Cold weather is setting in and fruit trees are acclimating to decreasing temperatures. Although the extent of cold damage fluctuates year-to-year, cold damage in tree fruit is “a perennial problem,” said Matthew Whiting, tree fruit specialist at Washington State University, during a recent webinar. Cold damage is becoming more problematic as climate changes affect seasonal temperature patterns, cause erratic temperature fluctuations and cause extreme high and low temperatures.
How does cold damage occur and how can it be assessed? What more can growers do to help trees survive cold weather?
Plants freeze if they cannot avoid ice nucleation – when ice crystalizes and propagates within the plant, either in between cell layers or within the cells, Whiting said. Non-lethal ice formation happens when water accumulates between the cells. Over time, however, enough water from within the cell is drawn outward, and cell death occurs due to dehydration.
The three periods when freeze damage occurs are late autumn, mid-winter and late winter/early spring. During late autumn and early spring, plants are acclimating or de-acclimating due to changing temperatures. Temperature fluctuations, even for a day or two, can change their hardiness levels, leaving them with increased susceptibility to cold.
The cessation of growth in autumn is signaled by reduction in daylength and temperature. This process leads to metabolic changes in the plant that cause hardiness. Buds and vegetative tissues that haven’t yet acclimated to cold temperatures are likely to suffer damage in late autumn if temperatures suddenly become cold.
During mid-winter, when plants have reached dormancy and are at full hardiness, they are stabilized and able to withstand temperature changes but can suffer cold damage if the temperature drops below their maximum level of cold tolerance. If there is a significant warm-up for several days, full hardiness is reversible.
“Trees and buds will lose their hardiness level much faster than they regain it,” Whiting said, so when mid-winter temperatures warm but then return to normal lows, cold damage can occur even at temperatures the trees previously withstood.
During spring, there is more water available to plant tissues, which means there is more water in the cells to freeze. Warming spring days make the trees increasingly sensitive to lower temperatures. When a cold snap happens, trees are vulnerable to cold damage.
De-acclimation is “almost exclusively driven by ambient air and tissue temperatures in the spring,” Whiting said. “Quite quickly, that hardiness is lost and tissues become sensitive to cold temperature.”
In the middle of summer, a severe cold snap could kill buds – they are no longer acclimated. But in the middle of winter, the same low temperature would be tolerated, due to cold acclimation which led to hardiness levels.
“You tend to have less variation between the temperature to kill 10% and the temperature to kill 90%” of the buds during mid-winter than in the acclimation of autumn or de-acclimation of spring, Whiting said. The temperature that will kill 10% of the buds (the LT10) of any tree fruit is variable depending on calendar date, stage of growth and cultivar. It also varies within cultivars.
Hardiness can be visually assessed by looking at the tree’s cambial tissue several days after low temperatures occur. Buds can also be sampled. Discoloration indicates tissue death. Conductivity of tissues can be studied, as freeze damages cell membranes, causing leaking of electrolytes.
Controlled freezing methods, such as differential thermal analysis (DTA), where buds or spurs are placed on thermo-electric modules in a freezer, tracks how cells respond to cold temperatures. Latent heat, from the fusion which occurs as water freezes, is detected and converted into an electric signal, which powers a data logger. DTA data have shown that there is significant variability in lethality depending on cultivar and differences in tree hardiness occur throughout the season. DTA has even found significant differences in lethal temperature within buds of the same cultivar sampled on the same day.
The Future of Frost Protection
Summers and winters are warmer. The autumn drop in temperature, which acclimates plants to cold, isn’t reliably happening. Warm winter spikes and cold spring spikes are happening more frequently.
The primary methods of frost protection in use today are heaters, which aren’t efficient and not very effective; wind machines, which can help during radiative freezes, when inversion occurs; and irrigation, either under tree or overhead, which requires a lot of water and may not be effective. Nucleation inhibitors such as kaolin clay, acrylic films or ice nucleation-negative bacteria prevent ice from forming and propagating in the cell tissues. Insulators, such as foam or low-density polyethylene, work by slowing down plant heat loss. But both have drawbacks.
Brent Arnoldussen, WSU Ph.D. candidate in the Horticulture Department, is researching a frost protection solution which is environmentally-friendly and able to offer more consistent protection from cold damage than current protective measures. These “plant-based dispersions to protect trees from frost damage” are aqueous solutions of nanoparticles derived from plant structural carbohydrates, he said. They have a high insulation value, are sprayable, form a durable layer when dried and are a renewable resource.
Cellulose nanofibrils (CNF) consist of physically broken down wood pulp. Cellulose nanocrystals (CNC) use pulp wood to create a nanocrystal structure. These products protect the buds both by decreasing heat loss and by inhibiting ice nucleation.
In 2019 field trials on one-acre plots of sweet cherry and apples, CNC was sprayed in a 3% solution using a electrostatic sprayer at a rate of 25 gallons/acre. Buds treated with CNC were able to withstand field temperatures three to five degrees Celsius lower than those left untreated, even at seven days post-application.
The 2020 studies took place during an actual freeze event. CNF was applied to two half-acre plots of sweet cherries at the tight cluster stage. A 2% solution was applied with an air blast sprayer. Temperatures at the first plot location were several degrees Celsius warmer than at site two, which had multiple frost events. Treated trees at site one had a 46% reduction in bud mortality compared to the untreated control, and also saw a 900-pound, on a per acre basis, increase in yield. At the second site, a 76% reduction in bud damage was seen in CNF-treated trees as compared to unprotected controls. There were no yield differences between the control and the treated plot at the second site, possibly due to the multiple frost events. Temperatures at site one reached -3.3º C, while they reached -5.5º C at site two.
The efficacy of the dispersions has not been studied past seven days, so it’s not yet known how long the protection will last. While unable to be used with irrigation frost protection methods, the dispersions could be used in conjunction with wind machines. “These sprayable cellulose nanofiber dispersions could be a new tool to mitigate cold damage,” Arnoldussen said.
New, effective and efficient frost protection tools, as growing conditions become less stable and frost events occur more frequently and erratically, can’t become available soon enough.