California wine grape growing has become as “narrowingly” uncertain as it has become more economically harrowing.
Vine balance starts with good vineyard design. Although that seems like a simple concept, it's actually where things start getting complicated, according to Jim Wolpert, Cooperative Extension viticulture specialist, University of California, Davis. Especially today with so many closely spaced vines where there is so much uncertainty.
“We do not know anything about the canopy based solely on row and vine spacing,” he told attendees at a recent UCCE grape grower meeting in Paso Robles. “We have soils that are all over the map, varieties that are all over the map, and we know very little about them.”
This gap in knowledge makes the notion of matching a rootstock to a soil very difficult, according to Wolpert. “Ford can tell you the towing capacity of Ford F-250, but we can't tell you the towing capacity of a rootstock.”
Factors such as soil fertility and scion are givens at the onset of vineyard design. Given those factors, decisions such as rootstock, spacing and trellis design are still largely a guess when designing and constructing California's wine grape vineyards.
When it comes to vineyard design, the unknowns are akin to rolling the dice. “Let's say I'm a grower and I want to choose the optimum rootstock and the optimum spacing, but I still have to deal with a lot of uncertainty because I don't know much about rootstocks,” he says. “We have a lot of narrow spaced vines today. We've changed the landscape considerably, and we're still trying to sort through an immense number of variables to come up with the answers.”
The working definition of vine balance is when grapevine growth is appropriate for trellising and spacing, and the leaf area and amount of fruit are in proper proportion. There are two major contributors to vine balance, according to Wolpert. There are the conditions that are set at planting in vineyard design that are permanent and secondly, there are the conditions that are influenced by cultural practices.
Vigor is a significant dynamic in vine balance and is determined by several factors including soil fertility, rootstock and scion. Those are permanent factors, not cultural. “You can't control vigor with pruning,” Wolpert says.
There are numerous scientific measurements to determine vine balance, but they are so far removed from the spectrum of practical commercial production that they are not terribly useful in determining vine balance. However, many of those measurements are correlated with pruning weight. Even more telling is pruning weight expressed per meter (or foot) and tied to shoot number and shoot weight.
In a Sangiovese study at Atlas Peak Vineyards in Napa, Wolpert looked at three scenarios that included 12, 20 and 28 shoots per vine. The rootstock was 3309C.
“Shoot number affects shoot length,” Wolpert says. “Longer shoots have more leaf area and have a greater percentage of leaf area as laterals. Manipulating shoot number per vine does not change leaf area per vine, but changes the percentage of primary vs. lateral leaf area.”
Lateral leaf area percentage decreased as shoot number per vine increased in the trial. Pruning weight was unaffected by shoot number.
“For vines of a given vigor, decreasing shoot number redistributes leaf area from shorter shoots to longer shoots and increases the percentage of lateral leaf area,” Wolpert says. “It also decreases the leaf area to fruit weight ratio and decreases the fruit yield/cane prunings ratio.”In an Oakville Cabernet Sauvignon trial, Wolpert looked at factors affecting shoot length and vine balance. In general, conclusions were that the average shoot length is related to the number of growing points rather than vine size. Additionally, not all rootstocks have the same response. With the same number of growing points on vines of the same size, 110R and 3309C will grow more, while 5C and O39-16 will grow less. Leaf area is not affected by pruning formula (buds/weight of prunings) but shifted from fewer longer shoots to more, shorter shoots. Rootstocks like 110R would be classified as “more vigorous,” or having more leaf area.
“For sustainability, balance is best achieved at vineyard design,” Wolpert says. “We don't know as much about this concept as we should. In my opinion, we are at a greater risk of planting vines too closely than too far apart.”
Annual practices can be tools to achieve balance, according to Wolpert. However, there are drawbacks to relying too heavily on annual practices rather than good vineyard design.
“It requires inputs that can be costly,” he says. “Additionally, growers need to keep in mind that pruning is not one of the practices to achieve balance.”
When growth is too great, excessive shoot growth and shading will result. When growth is too little, shoot numbers will be reduced, affecting yield per acre.