One of the most notable heralds of spring in the eastern United States is the profuse blooming of ornamental pear trees in front yards and along city streets. The Callery pear (Pyrus calleryana), and particularly its many cultivars such as ‘Bradford’, ‘Cleveland Select’, and ‘Aristocrat’, has become one of the most popular ornamental trees in North America. However, its commercial success has now become overshadowed by its tendency to spread along roadways and into natural areas through reseeding. Today this tree is considered invasive in many states, in stark contrast to how it grows in its native range in Asia. How did this tree become the scourge of land managers across North America? What has led to its fall from grace? To understand this fascinating story, we need to start at the beginning.
Seeds From China
Toward the end of the nineteenth century, farming began to replace ranching in the western United States and there was a growing demand for improved crops that could thrive there. The United States Department of Agriculture (USDA) began to focus on importing new plants for testing and, in 1898, created the Foreign Seed and Plant Introduction Office. The mission of this office, headed by David Fairchild, was to locate and import economically important plants from other regions of the world. Fairchild was especially interested in China, which was thought to possess a wealth of unexplored botanical resources. Chinese plants were also expected to grow well in the United States because China’s climate is very similar to that in the United States.
In the early 1900s, Fairchild began searching for plant explorers who had the dedication and stamina to tolerate the physical discomforts and social isolation of travelling for months in distant lands. He found Frank N. Meyer (1875–1918), a Dutch immigrant and former gardener who had a deep fascination with plants and saw nothing unusual about walking hundreds of miles on a botanical foray. Meyer eventually spent over ten years traveling across Asia looking for useful and valuable plants, seeking, in his own words, to “skim the earth in search of things good for man.” He eventually sent hundreds of shipments of cuttings and thousands of pounds of seeds back to the USDA. Many agricultural crops grown in the United States today, including certain grains, legumes, and fruits, resulted from Meyer’s collections.
But before he began his first Chinese expedition in 1905, Meyer visited several United States gardens to become familiar with Chinese plants in their collections. He was most impressed with the Arnold Arboretum’s collection and he met director Charles Sprague Sargent, who was keenly interested in Meyer’s travels but who also had a complex and often difficult relationship with Fairchild. Meyer clearly respected the Arnold Arboretum—he once requested that his Chinese material be sent there instead of to a USDA station because he felt it would receive better care. However, Meyer’s relationship with the Arnold’s plant explorer Ernest H. Wilson was somewhat uneasy at first, likely because they first viewed each other as competitors since Sargent had also asked Meyer to collect for the Arboretum during his Chinese expeditions. While Meyer was willing to oblige, this arrangement would sometimes place Meyer in an awkward situation because Sargent’s emphasis on capturing the diversity of the Chinese flora was often at odds with Fairchild’s directive to focus only on economically important species.
The search for new plants from other countries became more urgent in the early 1900s when valuable orchards of the edible French pear (Pyrus communis) were being decimated by fire blight in the Pacific Northwest. This bacterial disease blackened leaves and branch tips of infected trees as if they were scorched by fire, eventually killing large fruit trees, and it was quickly spreading throughout the region. In the hopes of breeding resistance to fire blight into P. communis, Professor Frank C. Reimer of the Southern Oregon Experiment Station hastily began testing all available Pyrus species and varieties for resistance to this devastating disease. The initial results proved disappointing, so a call was put out to locate Pyrus species in other parts of the world that might be fire blight resistant. Many plant explorers, including E. H. Wilson and Emil Bretschneider, traveled to Asia in the early 1900s, in part to locate new Pyrus species, often with the material sent back to the Arnold Arboretum. In 1908, Wilson first introduced P. calleryana into the United States with several seed lots accessioned and grown at the Arnold Arboretum.
In 1916, between his Chinese expeditions, Meyer visited the Pacific Northwest where he saw for himself the extensive fire blight destruction. He now understood the importance of his work because he learned from Reimer that resistance had only been found in the wild Chinese pear species P. calleryana and P. ussuriensis. However, Reimer needed more material for testing and Meyer agreed to collect and send back over 100 pounds of wild P. calleryana seeds during his next expedition. This was no small task since 25 pounds of cleaned seeds required at least 5,000 pounds of fruit.
