Straw-colored foliage is typical of spring frost injury.
This year, 2020, is already in the history books as the “Year of the Coronavirus,” but boxwood growers will also remember it for some of the worst frost/freeze damage seen in decades. Central Virginia experienced two abnormally late spring frosts, one in mid-April and the other on Mother’s Day morning, May 10. Although boxwood are typically hardy evergreens that can tolerate cold, they can be susceptible to injury when extreme weather events occur. There are two different types of cold temperature-related issues. Abnormally cold night temperatures in the fall or spring result in frost or freeze damage whereas extreme cold winter temperature events result in cold injury.
Frost or Freeze Damage
Running irrigation water over the plants during a cold event can minimize frost or freeze injury.
The terminology frost damage usually refers to a rather mild frosting of plants in the fall or spring, generally resulting in light die-back in the tops of boxwood. Freeze damage occurs at the same time but describes more severe damage as a result of colder temperatures or more prolonged cold temperatures. In the mid-Atlantic area of the United States, the damage from these weather events can occur in the fall from October through early November, or in the spring from late March through early May. The resulting damage is a “burning” of the most tender foliage, usually the tips of the newest growth, of the boxwood. The damage occurs when the temperature gets below 30 degrees Fahrenheit, with the worst damage occurring during a freeze event when the temperatures get into the low to mid 20’s. An extreme freeze event, with temperatures in the low 20’s in the fall or spring, can cause bark splitting, with resulting dead limbs of perhaps even the entire plant.
Frost blankets are specifically designed to be placed over plants during cold weather events. The fabric is breathable to prevent diseases, but warm enough to protect tender boxwood.
The discoloration and spotting on this Buxus microphylla cultivar could be mistaken for Boxwood Blight but is actually a common sign of frost or freeze damage.
In the fall, the extent of the damage of a frost/freeze event is related to the temperatures leading up to the cold temperatures, with the worst damage occurring with a very warm fall followed quickly by a severe and early frost or freeze.
In the Spring, the extent of the damage is related to the development of the new growth in the boxwood, with the worst damage occurring with a very early and warm spring fully pushing out the new growth of the boxwood, followed by an abnormally cold night.