Passive Measures to Reduce Impacts of Late Spring Frosts
Dr. Justin Tanner, UCCE Viticulture Farm Advisor
Dr. Horst Caspari, Colorado State University Professor and State Viticulturist
Grape Cold Hardiness and Freezing Injury
Cold tolerance in grapevine is a dynamic process that changes over the course of the season in response to preceding weather exposure. In the fall, as nighttime temperatures gradually decrease, the vines acclimate and become more resistant to cold temperatures. Late ripening varieties generally acclimate slower in the fall and may be more susceptible to cold damage when unseasonably warm and wet conditions precede the first freezing event in the fall. By midwinter, the vines have attained their maximum cold tolerance and are able to withstand very cold weather. Some varieties can withstand temperatures below -7⁰F without injury. In the spring, however, as temperatures warm, growth resumes in buds.
In California’s wine-growing regions, the most common cold weather threat we experience is late spring frost. Developing buds swell and begin losing cold tolerance making them vulnerable to frost. After bud break, actively growing shoots can be damaged by brief exposure to air temperatures a couple of degrees below freezing (<30⁰F). Cultivars that deacclimate earlier in the spring are the first to go through budbreak, putting them at risk of frost damage earlier in the spring. Each cultivar will vary in the start of acclimation/cold hardiness in the fall, degree of freeze resistance in midwinter, as well as the timing of deacclimation and resumption of growth in the spring.
Types of Freezes
The type of cold weather event will affect the effectiveness of frost protection efforts so it’s helpful to identify what type of event we are dealing with. Frost events are either radiative or advective (Figure 1).
Figure 1. Frost events can be classified as either radiative or advective.
Radiative events occur on calm clear nights when more heat is lost than is absorbed by the ground resulting in a cooling effect. During a radiative frost, the coldest temperatures occur on the vineyard floor. In calm conditions, cold air which is heavier than warmer air slowly flows and accumulates in low spots. As a cold air layer builds, it displaces warmer air that is forced skyward. The development of a colder air layer below a layer of warmer is called a temperature inversion and is typical of radiative frost events. Inversion strength depends on the difference in temperature between the upper and lower layers. If the sky is cloudy during the formation of an inversion the strength is likely to be weaker because water vapor in clouds will trap heat radiating from the ground in the air. Conversely, when the night sky is clear, radiation that accumulates in the soil from the sun during the day will radiate out of the area into space creating the potential for a strong inversion having much cooler temperatures closer to the ground. Radiative frost events are short events usually only lasting a few hours and reach their coldest point just before sunrise.
In contrast to radiative freeze events which are fairly common in California’s viticultural areas, advective freeze events are thankfully much rarer. Advective freezes occur when a large mass of very cold air moves into an area forcing warmer air out of the region. Advective events are accompanied by wind and lack a distinct temperature inversion. They can occur during the day or night and may last for several days giving them the potential to be much more damaging than radiative frosts.
There are many passive practices that can help to minimize the potential impact of radiative frost events such as site and cultivar selections, increasing cordon height, maximizing the drainage of cold air out of the vineyard, managing soil to increase heat exchange, and delaying pruning. These practices are classified as passive because they do not require active energy inputs during the event to mitigate frost. In areas such as the San Joaquin Valley where frost danger is low, active methods may not be economically viable. For this reason, we will focus on management strategies that aim to avoid frost through site selection, cultivar bud break date, pruning timing, trellis height, and maximizing the heat soil exchange capacity through vineyard floor management to reduce frost severity.
