13 minute read
Feature
Landscape Recovery following the December Flash Freeze in Tennessee
By Lee Rumble, Commercial Horticulture Extension Agent, UT/TSU Extension—Knox County
The Flash Freeze Event
At midnight on December 22, 2022, air temperatures in the Knoxville area were 46ºF. Across the next six hours, temperatures plummeted to 10ºF and as the day continued, air temperatures continued to decline to 3ºF by midnight on December 23, 2022. In a 24-hour period, Knoxville experienced a 43-degree temperature drop…and similar conditions were observed across the state. The rapid and dramatic decline in temperature was compounded by strong winds and sustained freezing temperatures (< 32ºF) that persisted across the state for three more days! It was not until noon on December 27, 2022, that temperatures would once again rise above 32ºF (Time and Date AS, 2023).
Understanding Dormancy in Higher Plants
Trees and shrubs have dealt with freezing temperatures for millions of years. Fortunately, researchers and growers have spent contemporary time focusing on how plants protect themselves from freezing conditions. At the dawn of the 19th century, Sir Thomas Knight pioneered early studies on dormancy in plants. In 1801, he conducted experiments that demonstrated the need of woody perennial plants to endure a period of dormancy before plants could resume growth in the spring (Fadón et. al., 2019). This research, and the work of many other scientists and scholars throughout the 20th century, helped to form the basis of our current understanding about dormancy in higher plants. Today, we recognize that four major conceptual stages in dormancy occur in deciduous plants (Fadón et. al., 2019):
Dormancy Induction – The first step in winter cold-hardiness of plants begins with annual leaf drop that follows in response to decreasing photoperiod and temperatures. Conditions stimulating dormancy induction vary greatly among species (and cultivar).
Endo-dormancy – Achieving the conditions that represent inherent dormancy requirements within the plant. After endo-dormancy needs are met, release from endo-dormancy will be most influenced by low nighttime temperatures that occur after an acclimation period that we now describe as “chilling hours”.
Eco-dormancy – A quiescent, or resting, state. Once a plant has become acclimated to cold temperatures, eco-dormancy is then controlled and maintained by environmental factors external to the plant. A period of sustained and mildly elevated temperatures could trigger resumption of plant growth. Often, however, growth is prevented by other unsuitable environmental conditions.
Growth Resumption – When plant-favorable environmental conditions again resume, phenological changes next begin that lead to renewed growth, including foliar bud break and initiation of floral blooming.
At their core, these four stages seem relatively straightforward. However, the number of interactions that occur within plants leading to emergence from dormancy are actually quite numerous and complex. Thresholds of photoperiod, temperature, and cold acclimation are met, then overcome, while transport blockage, phytohormones, genetic and epigenetic regulation, and carbohydrate reallocation all work tirelessly together in the background at each stage.
In east Tennessee, most of our mature deciduous trees showed no immediately noticeable damage. Individuals of many tree species were already fully acclimated to the freezing December temperatures. The same was not true, however, for many of our evergreen tree and shrub species. Throughout the winter months, evergreen species do not go fully dormant and continue to absorb light energy (photons). Under favorable environmental conditions, evergreen species are still able to photosynthesize; thus, utilizing their “evergreen” leaves for longer periods of time. Yet, under unfavorable conditions (i.e., freezing temperatures), the light rays that are absorbed by chloroplasts in leaves and needles cannot be used by the plant, resulting in plant damage. Freezing temperatures inhibit CO2 assimilation, leading to the overexcitation of photosynthetic machinery, and create light stress in evergreen foliage. This condition has been called chlorophyll bleaching or “solarization” (Öquist and Huner, 2003). The interactions between light, freezing temperatures, and also desiccation can be damaging (if not lethal) to evergreen leaves. In our area, broadleaf evergreens and needled evergreens really took a hit from the flash freeze event, and it was these plants that we received the most calls about.
