Garden Variety: Wild growth of Callery pears concerns conservationists
Callery pear trees are easy to spot this time of year in the landscape, on roadsides and in unmanaged pastures. They have stiff upright branches loaded with white blossoms that are part of what have made them a popular choice in landscape plantings since the 1960s. In recent years, the number of Callery pears growing wild is starting to concern conservationists and land managers in some regions in the U.S., including Kansas.
Callery pears are often referred to by their variety names, including Bradford, Aristocrat, Chanticleer, Cleveland Select and several others. They are all Pyrus calleryana, and the other names refer to specific cultivars of the species.
Plant breeders were initially interested in Callery pears for resistance to a disease called fire blight that affected orchard pears. As breeding and selection work went on, breeders recognized that Callery pears were also drought-tolerant, adaptable to a wide range of soil conditions, had few other pest problems and were pretty. In addition, the trees were small — only 20 to 25 feet at maturity — giving them appeal for use as street and urban landscape trees.
Unfortunately, when Callery pears grow in the wild, they form dense thickets and outcompete native plants. They make land unmowable and are difficult to remove or kill once established in an area. Land managers are especially concerned about the displacement of native plants and about the exponential effect of their establishment in wild areas. One tree can produce hundreds of seeds each year.
Why is this happening?
When Bradford pears (Pyrus calleryana ‘Bradford’) were first released, one of their desirable attributes was being sterile. If they did produce fruit, it was small and rarely contained seeds.
Botanically, Callery pears are self-sterile, meaning that on a single tree, the pollen cannot pollinate its own flower. Since Bradford pears are propagated from cuttings, they are all genetically identical, so all Bradford flowers are incompatible with each other.
Other cultivars are the same way — an Aristocrat cannot pollinate an Aristocrat. But an Aristocrat can pollinate a Bradford. Aristocrat, Chanticleer, Cleveland Select and the others gained popularity after people realized Bradford tended to split after 20 to 25 years of growth. These selections had better branch structure, fall color, more abundant blossoms, etc.
As the other cultivars became more widely planted, cross-pollination began. Then birds carried seeds to new places. As trees established in unmanaged land, they produced more seed, and the problem became exponential.
In rare cases, a single tree could also pollinate itself. Callery pears are grafted onto a hardy rootstock of the species. If sprouts emerge from the rootstock and produce flowers, they can pollinate the flowers of the main tree and vice versa.
What can you do?
If you have a yard or own land, keep your eyes open for seedlings and remove them as soon as possible. This is a manageable task in the landscape. In pastures, grasslands, ditches, roadsides, fencelines, and anywhere else wild Callery pear seedlings grow, get rid of the trees as soon as possible. Dig or pull small trees. Cut larger trees off at ground level. Treat stumps with systemic herbicide to prevent re-growth, or make plans to remove sprouts from stumps for several years following.
If you have Callery pear trees, remove suckers from the base of the trees if they appear to prevent possible cross pollination with flowers produced by the rootstock. If you already have one variety and want to plant another Callery pear, use the variety you already have instead of trying a new one to limit the amount of cross-pollination that occurs.
When Callery pear trees die, replace them with other small flowering trees such as serviceberry, fringe tree, seven-son flower, or with large shrubs such as viburnum.
Callery pears are getting a lot of criticism because of their growth in the wild in recent years. In many states it has been designated as invasive, although the designation means little other than a suggestion to landowners to be wary.
Ohio is the first state to enact an invasive plant law that gives regulatory authority. That law went into effect this year and includes a special clause that will prohibit the sale and distribution of Callery pear as of January 7, 2023.
— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.