Madeline Wimmer, Extension Educator, Fruit Production
Image: Young grape shoots damaged by late spring frost (May 11th, 2021). Photos taken by Madeline Wimmer.
Cold stress and frost damage have the potential to impact perennial fruit crops during different times of the year, and springtime in the Upper Midwest is no exception. While cold stress can happen at warmer temperatures, frost occurs when ambient temperatures fall below freezing (32°F). When a spring frost happens, it can harm vegetation, and negatively impact bloom and fruit set. This is problematic recognizing that many perennial fruits exit dormancy and begin growing in Minnesota before the threat of spring frost has passed. Crop loss due to frost damage can be devastating and many strategies that help annual crop growers (e.g., delayed planting) are not usually possible for perennial crops.
What happens when dormant chilling requirements are ahead of schedule?
During winter seasons when more chilling hours (total hours during dormancy where ambient temperatures are between 32°F and 45°F) have accumulated than normal, many fruit crops may break dormancy and “wake up” earlier. This means there is a larger window of time between the initial bud break stage and the last day when freezing temperatures could occur. Minnesota has generally accumulated between 100 and 600 more chilling hours this year than average, with the highest deviation of chilling hours documented near the Twin Cities metro, the Arrowhead region, and Eastern Minnesota.
Image: A map comparing this year’s chilling hour accumulation to the seasonal average (records as old as 1986/87).
Critical temperatures (Tc) impact how severely a frost impacts a fruit crop at different growth stages
There is variation in both winter chilling requirements as well as the temperature in which frost damage can occur (i.e., the critical temperature, Tc) between fruit species (e.g., apples vs strawberries, or elderberries) and across different varieties of the same fruit species. More importantly, each specific plant can be more or less vulnerable to frost damage based on its growth stage (see chart below).
Table 1 Critical spring temperatures for different apple growth stages where plants incur 10% or 90% kill.
Image: Chart showing critical temperatures (Tc) in which 10% and 90% frost damage leads to tissue death for apples at various growth stages (3). Note, these temperature listings are a generalization and not an exact threshold for all apple varieties.
What can be done to mitigate these situations? Keep reading to learn more about available strategies for frost protection, many of which have their own benefits and challenges. In order to be cost-effective, great consideration should go into weighing the cost of frost protection tactics, how frequently frost occurrences pose a risk, and the value of the crop being protected.
Spring frost protection strategies
Passive strategies
Passive strategies, including fruit crop variety, site, and training system selection and design, are proactive ways to prevent frost damage from happening. They’re typically in place long before frost protection is needed.
Fruit crop choice
Most fruit crops emerge from dormancy during spring and, even if they do not bloom during that time, their vegetation and fruiting buds can still be vulnerable to frost damage. However, by their natural growth habits, some fruit crops may be more likely candidates to incur frost damage than others. For example:
Peach buds have higher critical temperature thresholds than apples, making them inherently more sensitive to cold temperature damage.
Although emerging grape shoots tend to be highly vulnerable to frost damage, their compound buds offer back up shoots, in which the secondary or tertiary can take over if the primary shoot dies.
Other fruit crops, such as fall bearing raspberries, bear fruit on primocanes and may be able to escape frost damage to floral buds because of their late bloom timeframes, which can be advantageous.
Even more secure are unique crops like Day-neutral strawberries because of the tendency to establish transplants annually, making delayed planting an available method to escape frost damage.
Variety selection
There’s a reason the University of Minnesota has a substantial cold climate fruit breeding program- not only are the fruits bred for the Upper Midwest climate more adapted to withstand extreme winter temperatures, but many also have longer chilling requirements, which can delay bud break.
It is generally thought that ‘Honeycrisp’ apples require somewhere between 800-1000 chilling hours, whereas an apple variety like ‘Pink Lady’ (not recommended to be grown in Minnesota) only requires 200-500 chilling hours to break dormancy. However, chilling requirements are not always listed or easy to find for every apple variety. It is typically thought that an apple tree (and other fruit crops) bred for a colder zone will also require a larger amount of chilling hours, which can be advantageous in prolonging dormancy and delaying bloom. However, it is speculated that requirements can also change year to year based on the interaction between the plant, grow site, and environment.
When selecting new varieties, it’s recommended to ask the nursery about specific chilling requirements and average bloom times. Growers will also greatly benefit by recording bud stage and bloom dates for varieties on their grow site and create their own chilling requirement estimates by comparing these dates with accumulated chilling hours listed on the Midwest Regional Climate Center website.
Additionally, choosing to grow multiple varieties with variable bloom times can act as a passive tactic to protect an overall growing operation with the hope that one bad frost event will not affect all varieties present. Growers should still plan to incorporate some varieties with overlapping pollination, though, for fruit crops that require or benefit from cross-pollination (e.g., apples, blueberries, and honeyberries).
Interesting fact: Did you know that it is challenging to grow cold climate bred fruit varieties in warmer regions that lack adequate chilling hours requirements? If a fruit crop does not accumulate enough chilling hours, it is likely to experience uneven budbreak and growth, which can affect fruit set.
