Author: Annie Klodd, Extension Educator-Fruit and Vegetable Production
With mid-night freezes predicted for much of Minnesota tonight and tomorrow night, berry growers are assessing the risk of freeze injury to their crops and preparing to implement measures to mitigate freeze damage if deemed necessary.
In general, risk is relatively low in most cases throughout Minnesota, as bud development is not far enough along to be injured at the temperatures predicted. Nevertheless, it is important that growers monitor the temperatures at their specific site in case temperatures go lower than expected, and assess risk on an individual basis.
Below I have included information and resources for guiding this process:
To determine what bud stage your blueberries are in, and how much cold they can tolerate, see:
Blueberry Bud Stages - Michigan State University
Strawberries: Strawberry frost damage is of most concern once the blossoms are open, at which point they are damaged at 30 degrees F. We are not at this stage yet in Minnesota. Some growers may be at the tight bud stage, at which the buds can tolerate 20 degrees F. Frost damage can still occur to the leaf tips and edges.
See: Critical Bud Stages for Strawberry and Their Cold Tolerance - Cornell University
Grapes: Grapes are currently in various stages of bud swell. One grower in southwest Minnesota indicates some varieties are in late bud swell and about to break bud, while a grower in Alexandria reports early bud swell for most varieties. At early bud swell, buds of cold hardy hybrids can tolerate temperatures in the teens. At late bud swell, they are less hardy and can tolerate temperatures in the 20s. At bud break, freeze damage can occur at 28 degrees F.
Vinifera varieties, which are not hardy enough to be grown reliably in Minnesota, are less tolerant of cold temperatures than the cold hardy hybrids bred by UMN, Tom Plocher, and Elmer Swenson.
See:
Avoiding Spring Freeze Injury in Grapes - Purdue University
April 2020 Spring Freeze - Ohio State University Viticulture and Enology Newsletter
Tree Fruit: Amanda Green, tree fruit specialist at OMAFRA, just published a helpful article for the Ontario Fruit website, on tools for frost protection in apples and pears. Rather than me repeating her advice here, please visit the article:
Frost Warnings and Tools for Frost Protection in Apples and Pears
Growers should do diligent reading on this practice before implementing it. Not applying enough water, or stopping irrigation too early during a nighttime freeze, can cause more damage than not irrigating at all.
See:
Irrigation for Frost Protection of Strawberries - OMAFRA
Frost Protection: Tips and Techniques - Penn State University
Row Covers:
Grower Photos and Comments on Row Covers and Sprinkling - North Carolina State University
Cold Temperatures and Row Covers - University of Georgia
Copper: According to Ohio State University: "Copper has been shown to protect young shoots against frost injury by killing ice forming bacteria present on vine foliage. You may start spraying as soon as budbreak and repeat every 5-7 days (washes off easily and must be reapplied after an inch or more of rain) until you’re out of the frost threat period (2 – 3 weeks) in your vineyard. Read the label for the application rate. In California, 0.75 actual copper per acre was used. Read the label to avoid plant injury. To avoid injury, apply when not cold or wet (slow drying) and use formulation with lime. More info about copper is in the Midwest fruit pest management guide (p 90-91): https://ag.purdue.edu/hla/
Author: Annie Klodd, University of Minnesota Extension Educator - Fruit and Vegetable Production
With mid-night freezes predicted for much of Minnesota tonight and tomorrow night, berry growers are assessing the risk of freeze injury to their crops and preparing to implement measures to mitigate freeze damage if deemed necessary.
In general, risk is relatively low in most cases throughout Minnesota, as bud development is not far enough along to be injured at the temperatures predicted. Nevertheless, it is important that growers monitor the temperatures at their specific site in case temperatures go lower than expected, and assess risk on an individual basis.
Below I have included information and resources for guiding this process:
Assess risk: Determine what bud growth stage your plants are in, and what temperatures they can tolerate.
Blueberries: Photos I have received from Minnesota growers this week indicate that blueberries are between the early green tip stage and shoot expansion. Buds in early green tip can tolerate temperatures down to 15-20 degrees F. At shoot expansion, they can tolerate 23 degrees F. |
| Blueberry buds at early green tip stage on 5/6/2020. Photo: Tracy Linbo, Double T Acres. |
 |
| Blueberry buds at late green tip and shoot extension stages. 4/28/2020. Photo: Jim Luby |
To determine what bud stage your blueberries are in, and how much cold they can tolerate, see:
Blueberry Bud Stages - Michigan State University
Strawberries: Strawberry frost damage is of most concern once the blossoms are open, at which point they are damaged at 30 degrees F. We are not at this stage yet in Minnesota. Some growers may be at the tight bud stage, at which the buds can tolerate 20 degrees F. Frost damage can still occur to the leaf tips and edges.
