Ornamental pears offer new option: invasiveness

Although most plants introduced for horticulture are not invasive, some escape their confines and spread into natural areas. Like most ornamental tree species, the Callery pear was not considered a threat because it did not produce fruit.

In the last decade, wild-type Pyrus calleryana trees have begun appearing and multiplying in natural areas, especially in disturbed sites along railroads, roads, restored wetlands and park boundaries.

The wild-type Callery pear differs from ornamental cultivars in several ways. The wild-type tree grows quickly, flowers at a young age and produces fruit that can be spread, thus increasing its numbers. As the wild-type tree population expands, the threat of invasiveness increases.

Invasive plants inflict an economic and ecological cost to agriculture and natural ecosystems. Considerable work is being conducted on plants that have already established themselves as an invasive threat, but little work has been conducted on plant species beginning their reign in the invasive category.

A closer look at the threat

Scientists still do not fully understand why some species become invasive while others do not. Since the Callery pear evolved into a significant invasive threat, Assistant professor of Plant Biology Theresa Culley and PhD candidate Nicole Hardiman at the University of Cincinnati have used this seemingly nonthreatening plant to model how an ornamental species can evolve to become invasive. Their work, funded by USDA’s Cooperative State Research, Education and Extension Service (CSREES), focuses on several factors that contribute to the evolution of invasive traits in introduced plants.

“The Callery pear is an interesting case because its wide popularity as an ornamental has led to the unanticipated consequence of invasive spread. By comparing the DNA of different trees, we now know that invasive populations mainly originated from cultivated trees planted in surrounding residential and commercial areas,” Culley said.

The invasive wild-type Callery pear

Individual Callery pear trees are not an invasive threat by themselves. When multiple varieties are planted in the same area, cross-pollination can lead to production of a new and invasive wild-type.

The expansion of wild-type Callery pear over the past decade appears to stem from the number of cultivars planted in such close proximity. This environment encourages cross-pollination between several genotypes, creating cultivars capable of producing fruit.

The expansion may have also been propelled from improperly maintained Callery pear cultivar grafts. Callery pear scion wood may cross-pollinate with P. calleryana rootstock, if allowed to sprout and flower. Such occurrences have led to pear tree infestations in abandoned nurseries.

The same traits that were favorable for Callery pear cultivars as an ornamental are also key to the potential invasiveness of a plant. Callery pear cultivars grow rapidly, produce abundant flowers and possess a tolerance to a wide array of environmental conditions.

The impact of the wild-type pear is still not fully realized, but it is slowly being understood in newly restored wetlands and disturbed areas where the wild-type pear may impede the establishment of late- to middle-stage succession species. In addition, the formation of dense, thorny thickets at the base of wild-type pear makes removal difficult. From an aesthetic standpoint, the wild-type pear drops fruit, producing unsightly ground litter that attracts unwelcomed insects and birds that further promote propagation of this invasive species.

“The Callery pear story is an example of shooting ourselves in the foot. We thought we developed a fabulous urban tree -- well-behaved, explosive flowering display and sterile, so no mess or worry,” said Michael Bowers, national program leader at CSREES.

“It was planted widely, especially in the Washington, D.C., area, but nature has found a way to outthink the scientists. There is a lesson to be learned here.”

Control mechanisms

Because Callery pear varieties are so well-established as landscape trees in urban settings, there is an urgent need to monitor the impact of the newly evolved invasive populations of P. calleryana on the ecosystem.

There are limited options for controlling the wild-type pear once it becomes established in a region. The most effective control practice is to completely remove the tree and treat the trunk with an appropriate systemic herbicide, such as concentrated glyphosate or triclopyr, to prevent regrowth anywhere on the freshly cut trunk.

Mowing or tilling saplings and small trees is ineffective because the species readily sprouts from existing trunk or root systems. Suckers should be cut back to the base of the tree trunk or removed promptly to prevent growth, flowering and cross-pollination.

Callery pear cultivars that are not removed should be sprayed with ethephon when the tree is in full bloom. This treatment is 95 percent effective at preventing fruit set without causing premature blossom drop.

The next step

Consumer demand for unusual and colorful plants has increased the introduction of non-native plants, which are mass-produced, transported and distributed through nurseries, gardeners and the Internet. Introduced plants with certain traits, such as substantial fruit production, have some potential of escaping the garden confines and becoming invasive. In the case of Callery pear cultivars, this may occur through cross-fertilization between genetically distinct cultivars or possibly native species and also rootstock, resulting in viable offspring with unique and invasive genotypes.

Plant breeders and growers provide the first line of defense in this battle. Breeders and growers can help to identify potentially invasive species and cultivars. They can provide the public with suitable alternatives to popular and potentially invasive plants or provide completely sterile cultivars of highly popular species. It is up to scientists to develop accurate predictive models to map the expansion of invasive plants and identify invasive traits that will be useful to breeders and growers in screening their plant supply.

For more: USDA’s Cooperative State Research, Education and Extension Service, (202) 720-8188; http://www.csrees.usda.gov/newsroom/impact/2008/nri/01153_peartree.html. Theresa Culley, University of Cincinnati, (513) 556-9705; www.uc.edu.

- Stacy W. Kish

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Stacy W. Kish is a science writer with the USDA’s Cooperative State Research, Education and Extension Service.

May 2008