The introduction of exotic plant and tree species for the purpose of aesthetic gardens or utilitarian roles has been practiced within the fields of horticulture and forestry since ancient times. Our tendency to become dissatisfied with native flora all too often cultivates a desire to expand our landscape pallet to non-native varieties and cultivars. Such new specimens can produce vivid floral or textural displays, offering an arboretum or landscape renewed appeal.
Unfortunately, the cost of promoting newly discovered species that are out of place in our native ecosystem is rarely known until years after their introduction. Much of the time there are minimal impacts to the native ecosystem from well contained non-native landscapes. Occasionally, however, a plant or tree can escape and pose enormous consequences to native plant communities.
Pyrus calleryana is one such exotic species that have “escaped”. Ornamental (Callery) pear trees have been prized for their tidy form and magnificent spring display of snow-white flowers. Although now known to be weak-wooded and prone to storm damage, Callery pear cultivars such as ‘Redspire’, ‘Bradford’, ‘Capital’, ‘Cleveland Select’, and others are foundational to many landscapes because of their sure establishment and nearly pest-free acclaim.
Although earlier thought to be sterile and unable to reproduce, wild hybrids are now sprouting up across the country where landscape pear plantings have matured. These new pear hybrids are not only thorny but also have retained the weak branching structure of the parent cultivars. Hybrids from nursery-produced trees are also extremely vigorous and seem to tolerate numerous site conditions, eventually producing a thorn-filled stand of trees too thick for native regeneration. This spread may prove to be one of the most serious problems to date for land managers and homeowners alike.
Within our neighboring state of Missouri, Columbia Park Natural Resource Supervisor Brett O’Brien successfully partnered with the Missouri Department of Conservation and the Missouri Community Forestry Council to form an educational program called, “Stop the Spread!” In 2002, the Columbia Parks Department began noticing wild pear trees growing in a natural area. Within 6 years, thorny hybrid pear trees are now abundant and increasing within Columbia utility right-of-ways, natural areas, and even well-maintained park landscapes.
According to an article by Theresa Culley and Nicole Hardiman in Bioscience (December 2007 / Vol. 57 No. 11), Callery pear cultivars are now listed by 6 eastern states as invasive, with more to come. Culley/Hardiman notes that pear hybrids spread rapidly within restored prairie wetlands and are capable of producing “impenetrable” thorny thickets.
As ornamental pear populations increase from landscape plantings within our region, the danger for cross-pollination and the production of wild pear hybrids will increase. The importance of developing awareness to this problem and reduction of market demand for ornamental pear trees is critical. Homeowners, developers, nursery growers, and municipal forestry departments share the responsibility to promote the use of alternative species as substitutes for the Callery pear. Without such cooperation, Central Iowa could soon face the costly and ecologically devastating effects of hybrid pear colonization now being realized elsewhere.
