The first week of 2018 brought temperatures in the single digits to our part of the state. Some of our clients asked if they should be worried about their landscapes when plants are exposed to extreme cold. We did some research, and discovered some pretty incredible ways that plants adapt to sub-freezing temperatures. We also put together some tips to give your landscape the best chance to make it through a cold snap. Plus, ever wonder why Rhododendron leaves droop and curl up in the cold like they do? We’ve got an answer for that too.
Plants that are adapted to live in temperate and cold climates have developed a few key strategies to be able to survive the winter. The major threat to flora in these climates is freezing of the cellular water, which results in cellular rupture and death of the plant. To prevent this, species will generally employ one of the following:
Deep Supercooling: Plants that utilize deep supercooling are able to suppress ice formation on a cellular level. This means that down to temperatures as low as -40°F, water in the cells remains liquid. Any lower than -40°F, however, the water freezes and the cells burst. Species that use this technique include oaks, maples, rhododendrons, walnuts, and roses. In natural landscapes, this is a major factor in accounting for the clearly defined timber lines in northern climates.
Extracellular freezing: A great number of woody plants experience winter temperatures below -40°F. Instead of relying on supercooling to survive, these plants actually dehydrate their cells. This pushes cellular water out of the cells and into the space between, where it can be allowed to freeze without harming the plant. Examples in this group include paper birch, red twig dogwood and quaking aspen.
Complete Dormancy: Many perennials, and some shrubs, die back all together in the winter. This leaves no vegetation above ground to be exposed to extreme temperatures. These plants rely on the fact that the soil temperature will be warmer and more consistent than air temperature.
Acclimation and Our Role in Landscape Success
In our climate, plants tend to have problems not because they can’t survive the cold, but because they aren’t properly acclimated. Acclimation is the process that woody plants go through as winter approaches. As daylight hours shorten and temperatures grow cooler, plants receive signals that it is time to go dormant. This process can take up to 6 or 8 weeks in our area, meaning that an early (or late) frost can catch a plant when it is not fully dormant. This is when the majority of cold damage will happen to landscape plants. Although we are mostly at the mercy of mother nature when this happens, there are things we can do to greatly decrease the chances of damage.
Mulching: Maintaining a layer of mulch around the root systems of landscape plants (especially ones that are barely hardy in our area) can mitigate some of the negative effects of extreme cold. Mulch helps regulate soil temperature and provides insulation. Even if above-ground parts of a plant are lost to cold damage, mulch increases the chances that the plant itself will survive.
Pruning and Fertilizing: Both of these maintenance tasks usually stimulate a similar response in woody plants: new growth. Any new growth on a plant is young, tender, and not completely hardy. This growth will be the first to suffer in a cold snap. Pruning and fertilizing landscape plants in autumn should be avoided. (Note: Because transplanting does not have the same
effect on plant metabolism, fall is still one of the best times to plant new plants!)
The Rhododendron Phenomenon
Seeing drooping Rhododendron leaves on a chilly day is a familiar sight in Western North Carolina, and one that can leave homeowners worried. A plant that changes shape so drastically (and looks so sad) is surely suffering, right? Actually, you’re witnessing a few million years of evolutionary adaptation. Rhododendron have two different leaf-related reactions, though they often occur simultaneously.
Leaf Drooping: Rhododendrons typically inhabit the understory, meaning there is a canopy of shade above them for the growing season. In the winter, however, this canopy is non existent. Rhododendrons experience much more solar radiation in the winter, and it turns out that this is a big problem for them. One study (cited below) found that photosynthesis (the plant’s ability to make food) decreased as much as 50% in leaves that were prevented from drooping in the winter. The drooping leaves alter the angle of the leaf surface to the sun, decreasing the radiation exposure.
Leaf Curling: This reaction is directly correlated to air temperature. Rhododendrons are one of a few species in our climate who’s leaves undergo frequent (sometimes daily) freeze-thaw cycles. When leaf tissue freezes, it is not the initial freezing that is problematic. Damage occurs when it thaws too quickly, akin to a scuba diver with the bends. The same study mentioned above found that a curled leaf thaws more slowly than one that is frozen flat. This adaptation
allows Rhododendron to endure these cycles without sustaining damage.
We hope you enjoyed these insights! Keep up with our monthly blog for more. The links to our source articles are listed below, for further reading.
‘How Woody Plant Survive Extreme Cold’ https://hortnews.extension.iastate.edu/1996/3-1-1996/brr.html
‘Why Do Rhododendron Leaves Curl?’ http://www.arnoldia.arboretum.harvard.edu/pdf/articles/1990-50-1-why-do-rhododendron-leaves-curl.pdf