Much of the country and in particular the northern Great Plains have experienced colder than normal and even record breaking cold weather over first few weeks of December. Several locations have gotten down to -20° and even lower! Wind chills temperatures have gotten down to -35° and lower. I have spoken to so many people about how they just are not used to the cold yet and hope that the cold weather we have had so far is just the beginning of a much colder winter to come. So, what about our garden plants, trees and shrubs? Is this colder weather harder on them too? Should we have bundled them up with old parkas to protect them from the cold? Does wind-chill affect plants like it does us? These are all reasonable questions that I will try to address in this week’s column.
Cold temperatures certainly do affect our plants but there are some important differences. First of all we have to remember that our bodies produce heat and that our bodies try to maintain normal body temperature, around 98.6°. Plants end up being at about the same temperature as the surrounding air temperature. Parts of the plant exposed to the sun or a warmer, nearby surface, might gain a bit of heat but not much in most cases, especially during the winter. Parts of plants, particularly basal buds, the crown, roots or other underground parts of the plant can gain some cold weather protection from the soil they are growing in. We can use mulches to help trap that small amount of heat close to the soil surface or around the base of our plants. An old parka could work well too but it probably better to donate your old winter clothes to someone else that can wear it instead of covering your plants with it. Mulching also helps to reduce temperature fluctuations around the crown of the plant which is particularly important when we have open winters, and lots of temperature swings in the spring of the year.
Plants can actually also become “used” to the cold through various processes related to cold hardiness. Plants that develop in cold climates like ours have also developed means to survive extended periods of sub-freezing temperatures that would quickly kill plants that originated in warmer climates. Cold hardiness is a genetic trait, just like flower color or plant form. Some species of plants are hardier than others. Plants can adapt over time to gradually become more hardy through natural selection too so that two groups of plants that may have originated from the same place might develop different degrees of cold hardiness over the course of many years. It is also important to remember that different parts of a plant may have differences in cold hardiness. For example, butterfly bush acts like an herbaceous perennial plant here in Brookings, dying down to the ground most years. It grows to be a woody shrub a few hundred miles farther south and does not survive the winter at all a couple hundred miles north of here. Here the root system and crown are hardy but the above-ground buds and stems are not. Farther south the whole plant is hardy but farther north, none of the plant survives. Many of us grow Forsythia here for its display of lovely yellow flowers. But, many varieties will only have flowers as high as the snow was because the unprotected flower buds that were above the snow could not survive the cold. The vegetative buds that produced the leaves usually do survive so we get plants that turn green and grow but often do not flower. Plants can be selected or bred for hardiness. ‘Meadowlark’ Forsythia was selected to be hardy in our climate so we can enjoy the flowers all the way to the end of the stems most years.
Plants get ready for winter by undergoing a process called acclimation or hardening. Hardening is triggered by various environmental cues like the day length, cooling temperatures in the fall and in some cases moisture availability. The degree of cold hardiness generally increases from early fall until mid-winter so that a temperature of -20° might kill a plant in November, it might not cause any damage in January. Plants are generally able to withstand these colder temperatures by regulating the water content inside and around the plant’s cells. The water around the plant’s cells is usually the first water to freeze because the water inside the plant’s cells has various salts and other compounds dissolved in the liquid component of the cells. But, once the water around the cells freezes and forms ice crystals it can damage surrounding cells and draw out some of the water inside the cells which can lead to larger ice crystal formation and damage to the cells themselves. The various processes that plants use to withstand sub-freezing temperatures are quite complex and can vary dramatically from one species of plant to another. The important thing for us as gardeners is to select perennial plants that have sufficient cold hardiness to survive our climate. Typically we use a plant’s USDA hardiness zone as a guide in selecting plants that should be hardy in our particular zone.
Finally there is the question of wind-chill which is a way of quantifying how an air temperature affects our bodies in combination with wind, or how cold it feels versus what the actual temperature is. Since our bodies are warm, they radiate heat. The greater the temperature difference between our body and the air, the more quickly heat will be removed from our body. If there is wind, it will pull the heat away from our body faster so it will feel colder. Since plants do not generate their own heat, wind chill does not affect them. But, cold, usually quite dry winter winds can damage plants by drying them out. This is particularly a problem for evergreens that have leaves exposed to the wind during the winter. Sun exposure can cause the plant cells to become more active and can be more subject to winter desiccation damage. We often see that sort of damage on plants like yews, arborvitae and dwarf Alberta spruce, especially on the south and west sides of the plant.
So, yes it certainly has been cold lately but hopefully your plants were able to harden off sufficiently this fall and are able to withstand the cold while we huddle inside our warm homes or put on that snow suit to go outside.