By Fanny Iriarte
Iowa State University Extension
Boxwood (Buxus sempervirens) is a dark green evergreen native to southern Europe and northern Africa. It usually grows to a height of three to four feet in the Iowa climate, and is popular for borders and hedges because of its dense, dark green foliage. Boxwood requires fertile, well-drained soils and prefers wind protection if grown on an exposed site.
The past winter was tough on boxwoods in Iowa. We have received numerous reports of boxwood dying. Sometimes, entire hedges succumbed. Exceptionally low temperatures were to blame in most cases. Boxwoods are marginally hardy in Iowa, and can suffer from foliar burn and twig kill in severe winters in exposed locations. Leaves turn brown and twigs die back.
Plants subjected to subfreezing temperatures may exhibit ice formation in the vascular vessels, in the spaces between the cells and/or within the cells. If the freezing episodes are sufficiently extreme, as in the punishing winter of 2008-09, cells can die.
Typical signs of freezing injury are a blackened/brownish discoloration or bleaching of plant tissue. If the freezing injury kills a significant number of buds or cambial tissue, the plant may die or suffer so much crown die-back that it becomes unusable. If freezing injury is limited to flower buds and shoot dieback, it may require corrective pruning and time to allow the plant to grow out of the damage.
The level of damage depends on the cold hardiness of the plant; in other words, its ability to withstand cold temperatures without sustaining injury. The level of hardiness that a plant can attain depends on genetics, preseason conditioning and its current condition. The genetic origin of a plant contributes to its ability to "harden." Plants originating from warmer climates may not be as cold hardy when moved into northern regions as the same species grown in northern regions.
Preseason conditioning refers to the changes that occur in a plant as it begins to harden off. Short days and cool night temperatures of early autumn trigger metabolic changes that allow plants to withstand lower temperatures. In the Midwest, maximum levels of hardiness are usually reached by early January. As spring approaches, plants begin to deharden or lose cold tolerance with increasing temperatures. Late frosts can cause injury during this dehardening phase when plants begin to break bud and initiate shoot growth, losing their cold tolerance. Late-winter injury observed on some evergreen plants can be the result.
A plant’s state of health influences its ability to tolerate low-temperature extremes. As you’d expect, healthy plants are likely to be more cold-hardy than stressed plants. Plants stressed by drought, flooding, nutrient deficiencies, transplant shock or pest problems may not become fully hardy. So to give your plants the best chance to ward off cold inpury, keep them healthy the rest of the time. Make sure sufficient soil moisture is available during the fall, and add organic mulch around the base of the plants such as wood chips if needed to conserve moisture. Plants located at the east side of a building are better protected. Covering vulnerable plants to provide winter protection from wind also can be beneficial.
A vigorous, healthy boxwood is your best defense against winter injury. Drought stress is one of the main factors that predispose plants to diseases, so it is important to water plants deeply and regularly as needed. Even with the best precautions, unusually cold spells like we experienced in 2008-09 are likely to injure these somewhat tender plants. When this happens, prune back to the live wood and give them a chance to recover.
For more information about boxwood and frequently asked questions, see the United States National Arboretum webpage http://www.usna.usda.gov/Gardens/faqs/boxwoodfaq2.html or the American Boxwood Society, http://www.boxwoodsociety.org/FAQ.htm.
In addition, North Carolina Cooperative Extension Service has information about boxwood species and cultivars for the different USDA plant hardiness zones, and other important information http://www.ces.ncsu.edu/depts/hort/hil/hil-407.html .
(Mark Gleason, plant pathologist, and Jeffery K. Iles, horticulturist, contributed to this article.)