ANNAPOLIS, MD (April 1, 2007) - Maryland roadsides are lit up during April with the white blossoms of Callery pears (Pyrus calleryana). When the first cultivated variety, or cultivar, was promoted to the horticulture industry in the early 1960’s, the small flowering Asian tree seemed perfect for fast-growing suburban landscapes. Here was a well-behaved ornamental that could soften the harsh edges of newly built communities, with its neat formal shape, explosion of bloom and showy fall color. It was not thorny like many pears, and because it did not self-pollinate, it produced small sterile fruits. But subsequent introductions of additional Callery varieties that did allow cross-pollination have turned an April snow shower of bloom into a blizzard, leading the Maryland Invasive Species Council to name this tree as the April Invader of the Month.

Callery pear or ‘Bradford’ pear, as it is commonly called after the first commercial cultivar produced, grows quickly to about 20-30 feet tall. It has simple, alternately arranged, glossy dark green leaves with rounded bottom edges and small even teeth along the wavy margins. The leaves turn a deep mahogany, russet or even scarlet in the fall. The ½” flowers bloom in early spring, often before the leaves are out. They have five white petals and are borne in clusters along the stems, often so densely packed that the entire tree appears to be white. The bark is gray and smooth when the tree is young, eventually becoming ridged with shallow furrows. Bradford pears have an upright habit, and very tight branch crotch angles, making them susceptible to breaking or splitting in even mild wind storms. Even without breakage, the tree is relatively short-lived – 25 to 30 years.

Callery pears were originally cultivated in the early 1900’s as root stock for commercial pears. The ‘Bradford’ cultivar was introduced by the USDA to the horticultural industry for its rapid growth, dense foliage, and spring profusion of pure white blossoms. Although it seemed an ideal street tree, its tendency to split as it reached maturity made it less desirable. Alternative cultivars less prone to splitting were developed and introduced – trees like ‘Aristocrat’, ‘Cleveland Select’, and ‘Redspire’. With additional cultivars present in the landscape, cross-pollination occurred. ‘Bradford’ and other once sterile cultivars began to produce viable seeds. And that is when the trouble began – the offspring of these docile ornamental beauties became aggressive invaders.

‘Bradford’ pear seeds are now commonly sown by birds, and so spread rapidly from plantings into nearby meadows, pastures, roadsides, woodland edges, hedgerows and other sunny habitats. The effect is to choke out the native grasses, flowers and shrubs that would normally provide critical habitat for many of Maryland’s birds, insects and butterflies. These seedling clusters are beginning to dominate the landscape, frequently occupying the forest edge space where native serviceberry, redbud and dogwood normally grow. Pear seedlings have even been observed growing in open patches within mature forest.

While invasive pears can be controlled by both mechanical and chemical means, perhaps the most formidable obstacle to preventing the spread of wild seedlings of Callery pears is their popularity. Callery pear cultivars have been favorites for landscaping new developments, parking lots, and malls. In many communities, ‘Bradford’ or similar a variety, is the first and only ornamental street tree. In most neighborhoods where it is planted, it provides a welcome bit of color in both spring and fall. The ecological threat implied by the forests of wild Callery pears along our roadways is most often ignored; the April flower display is welcomed. But this seemingly innocuous invader is changing the native landscape. Wild pears can be cut down, although they will often resprout vigorously. They can be treated with triclopyr or glyphosate in cut stump, hack-and-squirt, or basal bark herbicide applications, according to label directions. Together, public education and wild seedling control efforts will ensure that Maryland’s April floral snows are serviceberry blossom flurries, not Callery pear blizzards.

For more information about other Invasive Species of Concern, visit www.mdinvasives.org or call the Maryland Department of Agriculture at 410-841-5920.

For more information on the Internet:

Plant Invaders of Mid-Atlantic Natural Areas

Invasive.org

Photos: Sara Tangren