Is this cold going to harm the trees? This is a common question when the mid-winter temperatures start to approach or dip below the -10°F mark (something we did not see last year). Much of the state has experienced temperatures in the -1° to -10°F range during January, and while these temperatures are hard on people, livestock, and cars, they are not a problem for most of our trees and shrubs. Many of our woody plants have hardened to tolerate temperatures to at least -30°F by this time and some can tolerate temperatures as low as -60°F in mid-January. Much of our typical “winter” injury is not due to cold temperatures in mid-winter, but from unseasonably cold temperatures in late autumn or early spring. Our cold weather injury occurs in October as woody plants are beginning to prepare for winter, and also during March as they are coming out of dormancy. At these times many plants are not able to tolerate temperatures even in the single digits.
The one exception to mid-winter temperatures not being harmful is when we look underground. Most tree and shrub roots cannot tolerate temperatures of even 10° or 15°F in midwinter. A winter with little snow cover and dry conditions so the ground is cracked allows cold temperatures to penetrate deep into the soil. The cold doesn’t have to penetrate very deep as most of the tree and shrub roots are in the upper foot or two of the soil. Usually the soil temperatures in this zone stay in the upper teens or even twenties, but bare-dry soil and no snow (a combination that is present in some new windbreaks, particularly in the Southeast) may result in root-killing temperatures for these young trees and shrubs. Woody plants that have roots killed or injured by cold temperatures often break bud in the spring and begin normal growth, but then stagnante and wilt as the injured root system cannot keep up with the water demands of the new leaves and shoots.