Figure 2. Relationship of air temperature and wind speed at 7am on April 12, 2022 from 38 weather stations located around Lodi, CA. Data provided by Lodi Winegrape Commission and Western Weather Group (https://lodi.westernweathergroup.com/)
Spring Frost Event of 2022 in Northern San Joaquin Valley
Last spring many vineyards around Lodi experienced frost damage during the early morning hours of April 12. According to weather data available on California Irrigation Management Information System (CIMIS) stations located in San Joaquin County, air temperatures at two of five weather stations briefly dipped below freezing. Linden weather station (CIMIS #262) recorded a low temperature of 28.3⁰F while the Staten Island station (CIMIS #242) reported a low of 30.8⁰F. CIMIS station air temperature sensors are located about 4’11” (1.5m) above the ground. Freeze injury from this event was unevenly distributed with low-lying and wind-sheltered areas receiving the most damage while many vineyards escaped damage completely.
Figure 3. Example of vertical temperature differences within a vineyard during a typical inversion night in April 2007 recorded in Western Colorado.
The Lodi Winegrape Commission provides weather data from the Western Weather Group for numerous locations around Lodi and highlights the diversity of conditions resulting from individual site microclimates within the Northern San Joaquin Valley region. Looking into data from 38 individual weather stations for the April 12 frost event shows that at locations where wind speeds were lower temperatures were generally colder (Figure 2). This relationship between wind speed and air temperature is typical for a radiant frost event. Wind allows for the mixing of cold and warmer air reducing differences in temperature between measurements close to the ground and higher up. In an example of a similar frost event in Western Colorado, differences in temperature at 6” and 64” are larger when wind speed is low and shrinks when the wind increases (Figure 3).
Site Considerations
Each vineyard will have its own unique features that will influence frost potential. It is helpful to identify cold spots that are more likely to freeze. As cold air is heavier than warmer air, it will sink and accumulate at the lowest point in the vineyard during a radiative event. In established vineyards, areas of concern may be identified by prior frost damage seen in previous years. Low areas will also accumulate water after heavy rains and highlight areas where cold air can collect.
Figure 4. Zoning of vineyard blocks management based on frost potential can be useful in simplifying operations to reduce risk. In this example a low-lying portion of the vineyard is identified as having the greatest potential for the accumulation of cold air and frost damage and designated as management Block 1.
When establishing a new vineyard, careful grading will reduce the development of cold air pockets. Cultivars that break bud later in the spring can be planted in the coldest spots in the vineyard to reduce frost risk. Slope direction can also influence temperature at a site. South-facing slopes will promote budbreak earlier than north-facing slopes. In a vineyard that contains various topographical features such as slopes, valleys, and hills, management zones can be established to simplify vineyard operations (Figure 4). As the coldest temperatures during spring frosts are at ground level, increasing cordon height will increase the temperature around sensitive young shoots and clusters. Low cordons located closer to the ground will be more likely to experience colder more challenging temperatures.
Minimizing Cold Air Accumulation
It is important to not block cold air from moving out of the vineyard. Inspect the perimeter of the site for physical barriers to cold air movement. Trees, tall vegetation, fences, and buildings can block the movement of cold air. On sloped sites, removing obstructions on the low side of a vineyard can reduce frost potential. Tall ground cover can also trap cold air and lower the area of the coldest temperature from the ground up closer to the height of the cordon. Upslope of the vineyards, well-placed obstructions such as hedges or fences can be useful to reduce cold air flow entering the site.
Maximizing Heat Capture by the Soil
During the day, sunlight falling on the vineyard floor will warm the ground, which will release that heat during the night. Any practices that increase the exchange capacity of this solar radiation can help to reduce frost severity. Bare soil will collect and store heat from the sun more efficiently than soil with vegetative cover. Cover crops insulate the soil like a blanket, reducing the amount of heat entering during the day and leaving at night. Where cover crops are used, mowing a few days before a frost event will allow the soil to store and release more heat than unmowed cover crops. Soil moisture also plays a beneficial role in improving heat storage, with moist soil holding more heat when compared to dry soil. Recently tilled soils are light and fluffy having more air pockets that will reduce heat storage capacity in comparison to firmer, settled soil. Recently tilled soil should be flattened and packed to reduce this insulation effect well in advance of an expected frost event.
