Article written by Madeline Wimmer, UMN Extension Fruit Production Educator and Amaya Atucha, UW-Madison Professor and Department Chair
Introduction to cold hardiness
Cold climate hybrid grapes grown in Minnesota (ex. Frontenac, Marquette, Itasca, and Clarion) are better able to survive the harsh winters in the Upper Midwest than European Vinifera grapevines, though they are still at risk of cold damage. How successfully grapevines make it through the winter relates to their cold hardiness.
Plant cold hardiness is the vine’s ability to survive freezing temperatures, and it changes throughout the season (Figure 1). In the fall, grapevines acclimate, gradually increasing their hardiness as they are exposed to cooler temperatures and shorter days, which causes a number of different physiological changes within the plant and its cells that make it more resistant to cold damage. They reach their maximum hardiness in midwinter, when some cold hardy hybrid cultivars can tolerate temperatures as low as 20 degrees below zero.
Because cold hardiness is dynamic, grapevines are vulnerable to freeze injury at many points from fall through early spring. Thus, it's important to note that vines can be injured any time air temperatures drop below the level of cold hardiness achieved.
- Fall: Damage can occur during a warm fall if vines have not yet acclimated and a sudden cold snap brings temperatures below their hardiness threshold.
- Midwinter: Injury can also occur in midwinter if an extreme cold event drops temperatures below even their maximum hardiness.
- Late winter/early spring: In late winter and early spring, warm spells trigger deacclimation, and vines begin to lose hardiness. At this stage, temperatures that would have been harmless in midwinter can cause significant damage.
Because the level of cold hardiness that a grapevine achieves depends on the cold temperatures it has experienced, as vines are exposed to progressively lower temperatures, their tissues develop greater hardiness, and this relationship allows us to measure and predict their tolerance to cold.
Methods to evaluate how cold hardy grape buds are at any point during the winter have been established since dormant buds are the most vulnerable tissues to freeze injury. One of the most widely used techniques is called differential thermal analysis (DTA), which measures the temperature at which buds freeze and are injured.
The LT50 is the temperature at which 50 percent of buds are expected to suffer cold damage, which can be estimated based on the DTA measurements mentioned above. The LT50 is used as benchmark that reflects the level of cold hardiness, because it provides a clear threshold: if air temperatures fall below the LT50 for more than about 30 minutes, we expect injury to occur. Just like cold hardiness is dynamic and changes, so does the LT50 throughout the dormant season as buds acclimate in the fall, reach maximum hardiness in midwinter, and then lose hardiness again during deacclimation in late winter and early spring.
Because cold hardiness is strongly influenced by air temperature, we can use weather data to develop models that predict bud cold hardiness throughout the dormant season. These prediction models have now been developed for many grape cultivars, including cold hardy hybrids, and they allow us to estimate the LT50 on any given day based on recent and forecasted temperatures.
For growers, these tools are extremely valuable. They help anticipate whether upcoming cold spells may cause damage, allowing growers to take protective actions when possible. They also help assess whether damage may have already occurred after an extreme event, which can guide decisions such as adjusting pruning levels to compensate for bud loss. Having access to real-time cold hardiness predictions gives growers a clearer picture of winter injury risk and improves their ability to manage vineyards during challenging weather conditions.
Grape Cold Hardiness Risk Assessment tool
A joint effort between researchers at the University of Wisconsin–Madison and Cornell University led to the development of the Grape Cold Hardiness Risk Assessment tool, also known as ColdSnap. This decision-support tool provides growers with real-time information about freeze risk for many grape cultivars and vineyard locations.
ColdSnap is available through the Network for Environmental Weather Applications (NEWA) and works with any NEWA-connected weather station. It can also be used by entering local coordinates, allowing the model to run on gridded weather data. This makes it accessible to vineyards across different regions, providing an easy way to track predicted cold hardiness levels and anticipate potential freeze damage throughout the dormant season.
To use the Grape Cold Hardiness Risk Assessment tool follow these steps:
Location details: Choose whether you want the model to use data from a NEWA weather station or from Grid Data.
If you select a NEWA station, simply pick the station closest to your vineyard and the model will use real-time weather data from that location.
If you choose Grid Data, enter your vineyard’s coordinates and the model will use interpolated weather information generated by the Northeast Regional Climate Center. This option is especially useful for growers who do not have a NEWA station nearby or for assessing a potential new site for a vineyard
Model details:
Select either Fahrenheit or Celsius temperature readings.
Then choose the date you want the model to run through. Every time you run the tool, it automatically starts on July 1 of the current growing season and calculates cold hardiness from that date up to the date you select. The model will display the LT50 for the entire period, allowing you to see how bud cold hardiness has changed over time. You can choose a past date if you want to check whether damage may have occurred, or you can select the current date to view today’s conditions. If your selected date includes forecasted days, the model will also project LT50 values into the future using the weather forecast.
Next, select the grape cultivar you want to evaluate; you can repeat the process for additional cultivars as needed. You also have the option to download the dataset for your records.
View results: After you have selected the location, time period, and cultivar, the visual results panel will display a graph with temperature on the Y-axis and dates on the X-axis. The graph shows the maximum daily temperature for the entire period as a dashed dark grey line and the minimum daily temperature as a solid line. You will also see a red dashed line labeled LT50, which represents the model’s prediction of the temperature at which 50 percent of the buds would be expected to experience cold damage.
If the minimum temperature line touches or drops below the red LT50 line at any point, this indicates that bud damage may have occurred during that period. A vertical blue dashed line marks the current date, helping you quickly identify where today falls within the selected period.
Freeze risk assessment: This section displays the forecasted freeze risk for the date you selected. A colored bar will indicate Low, Medium, or High risk for bud damage based on how forecasted temperatures compare to the LT50.
You will also see a series of small dots, each representing a future day; clicking on them will show the predicted risk for up to six days after your selected date. These risk levels are generated using the weather forecast, allowing you to anticipate potential cold events in the coming days.
When and how to respond based on the model
When you notice there is a high risk of bud damage, you might not need to check bud survivability at the moment, especially early in the season, but take note of it and make sure to do an assessment before you start to prune for the season.
We don’t recommend using this tool everyday unless you find it necessary, but refer to it as needed, like when you suspect a cold snap might be on its way. The UMN Fruit and Veg News is also planning to do several reports this winter at relevant times to keep growers informed and keep it in mind for your region and cultivars.
Important note about spring frost:
This model predicts the cold hardiness of dormant buds, meaning there is no active growth. Once buds begin to swell and break, tissues lose hardiness rapidly and become susceptible to a different type of injury: spring frost damage to green, actively growing shoots. The temperature thresholds for damage in these growth stages are not the same as those for dormant buds and are not captured by this model.For information on critical temperatures for green tissues after bud break, growers should refer to the resources compiled by Michigan State University:
https://www.canr.msu.edu/grapes/weather_climate/how-cold-can-grapes-go
To learn more in-depth about this tool, check out this previous webinar:
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