapevine is really quite simple. Trunks and arms are the rough-barked, semi-permanent woody parts of the vine. A trunk is easy to identify because it originates at or near the ground like the trunks on other woody plants (Fig. 1a). There can be one, two or several trunks on a grapevine. When a trunk divides into branches, these branches are called arms. Arms are rough-barked portions of the vine other than the trunk(s). Sometimes a trunk is bent so a horizontal arm runs along a trellis wire. This horizontal arm is then called a cordon (Fig. 1f). New vine growth begins each year with primary shoots, which are the elongating green tissues with leaves. Primary shoots often develop side branches, which are called lateral shoots. As a vine matures in the latter part of the growing season, shoots become woody from their base outward. After the leaves fall from the vine, these smooth, tan or brown woody vine parts are called canes. Lateral shoots that become woody are called lateral canes or simply laterals (Fig. 1f). As the shoots are maturing into woody canes in the latter part of the growing season, the canes from the previous year are developing a rough-textured bark. After the leaves fall from the vine in the fall, the rough bark characteristic of the 2-year-old or older portions of the vine distinguishes them from the smooth-barked current-year canes.
Nodes, Internodes, Fruiting Canes and Fruiting Spurs
Canes, the smooth-barked woody parts of the vine, are very important to the grower. They have enlarged areas along them called nodes (Fig. 1c). A node is a compound bud composed of: a primary bud, which typically produces two-thirds or more of the fruit; a secondary bud, which produces up to one-third of the fruit; and a tertiary bud, which produces little or no fruit (Fig. 2a). The smooth areas of a cane between the nodes are called internodes (Fig. 1c). Canes may be pruned to varying lengths. If they are pruned to one to three nodes, they are called fruiting spurs. Canes left longer and pruned to four or more nodes are called fruiting canes. Fruiting spurs are seldom used for table grape production in a temperate climate because the nodes at the base of canes often produce small, straggly clusters. Therefore, fruiting canes with at least 6 and up to 15 nodes are typically used for temperate climate table grape production (Fig. 1f).
Renewal Zone, Renewal Spurs and Vine Space
A vine must be managed to occupy its own specific space along a vineyard trellis. The portion of a trellis reserved for a vine is called its vine space. A vine is contained within its vine space through a combination of vine management practices including choice of training system, pruning, shoot positioning and fertilization. The renewal zone of a grapevine is the area within that vine space from which fruiting canes (or fruiting spurs, when these are used) originate. The precise location and shape of a renewal zone will depend on the vine training system being utilized. For example, when a modified 4-arm Kniffin training system is utilized (Fig. 3b), the fruiting canes originate from a renewal zone in the middle of the vine space near the top wire of the trellis. Therefore, in this example, an experienced pruner's eyes will concentrate on that portion of the vine to locate the desired fruiting canes. The concept of a renewal zone is important because it focuses the pruner's search for fruiting canes. There is no need to comprehend the entire tangled structure of a vine to prune it properly.
There is no guarantee that a pruner will find fruiting canes in the renewal zone if previous pruning practices have not encouraged shoot growth in that area of the vine. Therefore, a grapevine pruner must manage the structure of a vine not only for fruiting in the coming growing season but also to preserve the form of the vine for future years. Managing the structure of the vine for future years is accomplished by creating renewal spurs. These are canes in the renewal zone of the vine that are not chosen for fruiting but are pruned to one or two nodes (Figs. 1f & 3b) to promote shoot growth. The expectation is that some of the shoots arising from renewal spurs will mature into quality fruiting canes for the following season. Retaining ample renewal spurs makes pruning easier in subsequent years. However, it is also possible to go “spur crazy”. Saving too many renewal spurs causes shoots in the renewal zone to become crowded and develop poorly. Therefore, a general guideline is to create one renewal spur for each fruiting cane left on the vine.
Suckers and Trunk Renewals
The trunk of a grapevine may remain healthy for decades or become diseased or winter-injured after just one or two years. Therefore, the trunks of vines need to be managed for the specific conditions of a vineyard. Even under the best of conditions, it is good management to replace trunks every 10 to 12 years in a temperate-climate vineyard. Many situations will require more frequent trunk replacement. Establishing new trunks is accomplished by first managing suckers, which are shoots that develop from belowground or near the ground on trunks. (Some prefer to use the term “water sprout” for shoots that develop on the bases of trunks.) When a cane that has matured from a sucker is chosen to establish a new trunk, it is called a trunk renewal (Fig. 4). Suckers growing directly from the ground are preferred for trunk renewals on ungrafted vines. When they are not available, utilize canes originating on trunks as close to the ground as possible. On grafted vines, canes from suckers growing above the graft union must be utilized for trunk renewals. Grapevines grown in a temperate climate should typically be managed with two trunks because it is easier to combat the effects of winter injury to vines when each of two trunks services half of a vine space (Fig. 1d). Managing vines with up to four trunks per vine and with one or more of these trunks being replaced each year may be helpful when growing very cold-tender varieties (Fig. 4).