During his subsequent months in China, Meyer focused much of his effort on Pyrus calleryana. He painstakingly collected thousands of the marble-sized pear fruits in the field or bought them directly from local Chinese. He later wrote Fairchild that:
Pyrus calleryana is simply a marvel. One finds it growing under all sorts of conditions; one time on dry, sterile mountain slopes; then again with its roots in standing water at the edge of a pond; sometimes in open pine forest, then again among scrub on blue-stone ledges in the burning sun; sometimes in low bamboo-jungle … and then again along the course of a fast flowing mountain stream or in the occasionally burned-over slope of a pebbly hill. The tree is nowhere found in groves; always as scattered specimens, and but very few large trees were seen.
In 1917, Reimer himself joined Meyer in China and they traveled together for several days, with Meyer showing Reimer the locations of Pyrus calleryana trees he had found. In his report, Reimer’s amazement at this plant is evident, which also heralded his eventual emphasis on the species as rootstock:
In its ability to endure diverse and adverse soil conditions, this species certainly is a marvel … I found it growing in all the various soil types … ranging from heavy clays to light sandy soils and disintegrated rock. I found it growing in shallow ponds, along streams, well-drained moist loams, and on very dry poor hillsides and hilltops. In places it was observed where the layers of soil above the bedrock was not more than eight inches deep.
Reimer also saw that under favorable conditions in China, the tree “is a rapid, vigorous grower, has a long growing season, and its leaves remain green and lusty until very late in the fall.” In central China, the trees were often cut off for firewood every few years but they would put out “new sprouts from the stumps and continue to live for many years.” These wild trees also typically produced prominent thorns (actually sharp spur shoots) that effectively protected against herbivory. The species seemed to be adapted to mild climates, with Reimer suggesting, “It is quite probable that it will not endure very severe winter climates.” However, he also wrote that trees that originated from China had proved to be very hardy at the Arnold Arboretum over 10 years, despite the more severe winters near Boston compared to the native range in China.
Upon his return, Reimer continued to work with other plant explorers to obtain Pyrus calleryana seeds for further testing. Tragically, Meyer never returned to the United States, drowning in the Yangtze River in late 1918 just as he was beginning his trip home. However, his much-anticipated collection of P. calleryana seeds was shipped back in his absence, to complete the task that he had begun years earlier. It is from many of these seeds that our story continues.
‘Bradford’, the First Callery Pear Cultivar
Over the next few years, Chinese seeds collected by Meyer and Reimer were planted in large numbers—primarily at the USDA Plant Introduction Stations at Corvallis, Oregon, and Glenn Dale, Maryland—to test their resistance to fire blight. Over time, interest in the species turned to its ability to serve as rootstock for the economically valuable edible French pear. For example, Reimer wrote that “thousands of seedlings have been grown” in the Pacific Northwest from Meyer’s original seed “to test this species thoroughly as a stock for our cultivated varieties.” In Glenn Dale, Meyer’s seeds were also planted out in large numbers to test the plants for resistance to fire blight, overall vigor, and stock-scion compatibility with P. communis.
In 1952, one of the remaining 33-year-old trees of Pyrus calleryana from Meyer’s Chinese seeds that was still growing near the Plant Introduction Station in Glenn Dale caught the eye of John Creech of the USDA (Creech 1973). This tree had thick, glossy leaves and an attractive globular form, with a lack of sharp spurs so typical of the species. Recognizing its potential as a landscaping tree, Creech grafted scions of it onto P. calleryana rootstock. This method of propagation means that every tree is genetically identical to the original mother tree. Creech named this cultivar ‘Bradford’, in honor of F. C. Bradford, the former horticulturist in charge of the Glenn Dale USDA station (Whitehouse et al. 1963). (Incidentally, the original ‘Bradford’ tree was destroyed years later to make way for a parking lot).
