Bergmann, a field botanist for the Montgomery County Parks Department, extricates herself from the thicket and in the meadow shows me that what I take to be blades of grass are actually shoots of trees, mowed to a few inches high. There are countless thousands, hiding in plain sight in Great Seneca Stream Valley Park. If it were not cut back once a year, the meadow would become like the adjacent screen, wall upon wall, acre upon acre of black-limbed, armored trees worthy of Sleeping Beauty’s castle.

“You can’t mow this once and walk away,” said Bergmann, who began her 25-year career in the department as a forest ecologist but has been consumed by an ever-pressing need to address the escape of the Bradford pear and other variants of callery pear, a species that originated in China, along with other invasive exotics.

The U.S. Agriculture Department scientists who gave us the Bradford pear thought they were improving our world. Instead, they left an environmental time bomb that has now exploded.

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From the 1960s to the 1990s, the callery pear was the urban planner’s gift from above. A seedling selection named Bradford was cloned by the gazillion to become the ubiquitous street tree of America’s postwar suburban expansion.

The Bradford pear seemed to leap from an architect’s idealized rendering. But in this case, reality outshone the artist’s vision. It was upright and symmetric in silhouette. It exploded with white flowers when we most needed it, in early spring. Its glossy green leaves shimmered coolly in the summer heat, and in the fall, its foliage turned crimson, maroon and orange — a perfect New England study in autumnal color almost everywhere it grew. And it grew everywhere. It flourished in poor soil, wet or dry, acidic or alkaline. It shrugged off pests and diseases, it didn’t drop messy fruit like mulberries or crab apples. Millions of Bradford pears would be planted from California to Massachusetts and would come to signal the dream and aspirations of postwar suburbia. Like the cookie-cutter suburbs themselves, the Bradford pear would embody that quintessentially American idea of the goodness of mass-produced uniformity.

But like a comic book supervillain who had started off good, the Bradford pear crossed over to something darker. It turned from thornless to spiky, limber to brittle, chaste to promiscuous, tame to feral. Most of all, it became invasive. It is now an ecological marauder destined to continue its spread for decades, long after those suburban tract houses have faded away. Generations yet to be born will come to know this tree and learn to hate it.

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It is at its most conspicuous in early spring, when it bursts into flower. Suddenly whole rural landscapes — meadows, old pastures, woodland edges, ditches, the sides of highways and railroads — are lit up by its blossoms. From early March to mid-April, you can now track the arrival and progression of spring in the United States not as amber waves of grain but as a frothing tide of Bradford pear.

You might even enjoy its beauty, until you realize that it is squeezing out native flora and reducing biodiversity. As eye-catching as the flowers are, they are simply the start of the seasonal march of this invader. Six months after the blooms appear, clusters of seedy berries invite birds to fatten up for winter. In the bird’s droppings, the seeds will germinate and advance, becoming ever more genetically diverse in the process and making the pear ever more adapted to its own spread.

Its invasive tendencies became widely noticed by the late 1990s, and by the mid-2000s, it had become a weed in the District and 19 states, from Texas to New York. “While callery pear was introduced with the best of intentions, it now seems that a plague is truly upon us,” botanist Michael Vincent wrote at the time. It has now spread to 29 states.

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The roots of the Bradford pear fiasco go back more than a century, and had nothing to do with decorating suburbia. By the turn of the 20th century, Northern California and southern Oregon had become the centers of pear production in the United States. The common or European pear was a high-value fruit; in one Oregon county alone, Jackson, the pear industry in 1916 was worth a mind-boggling $10 million.

But the orchards were threatened by a new disease called fire blight. George Roeding, a nurseryman in Fresno, Calif., wrote in 1916 that “in the San Joaquin Valley, which sixteen years ago was one of the great pear producing sections of California, the pear has been absolutely wiped out of existence.”

In Talent, Ore., a plant scientist named Frank Reimer was using a test orchard to work on fire-blight control and found that the callery pear, first brought to the States in 1908, was highly resistant to fire blight and might be used as a rootstock onto which varieties of the European pear could be grafted. The much smaller callery fruit is used to make tea in China but is considered inedible.

But to take his research further, Reimer needed lots of genetically rich wild seed from China. He turned to David Fairchild, a dynamic young botanist in Washington. Fairchild is best remembered as the guy who helped bring the Japanese cherry blossoms to the capital, by first growing 125 imported trees on his estate in Chevy Chase, Md.

Fairchild headed the Agriculture Department’s Office of Foreign Seed and Plant Introduction. He traveled far during his career in search of new plants, but he relied on another explorer, Frank Meyer, in the quest for a super-pear. Meyer, an emigre from the Netherlands, had already explored extensively in northern and central China when Fairchild tapped him, in 1916, to go to southern China. (Meyer is best known for giving Americans the lemon variety named after him.)

In an age before passenger jets and digital communication, the quest for the callery pear would be no quick or easy excursion. A century ago, plant explorers needed an extraordinary set of skills to complete their missions and survive a range of perils.

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A haunting image of Meyer in China is captured in a 1908 photo. With a big black beard, sheepskin leggings and a staff, he is seen gazing into the distance from a mountainous, arid landscape. All about him, burlapped cuttings rest like tablets brought down from Mount Sinai. He is in his early 30s but seems older.

Meyer left Washington in mid-August of 1916, traveling west to see Reimer’s pear trials in Oregon and finally departing Seattle bound for Japan on the last day of summer. Within a couple of months, he had sent six large shipments of seed from Beijing containing pine nuts, walnuts and chestnuts, but no callery pear. Fairchild, exasperated that Meyer was hanging around Beijing, urged him to make his way up the Yangtze River to look for the callery. “You must not leave any stone unturned to secure it,” he wrote. The correspondence between Meyer and Fairchild form part of an archive of the Chinese expedition in the National Agricultural Library in Beltsville, Md.

