Factors Affecting Cold Injury In Perennial Landscape Plants
Published February 25, 2021 By GREG CHURCH, Ph.D. Plant Pathologist, Certified Horticulturist, Certified Arborist
The record-breaking cold temperatures, during the third week in February 2021, potentially caused cold injury to a number of plant species in North Texas. Plant cold hardiness is a term used to describe the extreme winter temperatures that a plant can tolerate without significant damage. The majority of plants are rated on their cold hardiness and every terrestrial location has a cold hardiness zone (1 through 13) corresponding to the range of extreme cold temperatures it can tolerate. North Texas is currently designated Zone 8a signifying that the region usually experiences extreme winter temperatures between ten and fifteen degrees Fahrenheit. The temperatures seen in February 2021 were similar to winter temperatures in Zone 6b. There are a large number of plants in North Texas that have been planted at the margins of their cold hardiness. These plants are the most likely to be severely injured or killed by these extremely low temperatures.
There are a lot of factors that influence the effects of extreme cold temperatures on plants. The dormancy of perennial, woody plants is a major factor. Plants that are either actively growing or beginning to emerge from dormancy are more likely to sustain injury from freezing temperatures. Many trees and shrubs protect themselves from cold injury during winter by dropping their leaves (deciduous) and going dormant in the winter. An exception to this are some evergreens, including the Live Oak, Mexican White Oak, Hollies, Photinias, Conifers, Junipers, Cedars, and others. Some landscape plants are evergreen in North Texas because they tolerate our relatively mild winters. These plants are likely to drop their leaves in response to freeze injury and may not sustain any significant injury to their above ground parts, including stems and buds. In the spring, these plants may regrow their leaves to recover their canopy. Other evergreen plants that are not as tolerant to sustained extreme temperatures may have been permanently injured to the point of death or may die back to the ground.
The dormancy of deciduous trees, shrubs, and vines is affected by air temperatures and soil temperatures. Many fruit and nut producing plants will be influenced by chilling hours and break dormancy when they receive a certain number of chilling hours. Certain varieties of these plants have a defined number of hours they need each winter to properly break dormancy and flower at the right time in the spring. However, there are some varieties that have been mistakenly planted in our region and have the incorrect number of chilling hours for our region. These plants had started to break dormancy prior to the extreme freezing temperatures and therefore are likely to sustain significant injury and dieback.
The cold injury in your lawn or turfgrass is likely to be highly variable. St.Augustine grass is known to experience cold injury during extreme winters. However, the layer of snow helped to insulate the grass and soil from the extremely low temperatures. Soil moisture was adequate prior to the storm and helped to buffer the temperature change and prevent cold injury. We will know more about the significance of cold injury in early summer. I recommend planning to topdress your lawn with a one-half to one inch layer of topdressing compost this spring to help it recover. Avoid pre-emergent herbicides so as to promote root growth in your grass.
The cold injury to landscape plants in North Texas can be predicted in some plants that do not have the cold hardiness to survive these temperatures. In other plants we will have to wait and see how the effects manifest themselves this spring and summer. I do not recommend making any assumptions and preemptively removing plants. Even if a plant has apparently died to the ground, there's a chance it will grow from its roots or the base of the trunk. It is best to wait until spring or early summer to assess your plants and determine the best course of action. There is no immediate action required at this time. However, these extreme cold temperatures will cause stress in many plants, even those that are tolerant to these temperatures. Plant stress can lead to disease and insect problems. In spring, consider starting a fertilization and soil health treatment program to help alleviate and manage the stress.