"Protecting Plants from Cold Damage"

LowTide Morning - December 15, 2018

"Protecting Plants from Cold Damage" (from my archives: Extension weekly article 12/21/04)

Local radio host, John Scott Kapaona recently introduced me to his listening audience on AM 720. Each week we chat about different gardening topics and John, not being a gardener, has learned to ask all of the right questions. Two weeks ago we talked about plant cold hardiness zones, which determines the minimum temperature that a plant can tolerate during winter and survive. A zone 7 plant that has acclimated to winter dormancy should tolerate temperatures as low as 0 - 10 degrees. Zone 8 will only tolerate between 10- 20 degrees. This week, as the temperatures dropped into the 20's, John asked about protecting plants against cold weather.

As I was pulling information together for the show, I started asking some simple questions at the Extension office that seem to have rather in-depth answers. I wanted to know if and when I really need to pull out the covers for the plants in my back yard and when I might just be wasting time. In my search for a more exact determination of when to protect plants, I gathered information from a number of sources. Cold hardiness refers to the genetic capacity of a plant to acclimate to freezing temperatures. A plant’s ability to acclimate refers to the ability to transform from a non-hardy condition during the warm growing season to a hardy condition prepared for winter temperatures. The process in reverse is called deacclimation. If a plant comes out of winter dormancy too soon in late winter/early spring, it may also be susceptible to damage. We often see flowers damaged in the spring on deciduous magnolias and forsythia (yellow bells).

I find it interesting that acclimation and deacclimation are triggered by hours of daylight and temperature. Hours of daylight are easily predicted from season to season. Here in Georgia, it sometimes seems, however, that there is no such thing as predictable weather patterns from season to season. So, the short-story-long is not only do you need to consider the hardiness zone, you need to consider the current season’s trend in temperatures and how slowly or how quickly the temperatures drop. After extended mild temperatures this fall, many plants may not be fully prepared for a sudden drop in temperature. Fluctuations in temperature during the winter months are also common in Georgia. Several warms days followed by a cold snap may result in injury.

There are other factors that will affect the process of acclimating to winter. Two things in particular will stimulate late season growth that may need to be protected from an early freeze. Late season fertilization with high nitrogen fertilizers and late season pruning both stimulate new growth, which is much more tender and susceptible to damage than mature foliage.

The location of plants in your yard and exposure to sunlight has an effect on susceptibility to damage. The coldest spots in your yard will be in low spots or on the north/ northwestern exposure of the house. Even though these are the coldest spots, southern or southwestern exposures may contribute to cold damage where plant cells warm up during the day and then may rupture as temperatures drop at night.

So, to sum up when protection is needed, it depends on hardiness, the current pattern of weather, acclimation, and general condition and location of the plant. When in doubt, I also throw in another category just to be one the safe side. If you have plants that have been passed along bysomeone special and cannot be replaced, those plants will probably be worth extra effort.

There are a couple of methods used for protecting plants. With lower growing plants, you can put a wire cage around it and fill the cage with leaves or simply mulch in the root zone to keep the plant at the base alive. Another method is 
to cover the plant with black plastic. This is preferred to clear plastic, which can create a greenhouse effect in the presence of sunlight. Black plastic is also preferred to fabric which allows cold air through the material. The key point in covering the plants is to secure the plastic at the base with something heavy such as brick, rocks, or soil to weigh it down. If the temperatures are expected to rise much during the day, remove the plastic cover so the plants do not overheat. I have also cover prized plants with cardboard boxes covered in blankets.

Another helpful reminder is to make sure the root zone is not dry prior to the arrival of freezing temperatures. Plants cannot take up water if the soil becomes frozen and moist soil is warmer than dry soil. If the root zone of your plant is dry, water before the freezing temperatures arrive. Water is particularly important for containerized plants because the root zone dries out quicker and containerized roots are more prone to freezing.

The foliage or flowers on some plants may be damaged by low temperatures or snow, but the plant itself will survive. At my previous home, I had Hedge Bamboo that turned brown after a couple days of snow on the foliage, but the plants flushed out in spring and were fine. You can check the condition of your plants if you suspect damage. Check the stems and flower buds by lightly slicing into the tissue to see if it is green or brown. I tend to wait until spring before pruning damaged tissue. You will know when things flush out in the spring if the plant or portions of the plant have not survived.

The basics of cold protection are knowing when to protect, knowing how to protect, and knowing how to assess for damage. The best protection is to select plants that are hardy for Georgia. Linda Alexander, one of our Coweta County Master Gardeners helped me come up with a list of plants that are only hardy to zone 8. A few of them that are grown in our area may be susceptible to cold damage are gardenia, confederate jasmine, angel trumpet, hibiscus, lantanta, pittosporum, and podocarpus. I have a couple of the plants on the list that are planted next to the foundation of my house in a protected area. I don’t plan on bringing out the blankets yet. I’ll let you know how they survive.

Post script: I have a Master Gardener friend who protected her palm tree by running outdoor lights through the tree, which helped keep the foliage warm.

~ LowTide explorer, Carolyn Fjeran

[LowTide is an eclectic collection of discoveries and reflective writings.]