In 1954, Creech planted two-year-old ‘Bradford’ clones in a nearby residential subdivision in University Park, Maryland, for a street tree study. ‘Bradford’ swiftly became quite popular for its rapid growth, attractive foliage that was retained into late fall, extremely showy and abundant flowers in early spring, and its overall hardiness. The cultivar was commercially released around 1961 and then planted widely across the eastern United States in residential areas.
However, by the early 1980s problems with ‘Bradford’ pears began to appear, especially a tendency for older trees to break apart during windstorms or under heavy snow loads. Its branching structure was to blame, as described by horticulturist Michael Dirr (1998), “‘Bradford’ tends to develop rather tight crotches and I have seen trees that were literally split in half … the plant will … fall apart because of the development of many branches around a common length of the trunk.”
More Callery Pear Cultivars Appear
Over the next few decades, additional P. calleryana cultivars were quickly introduced as improved replacements for older ‘Bradford’ trees that had begun to split. For example, ‘Whitehouse’, a narrow columnar form with a strong central leader, was selected in 1969 from seedlings still growing near the Plant Introduction Station in Glenn Dale. This cultivar presumably began as a seed from the original ‘Bradford’ tree that had been pollinated by one of the other wild P. calleryana plants at the station. Similarly, ‘Redspire’ also arose as a ‘Bradford’ seedling and was patented in 1975.
At the opposite side of the country, ‘Autumn Blaze’ was selected in 1969 from among several hundred seedlings of P. calleryana growing at the rootstock research nursery in Corvallis. This cultivar, known for its striking red to purple leaf coloration in the fall, originated from seeds that Reimer had brought back from China during his trip with Meyer in 1917 or later in 1919. Other cultivars such as ‘Avery Park’ and ‘Grant St. Yellow’ also originated in the late 1960s and 1970s near Corvallis, most likely from Chinese seeds imported into that area.
Over time, additional cultivars arose from other areas of the country, presumably from different seed sources. For example, ‘Aristocrat’ arose in Independence, Kentucky, while the cultivars ‘Cleveland Select’, ‘Chanticleer’, and ‘Stone Hill’ were all derived from the same street tree in Cleveland, Ohio. The cultivar ‘Valzam’ was found growing among ‘Cleveland Select’ trees in Perry, Ohio in 1975—most likely an offspring of that cultivar but with unknown paternity.
These and other Callery pear cultivars became exceedingly popular as ornamental landscaping trees for residential and commercial use. Not only were they beautiful, fast growing, and inexpensive, but they were also extremely tolerant of very difficult growing conditions. In commercial areas, for example, the tree could thrive in the harsh conditions of parking lot islands and between streets and sidewalks, where temperatures were excessively high and water was scarce. Following the tragic events of September 11, 2001, when the World Trade Center’s twin towers fell in New York City, a Callery pear tree at the site was found still alive but severely burned with damaged roots and branches. Known today as the “Survivor Tree,” it was rescued from the site, taken to a local nursery to recover, and later replanted back in the memorial park at Ground Zero as a symbol of resilience.
In 2005, ‘Chanticleer’ was chosen as the Urban Tree of the Year by the Society of Municipal Arborists, who noted that, “This tree has all of the character and quality of a sheared topiary specimen plus, of course, the magnificence of its spring, summer, and fall outer garments—the white flower, crisp glossy green summer foliage, and full fall color.” Some homeowners associations in areas of the United States even had a requirement that a specific Callery pear cultivar had to be planted in each front yard. In fact, the Callery pear had become so popular that Michael Dirr lamented in 1989 that “cookie-cutter Bradfords … inhabit almost every city and town to some degree or another; the tree has reached epidemic proportions and is over-planted.” In 2009 alone, the species generated over $23 million in sales across the country (USDA 2010), including continuing sales of ‘Bradford’. Although the majority of commercial sales occurred in the eastern and southern United States, cultivars were also available along the western coastal states. The tree had reached its epitome of fame and glory.