What Happened to Plants
Most often, freeze damage can be associated with ice formation within plant cells. This ice formation within the cell is what ultimately punctures the cell walls leading to a loss of function (Bumgarner et. al., 2023). Certainly, with the rapid and sustained temperature drops, cell damage was likely to occur in some plants. With such a rapid drop in temperature, plants were unable to implement internal defense measures to tolerate the sudden freezing temperatures. This, in addition to excessive wind gusts (averaging 19 mph here in Knoxville) helped to further desiccate (dry out) many of our plants. Within two weeks, noticeable signs of damage appeared across the state and the calls came swarming in. Coolseason turfgrasses, ground covers, conifers, borderline hardy shrubs, and both broadleaf and needled evergreen plants were most noticeably affected. Turfgrasses showed brown necrotic tissues, conifers almost immediately began to bronze, borderline hardy shrubs began to defoliate, and broadleaf evergreens started showing signs of solarization ( Figure 1 ). This flash freeze event appeared to be most destructive to those “borderline hardy” species listed as USDA Plant Hardiness Zone 7 or Zone 8 plants. Aucuba , Camellia, Cryptomeria , Distylium , Ficus , Indian Hawthorn ( Rhaphiolepis spp.), Loropetalum , and other genera all showed initial signs of damage. Yet it was not just borderline hardy plants that were affected; we also saw damage on Azalea, Arborvitae, Boxwoods, Chamaecyparis spp., Cherry Laurel ( Prunus laurocerasus ), Hollies ( Ilex spp.), Magnolia, Nandina, Crape Myrtle, Lenten Rose ( Helleborus spp.), and even English Ivy.
Interestingly, we observed that site location and exposure to microclimates within the landscape mattered. In some landscapes, species were seemingly killed to the ground, while the exact same species in another landscape may well have escaped the freeze damage. For those of us in the green industry, this serves as a clear reminder of the importance not only of careful plant species (and cultivar) selection, but also attention to where and how we place our plants within the landscape. We also observed that plants that had been pruned late into fall 2022, and that had developed new growth since pruning that had not hardened off, were more drastically affected (Figure 2). In many cases, plants that were already unhealthy and lacked vigor eventually succumbed to this freeze event—this was most noticeable in Cherry Laurels (Prunus laurocerasus) that went into the event already stressed (Figure 3).
What We Told People to Do
Almost immediately calls began from across the state—from Memphis to Johnson City. We encouraged people to be patient and stay calm. But with April and May still several months away, this advice was a tough sell given the drastic effects seen on many of our landscape plants. Most of the calls were initiated with “Should I cut my plants back now”? Due to this influx of calls, our University of Tennessee Extension Horticulture Workgroup collaborated to prepare and distribute a Fact Sheet: Handling the Holiday Freeze in Your Lawn & Landscape (Bumgarner et al. 2023) to help our clientele understand that follow up recommendations about these types of events are often a “wait and see” game. Only time can tell us which plants are able to weather a freeze event.
As plants continued to defoliate in the days that followed and calls continued well into January, we encouraged people to implement a “scratch test” on their favorite winter-damaged plants. By scratching bark from a small section of stems and twigs within the canopy, we could better see if the tissues beneath the bark were green (a viable plant) or brown (a dead plant). More experienced gardeners likely also understood twig pliability and a plant’s ability to rebound after slight pressure is applied to the uppermost twigs. Still, with many of our favorite plants defoliating rapidly, and spring temperatures rising, people were anxious to get out and try to fix the problem. We encouraged people to limit their pruning until the full extent of what was to come was made apparent with the arrival of spring bud break. A second added benefit of waiting to prune is that by leaving aboveground damaged tissues, especially for low-growing perennial plants, the stems and entwined leaf litter proved useful in providing a little more insulation to the crown and roots of these plants.
What Has Progressed Since
Since the initial damage we saw within days or weeks to many of our broadleaf evergreen plants, a more extensive portion of the foliage on these plant species ultimately succumbed to the damage that occurred. Initial solarization, loss of plant vigor, and subsequent defoliation were seen in most of the species listed previously in this article. While many plant species might have defoliated entirely, the ultimate survival and viability of many individual plants was still promising. Frequently, our scratch tests showed green (viable) tissues just beneath the bark. For a few species (particularly Indian Hawthorn, [some] Hollies, and Cryptomeria) we quickly saw brown scratch test results (dead stem tissues); it was these plants we became more worried about.
What We’re Seeing Now
By the end of May, we saw most plants rebound, while other specimens and species have succumbed to their injuries. It is important to remember that rarely does one single factor kill an established, mature woody ornamental shrub or tree. Instead, they die as the result of a cumulative series of stressful events that build up with time. Eventually, the combined losses of vigor, limbs, and foliage will lead to premature plant death.