Site selection and planting
Some growing sites are at higher risk for spring frost damage due to their location. For example, sites located within a valley may always face challenges and require more extreme measures for frost protection.
Even within a specific site, topological variation can lead to “frost pockets” within low spots, and other land features like forest edges can dam cold drainage and increase the risk of frost damage for surrounding plants. Being aware of these microclimates and either avoiding planting or changing the topography or landscape features can decrease the need for active frost protection tactics.
In addition to topography and other landscape features, how a fruit crop row is oriented when planted on a hill can affect how well cold air drains and should be considered before planting.
To reduce frost damage risk with site selection:
Avoid planting in low spots and/or level low spots when possible
Remove landscape features like hedges that might be daming cold air
Avoid planting near forest edges located at the bottom of a slope
Orient fruit rows parallel to the slope if conducive to other site factors (e.g., consider soil erodibility)
Training systems
For crops like grapes and hardy kiwis trained on a trellis, the height of the trained fruiting zone can greatly impact how a frost episode affects early season shoot growth. Because cold air sinks, fruiting zones trained at a taller height are more likely to escape frost damage. In fact, the temperature can increase as much as .64°F every 4 inches (5), translating to about 3.80-5.75°F difference in temperature between a Vertical Shoot Positioned and a High Wire Cordon vine, trained to ranges of 3-4 ft and 5-7 ft, respectively. This comes with a trade-off, as taller fruiting zones are more difficult to prune and manage manually. Specific to grapes and irrelevant of which system they are trained on, taking advantage of long pruning is another way to delay bud break for buds located closest to the fruiting zone. Growers who use this method, should be prepared to have enough time to come back and reduce bud count at a later time.
Images: From left to right, a High Wire Cordon (HWC) and Vertical Shoot Positioning (VSP) grapevine training systems demonstrate different fruit zone heights. HWC can be trained from 5-7ft tall whereas VSP is typically trained to 3-4ft. Illustrations drawn by Madeline Wimmer, 2017.
Active strategies
If it’s not possible to choose a variety with a longer dormancy requirement or raise the height of its fruiting zone, other methods may be implemented to help plants tolerate freezing temperatures, including overhead sprinkler irrigation, fans, passive heat, and growing inside of sheltered structures like a grow tunnel. These are referred to as active strategies, which typically incorporate energy or labor-intensive techniques to protect plants from frost damage in real-time.
Sprinkler irrigation
Sprinkler irrigation systems tend to require the least amount of energy inputs of any active frost protection method, but they also require a lot of water to be effective. Using sprinkler irrigation takes advantage of the heat released from water turning into ice upon the area it is applied (i.e., latent heat of fusion). Another way to think of it is to imagine cold as the absence of heat and, in order to become cold, heat must be given up by the object that is decreasing in temperature. Using this method leads to a layer of icy water mixtures on the plants upon which more water is continuously applied.
How effective a sprinkler irrigation system is in preventing freeze damage is dependent upon a few factors. Here are a few tips to successfully use this method:
Make sure the irrigation sprinklers are frequently passing by a plant and spreading water in a way that promotes uniform coverage.
Start irrigation systems when the temperature is above critical frost temperatures and keep irrigating until temperatures are above freezing and the icy water mixture melts away from the plants. Critical temperatures vary based on plant growth stage, plant type, and variety type, but for the most part it is recommended to start sprinklers at least 4°F above the estimated critical temperature where frost damage can occur (2). Refer to the Apple Growth Stage chart earlier in this article for more specific information on apples. For example, if apple buds were at “tight cluster” growth stage 10% of buds will likely be killed at 25°F and, for best protection, sprinkler irrigation would begin when temperatures are above or reach 29°F and continue irrigating until temperatures reached above freezing, allowing the ice to melt completely before stopping. Refer to the pdf Critical spring temperatures for fruit tree bud development to learn about other tree fruits.
If the application layer looks milky white (a sign of rime ice) it is likely that the application rate is too low and should be increased. However, increasing sprinkler pressure to change application rates should be done with caution to prevent damaging irrigation equipment.
If you plan to use this method, check your equipment now to make sure it is in working order. If a sprinkler head goes out and/or the cold temperatures go below a certain point (crop dependent) the method can be ineffective and lead to damage.
Recognize that this method’s effectiveness changes with wind speeds, requiring more water at a higher rate during higher wind speed conditions (5 mph or faster).
Image: Ice being formed over apple blossoms, leaves, and shoots as a frost protection strategy. Notice that the ice is clear. Photo taken by R. Crassweller and retrieved from https://njaes.rutgers.edu/e363/.
Wind machines
Agricultural wind machines mix air when an inversion layer has formed and are effective during calm nights with clear skies. Different styles of wind machines exist and many modern ones even have noise reduction features, which is beneficial for growers living near neighbors. However, the initial purchase and set-up costs can still be a barrier to accessing this frost protection method for smaller growing operations.