The Fall From Grace
As ‘Bradford’ and other Callery pear cultivars surged in popularity, early indications of problems began to appear. Pyrus calleryana had escaped cultivation as early as 1964 in Arkansas and 1965 in Maryland (Vincent 2005), but it was not until the 1990s that the species began to be more widely noticed in natural areas, especially in southern states. For example, Michael Vincent of Miami University (Ohio) examined 300 P. calleryana herbarium specimens collected across the nation beginning in 1964. He found that 1% of all specimens were collected in each of the periods 1964–1969 and 1970–1979 before a dramatic increase began in 1980–1989 (17% of all specimens), continuing through 1990–1999 (31%), and mounting rapidly in the last three years of the study, 2000–2003 (50%). By the late 1990s, members of several Internet gardening forums began noting the increasing numbers of wild pears beside roadways along the mid-Atlantic coast, largely in the Maryland area.
By the middle of the twenty-first century’s first decade, thousands of young wild pear seedlings were growing undetected in the roadside vegetation across the southern and eastern United States. But as they began to flower in their third or later year, their profuse early spring blooms started to give them away. As each successive year revealed more and more wild pears blooming, public alarm began to sound. Land managers began to notice wild pears appearing in all types of habitats—along forest edges, in wetland areas, and even within forests. This is not surprising given Meyer’s and Reimer’s remarks about the many different habitats where P. calleryana is found in its native range. In the United States, word eventually began to spread of not only the slippery mess caused by pear fruits littering sidewalks, the difficulty in removing dense stands of thorny trees in natural areas, and the putrid smell of the flowers, but also increasing concern of liability caused by falling tree limbs damaging property and injuring people. Pyrus calleryana (often indicated as just “Bradford pear”) began to appear on watch lists and invasive plant lists in several eastern and southern states. But why had this pear, which had behaved for decades as a popular landscaping tree, suddenly start to spread uncontrollably?
The answer lies in the reproductive system of the species as well as its horticultural history. As with most other pears, Pyrus calleryana has a genetically controlled self-incompatibility system that prevents individual trees from pollinating themselves, thus requiring outcrossing among unrelated individuals. When ‘Bradford’ was first introduced and became so wildly popular, ‘Bradford’ trees were unable to cross-pollinate (since they were all genetically identical) and fruits were never produced. As additional cultivars were introduced they were often commercially marketed as “self-sterile” or even “seedless”—this was true, as long as each cultivar was grown in isolation. Cultivar patents and promotional material included notes such as “[fruit set] very low (about 5 to 10%), usually only one fruit per cluster” (‘Autumn Blaze’); “little or no fruit and the fruit that is produced is small and hard” (‘Trinity’); and “self-sterile” with fruits typically not abundant and only produced when “planted near another clone”(‘Aristocrat’). The last point is the lynchpin of this story.
Although each Callery pear cultivar cannot produce fruits on its own, fruits can easily develop when two or more cultivars—which are genetically different and therefore cross compatible—are planted together (Culley and Hardiman 2007). Cross-pollination is promoted by insect pollinators, especially bees, which frequently fly over a mile each day visiting all flowers within their range. This has a large impact on the magnitude of the pear problem. For example, if a large residential area only contains ‘Bradford’ trees, no fruits would be formed. But if a new resident moves in and plants a single ‘Aristocrat’ in her yard, that new tree now has the potential to cross-pollinate all the ‘Bradford’ trees within a mile-wide range, and vice-versa. This could trigger a sudden outburst of fruit within a single year. Such massive fruiting may even go undetected at first because some people expect any pear, including P. calleryana, to produce large edible fruits like a ‘Bartlett’ pear, and they do not recognize the small fruits of Callery pear.
During the winter months, these fruits are consumed by birds that then defecate the seeds as they fly or roost in trees or along power lines, thereby spreading the species into new areas. In fact, genetic analysis of new wild Callery pear populations have confirmed that wild plants are typically F1 hybrids of cultivars planted in the surrounding residential and commercial areas (Culley and Hardiman 2009). In older populations, such as near the Glenn Dale station where ‘Bradford’ was first discovered, wild pear populations largely consist of advanced generation hybrids.
