Some landscapes are heavy on fluff but low on bones. While these gardens may have plenty of flowers and color through the growing season, they lack substance and a sense of place.
A mixture of hardscape elements, like stone walls and walkways, and the careful selection and placement of woody plants combine to create the garden’s bones. If a garden lacks substance and interest in the dead of winter, this space likely lacks bones.
Boxwood provides bones in the garden throughout the year. With a combination of finely textured leaves, deep green hues, sturdy forms and overall adaptability, boxwood adds what few other plants can add.
The quintessential hedge plant, boxwood has been used for centuries in formal European gardens, and fits well in herb and knot gardens. It is also well-adapted for sculpting or topiary. Best of all, boxwood is not preferred by deer (but it does have its share of pests — more on that later).
Boxwoods are best sited in full sun to partial shade in moist, well-drained soils. Protection from strong winter winds is important, although some cultivars are more wind-tolerant than others.
As a whole, boxwoods are very slow-growing and tolerant of pruning. They have shallow roots, however, and are not tolerant of cultivation or planting in and around the root zone.
Common boxwood, Buxus sempervirens — This boxwood is a wide-spreading shrub with dense, evergreen foliage. Leaves are glossy and waxy when they first emerge. Numerous cultivars are available, including the slow-growing ‘Suffruticosa,’ known as the “English” boxwood for its use in formal English gardens. ‘Suffruticosa’ can be tightly clipped, making it one of the most popular common boxwood cultivars. ‘Vardar Valley’ is another low-growing cultivar with dark, blue-green foliage. For tight, vertical spaces, try Green Tower, a very narrow columnar form that grows 9 feet high and only 18 inches wide.
Japanese or littleleaf boxwood, Buxus microphylla — This compact shrub is available in many cultivars, from the low ‘Green Pillow,’ only 18 inches wide by 12 inches high, to ‘Tall Boy,’ which grows to 5 feet in height. Commonly available cultivars include: ‘Winter Gem,’ a nicely rounded form reaching only 2 feet in height; and ‘Wintergreen,’ a mounded form with dark green foliage reaching 3 feet in height and 4 feet in width.
Hybrid crosses between the Korean boxwood (a variety of Japanese boxwood) and the common boxwood bring together the form and cold hardiness with good leaf color. Resulting cultivars include: ‘Green Gem,’ a tight 2-foot mound; ‘Green Mountain,’ an upright pyramidal oval growing 5 feet high by 3 feet in width; and ‘Green Velvet,’ with dark green leaves on a rounded plant reaching 4 feet high and wide at maturity.
Winter injury is one of the most common problems on boxwood. This manifests as yellowing or bronzing of foliage in winter and spring; in severe cases entire twigs will die back from this damage. Proper site selection and fall watering can help prevent winter injury, although some cultivars are simply more susceptible to injury than others. Through plant breeding, winter-tolerant cultivars are available that hold their color through winter, including ‘Green Velvet,’ ‘Green Mountain,’ ‘Winter Gem’ and ‘Winter Beauty.’
While anti-dessicants are sometimes used as a way to prevent winter damage, research shows these products have little lasting effect. Some gardeners shroud their boxwoods in burlap for the winter. To me, this is an ugly option for a plant that was likely planted in the wrong place.
Boxwood psyllid and boxwood leafminers are the most problematic insect pests of boxwood in Ohio. Psyllids are small, aphid-like insects that cause cupping of the leaves at the branch tips. These insects hatch out in early spring, as saucer magnolias are blooming.
Boxwood leaf miner adults lay their eggs in leaf tissue in early summer about the time weigela shrubs bloom. Immature leaf miners hatch out and eat the leaf tissue in between the upper and lower leaf surfaces.
Because they are protected under leaf tissue, contact insecticides are not effective against this pest once the immatures have hatched. Systemic insecticides can be used to manage this pest, but they should be used only after boxwood flowering since boxwood flowers are attractive to pollinating bees.
Boxwood blight is a serious and deadly disease of boxwood, first diagnosed in Ohio in 2011. This disease can cause repeated defoliation and death of shrubs. Symptoms include: brown spots or lesions on leaves, sometimes with dark borders; spots eventually growing together, often with a target-like appearance; areas of straw-colored foliage and overall stem dieback and blighting of shoots; dark-brown to black angular spots on stems; and significant leaf drop and stem dieback if the disease is severe.
Evidence of the fungal pathogen includes masses of spores present on the undersides of leaves and stem cankers with spore masses having a whitish, downy appearance.
The microscopic spores are easily dispersed by wind and splashing water, causing new infections especially when boxwood plants are growing closely together. The pathogen thrives at temperatures from the mid-60s to mid-70s and with high humidity. The fungus survives during and between seasons in cankered areas on the stems and in leaves and leaf debris as fungal strands and as fungal masses which allow the pathogen to survive long periods in the soil.
Volutella blight is another common boxwood disease. The volutella fungus causes salmon-colored spore bodies on undersides of leaves. Tips of infected branches turn red, changing to bronze and yellow. Infected branches die back. This disease is not as severe as boxwood blight, usually attacking shoots damaged by spring frost injury. Diseased branches should be pruned out when leaves are dry to avoid the spread of disease.
Sunscald and winter damage on boxwood are also common. Symptoms include blanching of leaves due to water loss in the winter and spring.
Root rot, crown rot and nematodes can cause long-term decline and death. Yellowing of foliage followed by wilting and death is associated with phytophthora root rot; plants infected with this fungus cannot typically be saved.
To avoid disease problems, select healthy plants and site them properly. Thin out overcrowded branches to promote air circulation. Do not plant boxwood in heavy, poorly drained soils or in dense shade with poor air movement.
For spectacular images of what can be done with boxwoods, visit Boxwood Garden’s website at www.boxwoodgarden.com. This Oregon nursery’s website has a photo gallery of formal and informal boxwood uses in every season.
The nursery display gardens feature over boxwood 50 cultivars. After a virtual visit, plan a spring trip to your favorite nursery and add some bones to your own garden.
Denise Ellsworth directs the honeybee and native pollinator education program for the Ohio State University. If you have questions about caring for your garden, contact her at 330-263-3700 or click on the Ask Denise link on her blog at www.osugarden.com.