Another negative effect that has since emerged following this freeze event is the appearance and expansion of vertical frost cracks in the bark and wood of some of our deciduous landscape trees (Figure 4a). Frost cracks most commonly occur on the south and/or west side of trees, on plant species that have particularly thin bark, and for tree trunks that may be exposed to direct sunlight. Warming daytime temperatures and cool nighttime temperatures in early spring combine to promote the expansion and contraction between the bark and inner wood—ultimately leading to frost cracks that appear as vertical splits on the main stems and scaffold limbs of trees and shrubs.
Unfortunately, with frost cracks, the damage usually gets worse before it gets better (Figure 4b). For smaller frost cracks, these should be left alone, allowing the tree’s natural wound response to attempt to compartmentalize (i.e., close) these injuries across time—do not use wound sealants (e.g., paint, tar, or expanding foam) on exposed wood tissues. If frost cracks are large and severe, you might be better off consulting with an ISA Certified Arborist to see if bark tracing, or carefully cutting away the peeling bark back to sound tissue using a sterilized surgical blade, may be a good option for your tree (Bark Splitting on Trees, 2018). Carefully removing (by surgical means) larger pieces of loose bark allows the tree a better opportunity to compartmentalize larger wounds. However, once the internal wood is exposed to the external environment, the elements, wooddecay Fungi, and various insect pests will all be able to access and further expedite decay within the exposed wood. Ultimately, the severity and extent of these frost cracks will determine the survival fate of these affected species.
While there is no clear cut answer to fix the damage caused by freezing temperatures, it is important to realize that, even for those specimens and species that appear to have survived this freeze event, they will still be susceptible and under stress well into 2023. Understanding this stress and managing it throughout this year will be a good way to help promote landscape plant vigor and a quicker, stronger chance of recovery.
How to Best Handle Future Freeze Events
While we get little say in future freeze events, taking a few precautions at the time of plant selection can go a long way in ensuring that the plants in our landscapes are hardy enough to handle another freeze event such as the one experienced in December 2022. Start by implementing a few best management steps that can go a long way in helping landscape plant selections to not only survive, but thrive:
Select cold hardy species (and cultivars) adequate for our USDA Plant Hardiness Zone 7.
Avoid late summer fertilization and/ or pruning, which might stimulate new growth late in the season.
Water trees and shrubs, especially evergreens, during dry periods until the ground freezes.
Use just a few inches of mulch to conserve soil moisture and insulate the roots from freezing temperatures. Older, decomposed mulch should be removed before adding new mulch to an established landscape tree or shrub bed.
Take appropriate steps to manage pests and encourage plant vigor throughout the year.
Identify and understand how to best utilize (or avoid) microclimates that occur within every landscape.
Resources Cited and Additional Reading:
Bumgarner, Natalie, Jim Brosnan, Amy Fulcher, Lucas Holman, Lee Rumble, Taylor Reeder, Celeste Scott, and Justin Stefanski. 2023. Handling the Holiday Freeze in Your Lawn & Landscape. University of Tennessee Extension Fact Sheet. https://westtn. tennessee.edu/wp-content/uploads/ sites/35/2023/01/Handling-the-Holiday-Freeze-in-Your-Lawn-Landscape.pdf
Fadón, Erica, Eduardo Fernandez, Helen Behn, and Eike Luedeling. 2020. A Conceptual Framework for Winter Dormancy in Deciduous Trees. Agronomy 10, 241.
Öquist, Gunnar, and Norman Huner. 2003. Photosynthesis of Overwintering Evergreen Plants. Annual Reviews Plant Biology 54, 329-55.
Plant Disease Diagnostic Clinic, Cornell. 2018. Bark Splitting on Trees: Various. Cornell University Fact Sheet. http://plantclinic.cornell.edu/ factsheets/barksplitting.pdf
Time and Date AS December 2022 Weather Graph – Knoxville, 2023. Time and Date AS 1995-2023. Web. 10 May 2023. https://www.timeanddate.com/weather/usa/knoxville/ historic?month=12&year=2022
This article was originally published in Tennessee GreenTimes, Fall 2023 and is reprinted with permission.