In open fields and forest edges along local roads in Maryland in early spring, the view of hundreds of small trees bursting forth with white flowers inspires a feeling of euphoria that winter has finally ended. What is not obvious is that many of these trees are non-native pears spread by birds that carry the seeds from our neighborhoods and city streets to forest margins, abandoned fields, and other areas. These trees are crowding out wild cherries, plums, and other native plants in Maryland and are recognized as invasive by the Maryland Invasive Species Council.
When we bought our house on Snowy Owl Drive in 2000, there was a medium-sized Bradford pear tree in the front yard. Alan Friedman, who has lived on the street since the houses were built in 1985, recalled that the developer planted either a Bradford pear or a pin oak in every front yard on Mountain Quail Road, Margate Road, and Snowy Owl Drive in the Kinsman Farm community. The result was a lot of Bradford trees in the neighborhood.
Many qualities made Bradford pears popular, including their tolerance to poor soils and drought, resistance to diseases and pets, compact form, beautiful white flowers, pea-sized fruit, and foliage that produced impressive displays in the fall. Thirty-seven years later, most of these trees, including the one in my yard, are gone, succumbing to structural problems that no one anticipated when they were planted. The branches of Bradford pears are arranged in a way that often results in their splitting away from the main trunk, destroying the tree. One of the only surviving Bradford pears on Snowy Owl Drive was wired early on to support the branches, as pointed out to me by Judith Horne, another original resident on the street. The poor structure of the cultivar has resulted in its losing favor for landscaping; but the environmental damage caused by the spreading of its seeds to natural areas is a much more serious problem that continues to accelerate.
The Bradford pear is a cultivar of Pyrus calleryana, the Callery pear, a species native to China, Korea, Taiwan, and Vietnam. Another species of pear (Pyrus communis) is the pear grown commercially for fruit. In the early 1900s, the U.S. pear industry, centered in the western coastal states, was in danger from a severe disease called fire blight. A plant breeder in Oregon, Frank Reimer, discovered that Callery pears already present in the U.S. had some resistance to fire blight and wanted to use them in a breeding program to address the problem, but he needed many more types to test. At his request, the USDA Office of Foreign Seed and Plant Introduction sent a plant explorer, Frank Meyer, to China for two years to search for Callery pears to bring back to test for the stronger resistance.
The seeds of the Callery pear were sent to the U.S. from China with the best of intentions — to save the pear crop on the West Coast from destruction. Once introduced, the search for resistance to fire blight began. Large numbers of seeds were planted both in Oregon where Reimer worked and at the USDA Plant Introduction Garden in Glenn Dale, Maryland. The resulting plants were purposefully infected with fire blight to see if they could withstand the disease. Plants were also tested to see if they would make good rootstocks for the cultivated pear trees. The Callery pear was found to be very resistant to fire blight and to have other useful characteristics, such as tolerance to drought. Its rootstocks began to be widely used for the common pear.
It was in one of plantings of Callery pears at the USDA station in Glenn Dale that the ornamental appeal of the species was first noted. In 1952, a few trees from one of Frank Meyer’s collections in China decades earlier were still growing at the site. One thornless tree with an attractive structure caught the eye of John Creech of the USDA. Creech grafted cuttings of the plant onto roots of other types of Callery pears. All the trees produced from the cuttings were genetically identical (clones) as they came from the same plant.
In 1954, Creech planted 180 of the pear clones along the streets of University Park in Prince George’s County as a study of its feasibility as an ornamental street tree. After eight years, the carefully pruned test trees were considered a success. The pruning of these trees undoubtedly helped to hide their unstable structure. The cultivar was named “Bradford” in honor of F.C. Bradford, a horticulturalist who had previously managed the Glenn Dale station; by 1962, the cultivar was available commercially.
The Bradford pear quickly became one of the most popular street trees in the U.S. and was planted in many residential and urban areas. It was not until the 1980s that the major structural flaw in the trees began to be apparent. The narrow crotch angles of the branches eventually caused most individual trees to split apart during windstorms or under snow loads after approximately 15 to 20 years of growth. After the revelation of flaws in the Bradford pear, other cultivars of the Callery pear with different branching patterns were introduced and planted. Now ornamental pear trees planted in urban areas are often a mix of the Bradford pear and other cultivars.
A different type of problem with ornamental pear trees, an environmental problem, did not become obvious until decades after Callery pears became popular ornamentals. In 1965, Callery pears were found to have escaped cultivation in Talbot County, Maryland. By the late 1990s, a number of gardening groups noticed increasing numbers of wild pears invading natural areas in the Mid-Atlantic region, especially in Maryland. By around 2005, wild Callery pears were growing uncontrolled throughout the southern U.S. in all kinds of environments. They soon appeared on lists of invasive plant species that required special notice and control. The U.S. Fish and Wildlife Service has listed them as a plant invader of natural areas in the Mid Atlantic.
I asked Richard Olsen, Director of the USDA National Arboretum, who has studied the history of the Bradford pear, how scientists could have gone so wrong with the Bradford pear and other Callery pears.
He explained that, “Science has come a long way since the Bradford pear was released. The possible effects of non-native invasive species only began to be realized about 30 years after this cultivar was developed.”
The reason this species became so invasive can be found in the way the Callery pear is pollinated and the history of the development of cultivars. Neither the Bradford pear nor any of the other Callery pear cultivars are themselves invasive because they cannot pollinate and produce seeds by themselves. Only when they cross pollinate with an unrelated tree will seed in the pear fruits develop, germinate, and have the ability to form new plants. The mix of different cultivars of the Callery pear, along with the occasional sprouting and flowering of the unrelated rootstocks, has led to cross pollination and the production of abundant viable seed for birds to plant far and wide. The resulting trees create a “dead zone” of dense thorny thickets of pears that prevent native species from competing for water, soil, and light.
The Callery pear is continuing its march into natural habitats in at least 26 states. The area into which it has spread in the U.S. is currently defined by its limited tolerance to cold weather but, as climate change shifts warmer temperatures north, the potential for spread will increase. It is listed as an invasive plant in several states, including Maryland. Discontinuing use of Callery pear cultivars as landscape plants will eliminate this source of seed for new invasions. Frostburg, Maryland, is one of several communities across the country that has a program to provide free native trees to residents who want to replace a pear tree. Many native trees — including redbud (Cercis canadensis), white fringe tree (Chionanthus virginicus), common serviceberry (Amelanchier arborea), Canadian serviceberry (Amelanchier canadensis), and sweet crabapple (Malus coronaria) — make excellent substitutes for the Callery pear and will help support healthy ecosystems. See “Landscaping with Native Plants” (www.mdflora.org/resources/Publications/GardenersGuidelines/Landscaping-Natives.pdf) for more information.
For more information, read the 2017 article “The Rise and Fall of the Ornamental Callery Pear” by T. Culley (arboretum.harvard.edu/stories/the-rise-and-fall-of-the-ornamental-callery-pear-tree).
[Williams, a USDA botanist, has lived on Snowy Owl Drive for 22 years and in the neighborhood for 35.] ■
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