Grapegrowers have traditionally fought seasonal frost with smudge pots and vineyard heaters, wind machines and water sprinklers, particularly if they had to plant in unfavorable sites like swales and valleys.
Yet increasing restrictions on pollution and noise, the cost of energy and shortages of water are making those options less attractive just as climate changes create new challenges. Growers in hot areas are feeling climatic changes, too, with some areas getting too warm for the vines that once thrived there.
One aspect of frost (and heat) protection little discussed is vine cordon height. It turns out that differences of even a few feet can create a 1° or 2°F difference in temperature—enough to make the difference between sustaining frost damage or not. The same difference could also help growers in hot climates.
Mark Battany, the University of California Cooperative Extension
viticulture/soils farm advisor for San Luis Obispo and Santa Barbara counties, has studied this phenomenon and recently reported on it at the Napa Valley Viticulture Technical Group.
He also described research that could optimize the use of wind machines. This is a summary of that research he presented in two papers.
Vine training height
Air temperature generally varies with height above the ground; the temperature is often colder near the ground at night and hotter near the ground during the day. For this reason, the choice of vine training height may significantly influence the temperature microclimates experienced by sensitive vine tissues during critical spring frost and summer heat periods.
Battany initiated a pilot project that measured temperatures above ground at one location in 2012 to help quantify how much the air temperature can change due to small differences in height.
He stated, “More thoroughly developed regional information will help growers choose the optimum vine training height for their growing conditions.”
Because it’s expensive to modify vine configurations once they are mature, it is best to do it right from the start. Battany’s work could help growers train their vines to the optimum conditions in their area.
Battany pointed out that growers have traditionally trained vines very differently in countries with long histories of wine grape production.
“These traditional practices likely evolved for a wide variety of reasons, which made good sense in that particular time and place; however, some of these traditional practices may currently be employed in areas where other designs would be more suitable considering the local climate conditions.”
He noted that air temperature has an important role on vine productivity and fruit quality. “Frosts damage young growth in the spring or mature canopies and fruit in the fall, while excessively hot summer temperatures can degrade fruit quality.
“During cold spring or fall nights, particularly when the skies are clear with little wind, temperature inversions often form, resulting in colder air temperatures near the ground surface. During the daytime with full sunshine, the opposite occurs, with significantly warmer air temperatures occurring nearer the ground surface.”
Vertical temperature gradients
Battany found that relatively little information exists about temperature variation above the ground at heights commonly used for training grapevines. Of particular interest is better understanding of the vertical temperature gradients.
These gradients are simply indications of how much the temperature changes for each increment of height above the ground, in units such as degrees per foot of elevation (°F/foot.)
On a spring frost night, the gradient will often be positive, meaning that temperatures become warmer with increasing height; on hot summer days the gradient will often be negative, meaning temperatures become cooler with increasing height.
Battany has conducted tests since late March at a vineyard west of Paso Robles, Calif. This site is located in the lower area of an open valley, surrounded by low hills. Three measurement masts, each eight feet tall, were outfitted with temperature sensors at 12-inch increments between 1 and 8 feet, with each set to read the air temperature every five minutes.
Measurements were initially taken in a mature vineyard block; after the spring frost period, measurements were taken in an adjacent fallow block with tilled bare soil.
Battany found a tendency for low-level temperature inversions to form at this site on the coldest frost nights of early April. The largest temperature gradient was observed between the lower heights (from 1 and 4 feet), while the temperature gradient was smaller at taller heights (between 4 and 8 feet.)
Thus, for the purposes of reducing frost damage, the largest gains in temperature would be attained by increasing the height of very low vines; the warming benefit becomes gradually smaller as the vine height increases.
He stated, “These small increases in temperature may be the difference between vines experiencing frost damage or not. Consider an example of raising the vine training height from 3 feet up to 5 feet; this would result in temperatures being about 1°F warmer for the taller vines on frost nights.
The opposite pattern was observed for the temperature gradients during hot afternoons in early August. “Notably hotter air temperatures occurred near the ground surface, with temperatures becoming cooler with increasing height above the ground.”
Again, the magnitude of the temperature gradients was greatest at the lowest heights, with diminishing magnitude as the height increased. Thus, for purposes of reducing high fruit temperatures, the largest gains will likely be attained by raising the height of very low vines, while less benefit will be achieved by increasing the height of taller vines.
The magnitude of the gradients under sunny daytime conditions was roughly twice as large as those observed under nighttime freezing conditions.
Fallow field relative to sparse canopy
As these summer temperatures were measured in a fallow field, there were no vines or other transpiring plants present. Under such conditions, much of the incomin g solar energy is converted to heat. If actively transpiring vines or other plants were present, a considerable portion of this incoming solar energy would instead be used in the evaporation of water, resulting in less heating of the soil surface and the nearby air. Thus, in vineyards with very sparse canopy coverage and clean tilled soil, the daytime temperature conditions may be similar to those measured in this fallow field. However, as the vine canopy coverage increases, and more of the incoming solar energy is used for evaporating water from the vines, the daytime temperature gradients likely will be less pronounced than those measured at this site.
Battany noted that with increasing adoption of mechanized farming practices, many growers are finding that the taller vine-training systems typically employed in these systems are more economical and efficient to farm, irrespective of any potential temperature benefits. He stated, “Many of these mechanized systems now use cordons around 5 feet high; it may be worthwhile to evaluate if even taller trellis heights are practical to farm with the currently available equipment, if future studies indicate that such heights provide useful temperature benefits.”
It should be noted that traditionally, growers in hot parts of Italy and Spain, for example, grew many grapes in overhead trellises, though the usual reason given was shading the grape clusters.
Battany’s research suggests that useful temperature benefits can be attained by avoiding excessively low vine-training heights in areas subject to spring frost damage and high summer heat.
In very cool coastal areas with little frost risk and insufficient summer heat, a lower vine-training height may be more desirable, as it may help increase the temperature to a useful extent in seasons when ripening is difficult to achieve.
However, for more inland areas with appreciable frost risk and concerns about too much summer heat, there may be considerable benefits to be gained by shifting toward vine training heights that are taller than those currently used in many vineyards.
To better understand how these temperature conditions vary by region and vineyard layout, Battany is planning to conduct similar measurements throughout 2013 at a larger number of locations.
Assistance in the project was provided by UCCE staff research associate Gwen Tindula.
UCCE regional temperature inversion study
The UC Cooperative Extension is also currently conducting a three-year study of springtime temperature inversion conditions in vineyard regions throughout Sonoma, San Luis Obispo and Santa Barbara counties. The project is funded by grants from the American Vineyard Foundation
and the CDFA Specialty Crop Block Grant program.
The purpose of this study is to quantify the temperature inversion conditions that occur during the springtime frost period and to use this information to estimate the potential efficacy of wind machines for frost protection in these counties.
With water supplies for sprinkler frost protection becoming less available in many areas, growers need to look for suitable alternatives. Improved information about inversion conditions will help ensure that industry investments in wind machines provide a predictable benefit.
Sonoma County viticulture farm advisor Rhonda Smith is assisting Battany in this study, as is Rick Snyder, extension biometeorology specialist with the U.C. Davis Department of Land, Air and Water Resources.
Get more information including the full papers at cesanluisobispo.ucanr.edu/viticulture