Cross pollination between mature specimens of Callery pear cultivars is not the only way fruit production can occur. To maintain their genetic identity, Callery pear cultivars are clonally produced by vegetative propagation. Stem cuttings of Callery pears are difficult to root so most trees are propagated for commercial sale by grafting. In this process, the scion material of the selected cultivar is grafted onto rootstock, which is usually P. calleryana seedlings. The two sections grow together, resulting in a tree composed of two genotypes. Occasionally, the rootstock of a planted Callery pear cultivar may develop shoots that eventually flower. In such a case, the rootstock now has the potential to cross-pollinate the upper scion of the same tree, triggering fruit production. So it is possible for a wild population of pear trees to come from a landscape planting of multiple cultivars or even from a single grafted tree. In a genetic parentage study of a wild Callery pear population in southwestern Ohio, it was discovered that at least 17% of wild trees had a rootstock parent (Culley et al. 2011).
The introduced range of Pyrus calleryana in the United States is currently restricted by the species’ limited cold tolerance, as predicted decades ago by Reimer and Meyer. However, hardier cultivars are now being developed that will expand Callery pear’s landscape presence. In addition, wild Callery pear is expected to continue to spread northward as global climate change causes shifts in warmer temperatures. In fact, wild Callery pears have already been observed around Madison, Wisconsin (USDA Hardiness Zone 5a), an area where they were thought never to survive.
What can be done to prevent the continued spread of the wild Callery pear? First, Callery pear cultivars need to be carefully phased out of commercial production and replaced with suitable alternatives. The latter is critical, as it would allow plant breeders and the nursery industry to recoup any economic loss and remain profitable. In fact, sterile varieties of P. calleryana are currently being developed. Second, as cultivated trees within the landscape break apart or decline they should be replaced with these alternatives. This is something already happening in many towns nationwide, as Callery pears are being replaced with different tree species. Finally, if homeowners choose to keep cultivated Callery pears growing in their yards, they must take responsibility for ensuring that fruits are not produced. Callery pear fruit production can be reduced as much as 95% by spraying flowering trees with ethephon, a plant growth regulator that does not affect the flowers’ appearance but does make them incapable of developing into fruit. But even if all these suggestions could be accomplished, the sad reality is that wild Callery pear will continue to be a growing problem in the years to come because so many cultivars and their rootstock are already established in the landscape.
Today, the Callery pear story is another example of how even the best of human intentions can go awry. The pear was introduced into the United States for the best of reasons—to save the valuable crop of P. communis on the West Coast from fire blight in the 1920s. Decades later, ‘Bradford’ and other Callery pear cultivars were selected and promoted to give gardeners and landscaping professionals additional highly tolerant and attractive trees for the landscape. These were all good and sensible ideas at the time, especially since the majority of introduced species in the United States never become invasive. The resulting spread of wild P. calleryana into the American landscape was unanticipated and completely unintentional. The best that we can do today is to view the Callery pear as a lesson on the importance of considering how mixes of ornamental cultivars may contribute to invasive spread of certain species. By learning from our past history, we can better understand why certain species become invasive, and thus we can work more effectively to prevent invasive spread of species in the future.
The author would like to thank the many researchers, land managers, nursery professionals, and students who were invaluable in uncovering this story. Most notably, this project was sparked by Marjie Becus, who first pointed out wild Callery pear to the author during their vegetational rambles, and Robert Naczi, who mentioned the Internet discussion on the topic shortly after. The author also thanks Joe Boggs and Michael Vincent for their willingness to share their thoughts and expertise, as well as Larissa Glasser for locating Frank Reimer’s original notes in the Arnold Library. This article is dedicated to the memory of Sarah Elizabeth Reichard, who was instrumental in highlighting the importance of horticultural introductions in plant invasions as well as the necessity of working with nursery interests to create practical and effective solutions. She will be missed.
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Meyer, F. N. 1918. South China Explorations: Typescript, July 25, 1916–Setpember 21, 1918. The National Agricultural Library. Available online at: https://archive.org/details/CAT10662165MeyerSouthChinaExplorations
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Theresa Culley is a Professor of Biological Sciences at the University of Cincinnati. She also serves as a board member of the Midwest Invasive Plant Network and is a past president of the Ohio Invasive Plants Council (OIPC). She currently is the Chair of the OIPC’s Invasive Plant Assessment Team for the state of Ohio.