Covers and shelters
Using covers and shelters can help increase downward radiation and trap heat. Some types of fruit crops are more conducive to covering and sheltering structures as a means to reduce frost damage risks. For example, June-bearing strawberries are traditionally covered with straw each fall and uncovered again in the spring. Timing when to uncover June-bearing strawberries is important as plants should be uncovered as they start to grow, but this also increases their potential to incur frost damage.
Methods for covering various fruit crops include using organic materials like the straw listed above, row cover cloth when applicable, or even utilizing larger structures like grow tunnels for high-value fruits or fruits that will also benefit from season extension (e.g., Fall-bearing raspberries).
Plant growth regulators (PGR)
Plant growth regulators (PGR) are compounds that send messages throughout a plant and influence biological processes. Some products utilize different PGRs to prevent fruits from dropping after a frost event. One product applied in apple production, known commercially as Promalin, contains gibberellins (typically present in apple seeds) and cytokinins, and is applied within 24 hours of a frost event to prevent fruit abortion and mitigate frost damage on a cellular level.
Note on heaters: various styles of heaters are some of the oldest methods of frost protection in fruit production. Depending on their power source, heaters can be energy intensive as well as inefficient- putting out more heat than needed, which ends up being lost to the atmosphere (4). Older smudge pots can also release undesirable smoke into the environment. Ideally, to effectively use this method, growers benefit from placing more heaters that release less heat within an area. However, this increases the upfront costs.
Other factors that impact frost damage
Plant health
Plants that are in good health and have balanced growth are more likely to have the reserves and ability to tolerate freezing stress more than plants that are stressed or overly vigorous. Grapevines are a great example of this, where overly vigorous shoots, known as bull shoots/canes, tend to be larger in size with larger plant cells. Because frost damage starts on a cellular level and the ability to supercool water (a state where water does not turn into ice despite existing in freezing conditions) decreases as cell size increases, these larger shoots are at higher risk of frost damage.
Other management conditions, such as soil moisture levels, soil composition, and whether the soil is bare or has vegetation also alter the impact of frost. For example, soils with a high moisture level tend to retain and transfer heat better than drier soils. Additionally, bare ground, while having negative connotations related to other factors like land erosion, can be beneficial in its ability to retain heat.
Weather conditions
Not all cold temperature events are created with the same environmental conditions. Here are some questions to navigate how various weather conditions impact the chance of frost damage.
What was the temperature on days prior to the frost incidence? Many plants will be less hardened and unable to tolerate stress from freezing temperatures if there is a drastic drop in temperature within a short period.
Is there a clear sky or is it cloudy during the frost period? Clear skies allow for heat to escape into the atmosphere through radiation. This creates an inversion layer with a clear layer of warm air above and cold air below. During situations like this, active methods of protection that mix the cold and warm air can be effective in preventing frost damage.
How fast is the wind speed during the frost period? Despite already discussing how wind machines can benefit fruit stands when an inversion layer exists, high wind speeds can increase the rate of heat loss for non-radiative frost events, known as advective frost. Advective frost conditions generally happen when a large amount of cold air is moving in from another region, in which an inversion layer is usually absent. This usually poses a problem for methods like using irrigation, which becomes challenging when wind speeds go above 5 miles per hour.
What is the dew point during the frost event window? The dew point is the temperature at which humidity in the air will create water (or ice if below freezing temperatures). If the dew point happens to be above the critical temperature at which a plant freezes, the extra water released during that temperature window and eventual frost thereof will release heat near surrounding plant tissues and slow the rate of frost damage (2).
Frost protection is no joke for growers working with perennial fruit crops. Considering passive frost protection strategies before establishing a fruit stand can help decrease the need for active methods. Active methods are available, but often come with a heavy price to operate whether requiring high water inputs, energy, and/or the initial cost and installation of equipment. For many growers, the cost-benefit analysis must be considered before implementation.
This article was reviewed by Natalie Hoidal and Matt Clark.
Resources Cited:
EIP-AGRI Focus Group. (2019). Protecting fruit production from frost damage. European Commission. Retrieved from https://eu-cap-network.ec.europa.eu/sites/default/files/publication/2023-05/eip-agri_fg_frost_damage_final_report_2019_en.pdf on 03/25/24.
Harbut, R. Understanding frost in fruit crops. Retrieved from https://fruit.wisc.edu/wp-content/uploads/sites/36/2011/02/Understanding-Frost-in-Fruit-Crops1-2.pdf on 03/25/24.
Proebsting, E. L., Jr., & Mills, H. H. (1978). Low Temperature Resistance of Developing Flower Buds of Six Deciduous Fruit Species1. Journal of the American Society for Horticultural Science, 103(2), 192-198. Retrieved https://doi.org/10.21273/JASHS.103.2.192 on 03/25/24
Snyder, R. & Paulo de Melo-Abreu (2005). Frost Protection: fundamentals, practice, and economics. Food and Agriculture Organization of the United Nations.
Trought, M.C.T., Howell, G.S., and Cherry, N. 1999. "Practical considerations for reducing frost damage in vineyards." Report to New Zealand Winegrowers.
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