Delaying Budbreak
Delaying winter pruning can have a beneficial effect on delaying budbreak and extending cold hardiness. Late pruning can delay budbreak by as much as 15 to 20 days and increase the chance that sensitive tissues will avoid cold exposure by developing after the risk of frost has passed (Poni et al. 2022).
Figure 5. Prepruning of dormant vines in Fresno County. Photo by George Zhuang
Mechanical prepruning to seven or eight nodes can be accomplished at any time during the dormant season, followed by final pruning closer to budbreak (Figure 5). This practice is helpful in reducing labor requirements and is especially useful for managing large vineyards in a timely manner. Final pruning can be delayed up to the unfolding of the second or third leaf on apical shoot positions with little impact to yield. If pruning occurs after this stage however, it can result in reduced yield, delayed harvest date, and additional strain on the vine’s stored carbohydrates reserves which may affect its ability to respond to stress in the future. Despite the challenge of accomplishing the final pruning in a timely manner, delayed pruning carries the additional benefit of reducing the chances of infection by trunk diseases which will enter through pruning wounds. These wounds heal slower in cool conditions which allows them to be susceptible to infection for a longer period. In large vineyard operations where labor constraints may affect the ability to prune the entire vineyard on time, pruning can begin with the least at-risk areas reserving low-lying and wind-sheltered locations for pruning last. In flat terrain, consider pruning earlier bud-breaking cultivars after cultivars that break bud later.
Figure 6. The extent of frost damage is difficult to fully evaluate visually immediately after a late spring frost event. A) Wilted shoot tip after freezing and thawing is still green. B) A few days after a frost event damaged tissues show oxidative browning on leaves and clusters. Photos by George Zhuang
Assessing Damage After Spring Frosts
After a spring frost event, the damage will not be immediately noticeable. Once temperatures return to seasonal norms, the injury will usually take two to three days to fully express itself (Figure 6). Freeze damage on green tissues such as shoots, leaves, and clusters occurs due to ice formation in cells damaging cell walls. This allows phenolic compounds to leak out of cells and oxidize resulting in brownish discoloration of affected tissues. Once evident, damage can be assessed by counting the number of undamaged shoots of each cultivar in the vineyard. If damage is uneven across the vineyard, assessment based on zones grouped by damage level can give a more accurate account of potential losses. When frost injury occurs on shoots shorter than five inches, clusters are more likely to be damaged, reducing yield potential. Longer shoots, if not completely damaged, can still produce a crop if the bottom four to six nodes are unharmed. Death of the shoot tip however can initiate the growth of lateral buds that may crowd the fruiting zone posing additional complications.
Summary
Spring frost damage occurring after budbreak is the most common cold weather hazard faced in California’s wine-growing regions.
Damage to young shoots can occur at temperatures just below freezing.
While each site is different, understanding the unique features of your vineyard can inform management.
Promoting cold air drainage and increasing soil heat exchange capacity will reduce frost severity.
Delaying final pruning can reduce frost risk by delaying budbreak and extending cold hardiness a few days longer.
Additional Frost Resources Online
Department of Land, Air and Water Resources, UC Davis Frost Protection Video https://lawr.ucdavis.edu/cooperative-extension/frost-protection
CalAgroClimate http://agroclimate.org/
California Irrigation Management Information System https://cimis.water.ca.gov/
Lodi Winegrape Commission and Western Weather Group https://lodi.westernweathergroup.com/
Further Reading
Moyer, Michelle, et al. "Assessing and managing cold damage in Washington vineyards." Washington State University Extension EM042E (2019). http://pubs.cahnrs.wsu.edu/publications/wp-content/uploads/sites/2/publications/em042e.pdf
Poni, Stefano, et al. "Facing spring frost damage in grapevine: recent developments and the role of delayed winter pruning–a review." American Journal of Enology and Viticulture 73.4 (2022): 211-226.
Snyder, Richard L., and JP de Melo-Abreu. "Frost protection: fundamentals, practice and economics. Volume 1." Frost protection: fundamentals, practice and economics 1 (2005): 1-240.