See: Critical Bud Stages for Strawberry and Their Cold Tolerance - Cornell University
Grapes: Grapes are currently in various stages of bud swell. One grower in southwest Minnesota indicates some varieties are in late bud swell and about to break bud, while a grower in Alexandria reports early bud swell for most varieties. At early bud swell, buds of cold hardy hybrids can tolerate temperatures in the teens. At late bud swell, they are less hardy and can tolerate temperatures in the 20s. At bud break, freeze damage can occur at 28 degrees F.
 |
| Late bud swell on a cold hardy grape variety, 5/6/2020, Worthington, MN. Photo: Scott Ellenbecker, Round lake Winery |
Vinifera varieties, which are not hardy enough to be grown reliably in Minnesota, are less tolerant of cold temperatures than the cold hardy hybrids bred by UMN, Tom Plocher, and Elmer Swenson.
See:
Avoiding Spring Freeze Injury in Grapes - Purdue University
April 2020 Spring Freeze - Ohio State University Viticulture and Enology Newsletter
Tree Fruit: Amanda Green, tree fruit specialist at OMAFRA, just published a helpful article for the Ontario Fruit website, on tools for frost protection in apples and pears. Rather than me repeating her advice here, please visit the article:
Frost Warnings and Tools for Frost Protection in Apples and Pears
Orchard floor management impacts risk of freeze injury
How the ground is managed prior to a freeze impacts the temperature that the plants will experience during a freeze, and therefore affects the risk of freeze injury. Specifically, tilling and long cover crops cause soil to be colder. In contrast, bare, firm soil with or without a shredded cover crop are warmest.
Moistening the soil should be done at least a few hours before nightfall, so the ground has a chance to absorb heat again before the freeze event. The soil needs sunlight to accumulate heat. See the table below.
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| Table: Courtesy of Mark Longstroth, Michigan State University. Source: Swanson, F., Christensen, P., and Jensen F. 1974. Preventing Vineyard Frost Damage. An old California Coop. Ext. Bulletin |
Practices to Minimize Freeze Injury
Irrigation: Overhead irrigation is frequently used to protect berry crops from freeze injury throughout the US. It can be a very reliable and effective strategy to prevent freeze injury. Therefore, there is an expansive collection of research and Extension resources available for learning how to do it successfully.Growers should do diligent reading on this practice before implementing it. Not applying enough water, or stopping irrigation too early during a nighttime freeze, can cause more damage than not irrigating at all.
See:
Irrigation for Frost Protection of Strawberries - OMAFRA
Frost Protection: Tips and Techniques - Penn State University
Row Covers:
Grower Photos and Comments on Row Covers and Sprinkling - North Carolina State University
Cold Temperatures and Row Covers - University of Georgia
Spray Products for Freeze Protection:
KDL (0-0-24) fertilizer: I understand that some growers want to use this product for frost protection, and that some believe that they have had success with it. However, research has shown that KDL does not protect shoots against frost injury once vines resume growth. This is based on controlled, published research at Penn State University that compared the treatment against untreated vines. Therefore, I cannot recommend KDL for frost protection, as there is not adequate research-based evidence supporting it. Dr. Maria Smith at Ohio State University researched this product during her PhD work with Dr. Michela Centinari, and can be contacted directly for more information at smith.12720@osu.edu.Copper: According to Ohio State University: "Copper has been shown to protect young shoots against frost injury by killing ice forming bacteria present on vine foliage. You may start spraying as soon as budbreak and repeat every 5-7 days (washes off easily and must be reapplied after an inch or more of rain) until you’re out of the frost threat period (2 – 3 weeks) in your vineyard. Read the label for the application rate. In California, 0.75 actual copper per acre was used. Read the label to avoid plant injury. To avoid injury, apply when not cold or wet (slow drying) and use formulation with lime. More info about copper is in the Midwest fruit pest management guide (p 90-91): https://ag.purdue.edu/hla/
Author: Annie Klodd, University of Minnesota Extension Educator - Fruit and Vegetable Production
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