by Melinda Heigel, Extension Master GardenerSM Volunteer of Durham County
Regardless of where you live, if you’re in the US, you know what unusually prolonged cold much of the country experienced in late December 2022 as a bombogenesis (“bomb cyclone”) made an unwelcome appearance. Here in Durham, NC, December nighttime temperatures typically range in the low 40s. But during the “freeze out” of December 2022, our nighttime lows plummeted quickly ushered in by strong sustained winds. Beginning on December 24th, temps averaged a mere 14.3 degrees over 3 nights and averaged a frigid 20 degrees over 6 consecutive nights. While this is not arctic by a long shot, it is for our plants, trees, and shrubs, many of which are neither hardy nor acclimatized to these constant low temperatures. With such weather extremes, even zone-appropriate plants can come under great stress.1 Needless to say, many gardeners had some nail-biting nights. While this extreme weather event has passed, you may just be seeing its effects in your landscape. Identifying the damage and understanding how to treat cold injuries will likely come in handy about now. In a later blog post, we’ll address how to protect your plants best when cold temperatures hit, including those late-season frosts that always tend to zap blooming hydrangeas in late April and early May. Right now, though, let’s get down to brass tacks about cold injuries and how to evaluate and tend your plants.
Understanding and Identifying Cold Injuries
There are several types of cold injuries plants can experience. A chilling injury is damage to plant parts like leaves, flowers, or fruit that happens when the temperature is above the the freezing point. This generally occurs between 32 -55 degrees depending on the cold-hardiness of the plant. This kind of injury is often found in tropical or subtropical specimens. This damage often shows up quickly after the exposure. Signs of chilling injuries can appear as leaves that turn red, purple or black in color, leaves that wilt, and flowers that die. This damage may stress plants, lead to some dieback, or slow future development, but it isn’t always fatal.
Frost and freeze injuries are more in line with what most of us experienced in the recent deep freeze. When the conditions are like those we experienced in December, we have what is known as an advective freeze.2 Frost and freeze injuries are very similar and tend to affect plants in the same way. With these injuries, water inside the plant cells basically crystallizes with rapid temperature fluctuations. As these ice crystals expand, they cut and burst cell membranes, and fluids leak from the cells leading to plant death. Leaf and stem tissues are most at risk with this type of fast freeze.
The timing for identifying frost and freeze damage to plants can be tricky. Sometimes, as in the case of the photo of the potted pansy, the evidence of injury appears quickly. This photo is after only one night of severe cold. However, in other cases, frost and freeze injuries can take days, weeks, or even months to show up. Immature plants are more susceptible than mature ones. Damage often starts with the softest, actively-growing plant parts–like the basal leaves of the pansy. Some signs of damage may show up after a week or two. For example at my house we have mature yucca plants (Yucca spp.) and after the 6 nights of extreme cold, they appeared to have weathered fairly well. One week later, they began showing signs of distress. And today, nearly one month later, the extent of the injuries is becoming more and more evident as the photos below demonstrate. Overall, damage to flower buds, vegetative buds, stems, and entire plants may remain hidden until active growth begins again many weeks later in the springtime. When it comes to evergreen shoots, the extent of impact is usually evident within 2 weeks.
(Left to right) Freeze damage may take time to materialize. This once-evergreen mature yucca began to show discoloration and necrosis about a week after the deep freeze. It continues to evolve nearly one month later as what appears to be significant dieback is now occurring in the apex of the plant. (Image credit: M. Heigel)
Frost and freeze injuries can impact all of the parts of a plant and show up in myriad ways. Leaves can appear wilted, black or otherwise discolored, have a mosaic-like appearance due to damaged or dead cells, and even look wet or waterlogged from the bursting of plant cells. Later signs might be misshaped or curled leaves. In evergreens, coloration changes can occur. Needled evergreens like junipers (Juniperus spp.) may turn uniformly brown to bronze; broadleaf evergreens such as boxwoods (Buxus spp.) and hollies (Ilex spp.) may show leaf burn or scorching at their margins from cold damage. In terms of vegetables, many cool-weather crops like kale, collards, and Brussels sprouts and can handle temperatures below 26 degrees briefly, but damage can materialize in the form of affected foliage and potentially reduced yield.
