Pretty Invasive

Published: April 1, 2009

At this time of year, it is easy to see why ‘Bradford’ pear and newer cultivars of Pyrus calleryana are among the most widely planted urban trees in the Midwest. They are fast-growing, have a spectacular bloom display and pleasing, symmetrical form. The summer foliage is glossy and attractive and they can get an eye-popping crimson fall color if conditions are right. Despite their more upright forms, some of the newer cultivars have wider branch angles than ‘Bradford’ and are somewhat less susceptible to storm damage. However, some recent developments have begun to make some people re-think their plans for extensive plantings of ornamental pears.

Fire blight is a bacterial disease that commonly affects pears. Until recently, P. calleryana was considered highly resistant to this problem. However, in the past ten years there have been increasing reports of dieback and disfigurement of trees of many of the P. calleryana cultivars. While fire blight can be managed by pruning and spraying with streptomycin, improper use of either of these measures can have disastrous effects, including development of antibiotic resistant organisms.

In the past few years, there have been numerous reports of invasion of natural areas by P. calleryana seedlings. ‘Bradford’ and other cultivars have generally been considered self sterile and, therefore, not likely to produce seed. However, recent evidence indicates that cross pollination between trees of different cultivars can lead to development of fruit with viable seeds. Since fruits are relatively small, they can be eaten by birds, which then distribute the seeds widely. Since seeds seem to germinate readily and trees grow quickly, there are currently some alarming populations of P. calleryana that are showing their potential to shade out plants in native areas and create power disruptions from limbs falling on power lines in unmanaged areas.

This is yet another example illustrating the need for more diversity in urban landscapes. There are good reasons why Mother Nature rarely creates a pure stand of any plant. We should follow her example more often.