Good to Know - Julie Tarara
Wind—friend or foe?
Trellising and training in vineyards represent money and effort invested to maximize canopy photosynthesis and to optimize solar radiation exposure at the fruit. Canopies that are symmetrical around the cordon are desired, and vineyard managers often follow conventional wisdom of orienting vine rows north-south to balance total solar radiation on both sides of the canopy. Even in symmetrically trained vines, exposure to high or sustained winds can stunt shoots and redirect growth windward, creating an unbalanced canopy architecture. Intuitively, we perceive these effects in windy locations. Previous research showed smaller fruit clusters on the windward side of vines, hypothesized to be caused by disrupted berry set. This article summarizes an assessment of shoot geometry and wind-shaped canopy architecture, and their effects on solar radiation at the fruit.
We measured shoot geometry, solar radiation at the fruiting zone, wind speed and direction at two contiguous Merlot vineyard blocks (12 years old, spur-pruned) near Paterson, Washington, a location known for consistent winds. Rows in Vineyard A were oriented northeast-southwest, about parallel to the prevailing wind, and rows in Vineyard B were oriented north-south, at an angle to prevailing winds.
Four classes of shoots were defined: north and south in Vineyard A; east and west in Vineyard B. After leaf fall, three-dimensional geometry of every shoot on five vines per block was defined using a custom compass-protractor and standard trigonometric equations. Variables included location of the shoot base along the cordon; initial and final shoot angles; initial and final shoot tip compass directions; shoot length; chord length (the shortest distance between shoot base and tip); and number of nodes. The change of shoot curvature due to gravity and wind was calculated as the difference between the shoot tip's final location and a hypothetical final location had it grown in the absence of external forces like wind.
Prevailing winds at the site are southwesterly, with gusts up to 45 miles per hour. During the experiment, 40 percent of mean wind speeds exceeded 7 mph and about 15 percent exceeded 11 mph (day and night). At about 10 mph, stomatal conductance in grapevine leaves decreases significantly. Intermittent stomatal closure can limit canopy photosynthesis and thus the supply of carbon to the fruit.
On average, shoots in Vineyard A, with rows parallel to prevailing wind, were longer than those in Vineyard B that had rows at an angle to wind, an outcome driven mostly by stunted shoots on the windward or west side of Vineyard B rows. About two-thirds of west shoots were less than 20 inches long, whereas nearly two-thirds of all shoots in Vineyard A were more than 20 inches long. Overall, windward shoots were from 26 to 29 percent shorter than leeward shoots because of fewer nodes per shoot. Consequently, one would expect proportionately less leaf area on the windward side of Vineyard B. In a dense canopy like that of juice or table grapes, stunting on one side of the canopy may be of minor concern because there are several leaf layers. But in porous canopies like wine grapes under deficit irrigation, approximately 30 percent less leaf area on the windward side of vines could result in lower daily photosynthesis and different radiation exposure at windward and leeward fruit. The windward or west canopy of Vineyard B intercepted 52 percent of the total daily solar radiation, meaning 48 percent penetrated to the fruit zone. The leeward or east canopy intercepted 75 percent of solar radiation, meaning 25 percent penetrated to the fruit zone.
Regardless of row orientation or side of the canopy, wind altered shoot growth. In Vineyard A, the rows parallel to wind, shoots were streamlined down-row roughly symmetrically about the cordon. In contrast, Vineyard B, with rows oblique to wind, the west or windward shoots grew more upward and easterly, while the east or leeward shoots grew away from the cordon and mildly northward.
We plotted wind roses, which are diagrams used by meteorologists to show the distribution of wind directions and speed, and companion rose diagrams that showed the distribution of the magnitudes and directions of shoot growth in both vineyards. The rose diagrams supported our field observations. About two-thirds of all shoot tips were displaced eastward of the hypothetical final locations where they should have grown, consistent with the west-southwesterly prevailing wind. Nonetheless, in both vineyards, most shoots did terminate on the same side of the canopy from which they had originated.
By orienting vineyard rows parallel to prevailing winds, growers may achieve a canopy symmetrical about the cordon, without expensive trellis or shoot manipulation. However, if prevailing winds prompt an east-west row orientation, nonuniform exposure to solar radiation may occur between north and south fruit.
Practical considerations of topography aside, growers must strive for a balance between managing canopies for symmetry in a windy location and sun-earth geometry or compass direction. Absent strong winds, in north-south oriented rows, daily peak and daily total solar radiation at the fruit are equal on east and west sides of the vine (see "Comparison of solar radiation received"). With asymmetrical canopy architecture, peak solar radiation at west or windward fruit was 83 percent higher than at the east or leeward fruit. Total daily solar radiation was 90 percent higher at the west fruit than at the east fruit. By contrast, in rows parallel to wind, peak solar radiation at the south fruit was 150 percent higher than at the north fruit. North fruit received approximately equal solar radiation per day as heavily shaded fruit, and less than half of that received by south fruit. The solar radiation environment of Vineyard A was more indicative of sun-earth geometry or compass direction than of direct wind effects because of the relatively uniform streamlining of shoots in this block.
What do these results mean? First, peak solar radiation at the west fruit of Vineyard B, with north-south rows, occurs at a similar time of day as maximum air temperatures. This creates potential for fruit temperature to exceed optimum temperatures for high quality fruit, although exact temperatures and heat duration for targeted fruit quality have not been established. Where excessive radiation exposure of west fruit leads to sunscald, practical solutions for sites with prevailing west-southwest winds could lie in orienting an upper wire or trellis member with a westward lean to counteract shoot tip displacement. Where prevailing easterlies cause westward shoot displacement and a greater likelihood of partially shading west fruit, wind-induced reshaping of an otherwise balanced canopy could be advantageous, possibly reducing some of the labor involved in shoot-level or foliage wire manipulations. At windy sites where shoots are stunted, growers might consider counteracting diminished growth by applying more irrigation early in the season. Alternatively, because shoots are renewed annually, canopy symmetry could be achieved by windward/leeward-biased pruning. The occurrence of sunscald on exposed south and west fruit also could be approached with targeted cluster thinning, a labor-intensive though common practice when fruit is grown for the premium market. Because row orientation should be considered both in terms of sun-earth geometry and in terms of environmental perturbations like wind, growers need to consider their canopy management comprehensively, as there is not a single prescriptive solution for vineyards at consistently windy sites.