Boxwood (Buxus spp.) following December’s freeze exhibiting damage with marginal leaf burn and possible desiccation.3 (Image credit: M. Heigel)
The bark of woody plants and trees can also be impacted by rapidly freezing temperatures. Long cracks that are deep and narrow called frost cracks can occur in trees that experience wide-ranging temperature changes. The crack can expand and contract after the initial injury and callus over in the summer. Frost cracks can reoccur. In other woody plants such as azaleas, rapid temperature changes can cause the bark and/or stems to literally split near the plant’s base. Unfortunately, this can be a tough injury for plants to overcome. Damage to roots, while harder to see, can definitely occur. This is especially true if plants are in containers where their roots aren’t protected below ground. Likewise roots of ball-and-burlap plants are also at higher risk. According to Clemson University’s Home and Information Garden Center, even zone-hardy plants planted above ground in planters with “unprotected roots” can experience lethal root injuries at temperatures beginning at 28 degrees.
Caring for Plants
Pruning –Once you look for and identify signs of frost/freeze damage, exercise restraint. Some plants may appear dead, but they aren’t. While you might not like the looks of your plants, think twice about reaching for the clippers. Dead foliage might protect the plant’s crown and roots from further damage by providing insulation, especially with evergreens and perennials. As mentioned, damage is not always apparent and can take months to surface. Any pruning or cutting back now may mean you would be leaving potentially unhealthy plant tissue behind. Wait until you can fully determine the extent of damage before taking action like cutting out dead wood and plant material. This could mean waiting until the spring. One exception to this would be herbaceous plants (plants that have no woody stems above ground) like the pansy above whose cells have essentially collapsed. Removing this specific kind of dead plant material is recommended as there is little hope for revival.
Fertilizing–While it might seem tempting to help nurse your ailing plants with added nutrition, don’t apply fertilizers following these types of events. Your actions may indeed encourage the plant to put out some new growth, but if we are in the dead of winter or in late fall or early spring when frosts happen, you might be harming your plant further. Tender new growth is especially prone to cold injury. Wait until the any threat of additional frost/freeze has passed before your fertilize.
Watering–Pay close attention to moisture. After an extreme event like a hard freeze where the ground is potentially frozen around the plant, its roots are not able to take up adequate moisture. Likewise, very windy conditions like we experienced with the December cold snap also contribute to plants’ inability to capture and use moisture when under stress. Water your plants at the soil line after harsh cold-weather events to ensure proper hydration. Likewise, make sure the plant receives adequate water if nature doesn’t provide it throughout the upcoming growing season. The cold pressure plants experience in these freeze events means that gardeners need to pay special attention to keeping them healthy in the months that follow.
Mulching–Some experts suggest that keeping the base of plants well mulched is a way to help protect roots of plants, moderate soil temperatures by lessening heat loss, and hold moisture–all things especially critical during extreme freeze events.
If you haven’t surveyed your landscape thoroughly after the December weather–or even if you have, go out and assess what you see today using these descriptions of cold injuries as a guide. Be patient and remember that spring is on the way!
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Notes
1–Knowing your area’s USDA Plant Hardiness Zone is an important tool in plant selection and success year round in outdoor growing. However, it’s not a bulletproof guide. These zones are based on the average minimum temperature your area has historically experienced. In extreme weather, how quickly and low temperatures fall can negatively impact even zone-appropriate plants.
2–An advective freeze is a wind-borne event that happens when an air mass with below-freezing temperatures moves into an area and displaces warmer air. With this type of freeze, windy conditions tend to persist throughout the event. This influences the temperature of the plants and makes conditions right for ice crystals to form within the plants’ tissues.
3–Desiccation can also be a type of cold-related injury to plants, and evergreens, both narrow and broad-leafed varieties alike, are especially susceptible Our December 2022 weather event presented the perfect example of conditions that easily lead to this type of damage. During very windy conditions, and especially when they are exacerbated by frozen ground, plants can lose water faster than they can take it in. This often results in damage that appears like leaf burn on scorch on leaves. This damage pattern can also come from extremely low temperatures. Root injury can also result.
Resources and Additional Information
To determine your USDA Plant Hardiness Zone, plug in your location on USDA’s interactive map.
https://planthardiness.ars.usda.gov/
NC Cooperative Extension offices have several sites with articles featuring cold-related plant injuries and possible methods for protecting plants during extreme weather.
https://lenoir.ces.ncsu.edu/2018/01/plants-and-cold-temperatures/
https://lee.ces.ncsu.edu/2018/04/identify-frost-damage/
https://forsyth.ces.ncsu.edu/2021/04/protect-your-plants-from-frost/
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