1. Pyrus calleryana Decne. (Bradford pear)

Map 2483

Plants trees to 15(–20) m tall, usually thornless, rarely thorny. Twigs glabrous or rarely short-hairy, the winter buds relatively large, the scales densely cobwebby-hairy and fringed. Petioles 20–45 mm, moderately hairy when young, glabrous or sparsely hairy at maturity. Leaf blades 4–9 cm long, elliptic to ovate or more commonly broadly ovate, occasionally nearly circular, mostly short-tapered to a sharply pointed tip, rounded to shallowly cordate at the base, the margins finely scalloped or finely and bluntly toothed, the surfaces somewhat cobwebby-hairy during development, glabrous and shiny at maturity, green to dark green. Inflorescences variously umbellate clusters or short, broad racemes with 5–12 flowers. Sepals 2–4 mm long, triangular, the upper surface densely hairy, the undersurface glabrous, not persistent at fruiting. Petals 5–7(–10) mm long, white. Styles 2 or 3. Fruits 0.8–1.5 cm long, globose to slightly oblong-globose, the surface green to yellowish brown or blackish brown, with pale dots. Seeds 1 or 2(–4). 2n=34. March–May, rarely also September–October.

Introduced, scattered in the southern half of the state and continuing to spread (native of Asia, introduced widely in the eastern U.S. west to Illinois, Kansas, and Texas, also California). Banks of streams and mesic upland forests; also pastures, old fields, ditches, roadsides, and disturbed areas.

The history of the introduction of this species was reviewed in detail by Vincent (2005) and Culley and Hardimann (2007). Pyrus calleryana was imported into the United States in 1917 by the U.S. Department of Agriculture, which was tasked with locating Pyrus germplasm that was resistant to the fungal disease, fire blight (see also the treatment of Pyracantha in the present volume). It was used in breeding programs with the cultivated pear and also was used as root stock onto which other pears could be grafted. It was not until the 1950s that breeding of P. calleryana as an ornamental was begun, and the popular cv. ‘Bradford’ was released to the nursery trade in 1960. Bradford pear was touted as having low fruit set and was rated highly as a hardy street and yard tree tolerant of a wide variety of environmental conditions. It soon became one of the most popular ornamental trees in horticulture in many parts of the country, with an estimated 300,000 trees planted by about 1980 (Dirr, 1981). Concerns about the overuse and mass-planting of the trees initially was aimed at the potential impacts of new diseases on them, as well as the observation that the brittle wood tended to cause excessive breakage and trunk-splitting following ice- and wind-storms. By the late 1990s, flags were being raised about the invasive potential of the species into natural plant communities. In addition to the documented escape of plants into natural habitats, cases were noted in which trees reverted to the so-called wild-type for some characters, including production of more fruits and thorn-tipped branches.

Although the earliest specimen documenting the escape of P. calleryana in Missouri dates to 1952 in Barton County, virtually all of the other specimens in the state’s herbaria were collected on or after 1990. In portions of southern Missouri, the species has developed extensive populations during the past two decades, particularly in area that are recovering from heavy disturbance, such as highway improvement projects. Recent observations suggest that abundant fruits are produced in some populations, which are spread by birds and mammals. Culley and Hardimann (2007) proposed a broad potential range for the species nearly throughout the United States and considered it to represent an invasive exotic that was in the early phases of its epidemiology.

An unusual variant with brownish yellow, globose fruits to 2 cm in diameter has been collected sporadically in fencerows and pastures, thus far in Howell, Jefferson, Phelps, and Washington Counties. This was originally noted by Doug Stevens of the Missouri Botanical Garden, who says that the fermented pomes can be used to make a superior fruit wine. Flowering and fruiting specimens appeared initially to key to P. serrulata Rehder. However, following his studies of the Bradford pear complex in North America, Michael Vincent of Miami University (personal communication) redetermined these as probable hybrids of horticultural origin developed from crosses of P. calleryana and some other species, perhaps P. serrulata.