Woody ornamentals can be damaged by cold temperatures due to three main factors: lack of hardiness, early or late-season frosts or unprotected root systems. A lot of the damage that I have personally encountered has been due to low temperatures causing bark splitting. This type of damage is often difficult to diagnose because the effects are usually very delayed. Low temperatures typically either cause damage in the fall or late winter/early spring in our region. In the fall, we typically see damage occur when temperatures dip unexpectedly low before plants have a chance to harden off their tissues before the cold. Fertilizing late and fall pruning can make this worse, as both practices stimulate new tender growth that is vulnerable. Similarly, when we have periods of warm temperatures that sharply contrast with cold temperatures in late winter and early spring, we see lots of damage. The warm temperatures coax the plants out of dormancy and stimulate new growth that is easily damaged by cold temperatures. A common symptom that I often see caused by cold damage is bark splitting. This is when the stem or bark splits, usually near the base of the tree due to a wide swing in temperatures. If this happens at the crown, the plant may not survive. Another type of damage is known as frost cracks. These are long cracks that are typically linear along a trunk or branch, and affect not only the bark but also the wood. These are commonly seen on the southward or southwest side of young and/or older trees with smooth bark (e.g. red maple). This is because the sun can warm the tree on that side and cause the outer wood to rapidly expand. Once the sun goes down and temperatures drop, the outer part contracts faster than the inner tissues. This difference can cause the outer trunk to crack. Young and thin-barked trees can and should be protected by wrapping the trunk with paper tree wrap or burlap. The wrap should cover the trunk from the ground up to the first lowest branch. If you have ever seen young fruit or even ornamental trees with their lower trunks painted white, this is why. The white latex paint reflects the sun’s rays and helps to prevent frost cracks.
An important thing to remember is to always wait and allow ample time to see the full extent of the winter damage before corrective pruning is done. A couple of tips for avoiding this type of injury include:
Avoid fertilizing trees and shrubs in the fall. Fertilizing may stimulate the tree to put on new growth, which is vulnerable to cold temperatures.
Avoid pruning trees and shrubs in the fall. Pruning is known to stimulate flushes of new growth, which is vulnerable to cold temperatures.
If the summer and/or fall are unusually dry, it is recommended to provide supplemental water. Drought stress predisposes trees and shrubs to winter injury.
Pack potted plants close together to reduce air movement between the containers. Mulch or soil can also be packed around the containers or root-balls to help insulate them. If potted plants are covered with plastic in cases of extreme cold, be sure that the plastic does not touch the plants or is removed promptly the next day as plants can be scorched by the sun.
For more information see HGIC2350, Cold Damage.