Meyer eventually made his way to Yichang, a four-day boat ride from Hankou (now Wuhan) up the Yangtze, and he soon found wild-growing callery pears, if not in the extravagant amounts Reimer was seeking. “Altho’ not rare in the hills around here, the trees are very widely scattered, they are often quite small and as such produce individually but little fruit,” he wrote.

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Many of the trees were stunted in dry, poor sites, but one remarkable aspect of this species soon became apparent. It could grow virtually anywhere. Meyer recorded pear trees on sterile mountain slopes, on the edge of a pond in wet soil, on screes, in bamboo stands and even in running water.

Having made arrangements for locals to collect and process the seed the following fall, he set out to southern China in search of other plants but got only as far as Hankou, where he took to his bed with “nervous prostration.”

After several weeks of rest and therapy he was back on his feet, and by late summer, he had made his way to the area around Jingmen, today a large city in Hubei province.

He sent his first shipment of callery pear seed to Fairchild in September, and the following month he teamed up with Reimer and gathered another 25 pounds of seed — enough to grow a small forest of callery pears. Reimer also collected on his own, before the two men went to Yichang in early November.

Another calamity was about to overtake Meyer, however, as China had become gripped by spreading internal strife. Factional warlords fought for dominance. This kept him confined to Yichang while seeds and notes were isolated in Jingmen. “As I am writing,” he wrote in February 1918, “we hear the rickety noise of rifle fire.”

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That spring, he made his way to Jingmen to retrieve his pear seeds and baggage, and then took a steamer down the Yangtze back to Hankou.

On the evening of May 31, 1918, Meyer boarded another ship sailing downriver to Shanghai, where he planned to stay a month to escape the heat of Hankou and to ship his pear seeds to Fairchild.

Meyer had been suffering from a stomach ailment but appeared to be getting better the next day, a Saturday. His mental state may have been getting worse. Meyer told his servant Yao Feng T’ing that his father and some old friends had come to him in a dream, and that he considered this an omen.

That Saturday night, when the boat was about 30 miles upriver from the city of Wuhu, Meyer was seen by a cabin boy making for the deck. The lad thought Meyer was going to the toilet, but he was never seen alive again. His body was fished out of the Yangtze four days later near a settlement named Ti Keng.

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Samuel Sokobin, the American vice consul in Shanghai, conducted an investigation and reported that although it seemed Meyer had not been killed, it was “impossible to state” whether Meyer’s death was an accident or suicide. Signs point to the latter. After talking to the explorer’s servant and fellow passengers, Sokobin wrote: “It appears certain that Mr. Meyer had been depressed for some time.” He was 42. Sokobin had Meyer’s body moved to the Protestant Cemetery in Shanghai and his seed collections sent to Washington.

Reimer, in his time with Meyer in Jingmen, had come to see just how low he was. “He told me of his life’s struggles,” he later wrote. “Few people ever realized the tremendous battle that was raging in his soul.”

The seed that Meyer and others would collect ended up in Reimer’s test orchard in Oregon and the U.S. Plant Introduction Station in Glenn Dale, then a rural enclave in Prince George’s County.

In the early 1950s, a horticulturist at Glenn Dale named John L. Creech began to see the callery pears there not as a rootstock for the common pear but as an extraordinarily handsome and tough street tree in its own right. He latched on to a single specimen that had been grown from seed that was not part of Meyer’s shipment but acquired soon afterward in Nanjing as part of Fairchild’s same feverish search for callery pear seed.

The tree was 30 years old when Creech first evaluated it, and judging from his later writings, he was besotted by this specimen. He was struck by its vigor, its handsome, mature spread and its evident ornamental qualities. But a plant’s value lies too in what it is not. This one tree did not have the thorns of other callery pears; it was free of diseases and pests and held together in storms. In selecting this individual to mass-produce, Creech named it Bradford after the station’s former head, F.C. Bradford. This specimen’s resilience in storms belied what would become a major problem with its mass-produced Bradford clones: Tight branch-to-trunk angles and congested branching invited the limbs to break apart.

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Before releasing the Bradford ornamental pear to the nursery trade, Creech decided to trial it in the nearby Washington suburb of University Park, which was then treeless and had difficult soil — perfect for putting this wondertree to the test. He planted 180 saplings in 1954. He pruned them to keep lower branches out of the way of pedestrians and cars, but also to give each tree a handsome profile. This undoubtedly produced more attractive and useful trees but further hid the Bradford’s weak-wooded Achilles’ heel.

Happy with its performance in University Park — some of the original Bradfords are still there, as lanky, open shade trees — he officially released the Bradford pear to the nursery trade in January 1960 and invited growers to obtain shoots for grafting onto callery pear seedlings. Each Bradford scion would be genetically identical, but the rootstocks each had their own DNA. This would come back to bite us.

For a few innocent years, the only thing working against the Bradford pear was its ubiquity. It was so golden that every nursery grower wanted to propagate it and every home builder and highway department wanted to plant it.

In 1966, Creech and his colleague William Ackerman wrote that the University Park plantings had displayed very little fruit, in contrast to the situation back at Glenn Dale, where the whole zoo of callery pears — 2,500 seedlings — had produced “abundant fruit development” on similarly aged Bradfords. Their point was that if you kept Bradfords away from other pears, messy fruiting wouldn’t be a problem. But they clearly missed the ecological repercussions. And in the University Park planting, they noticed something else that should have raised alarms. On a few grafted trees where the scion had failed, the rootstock had produced suckers that then bloomed. These flowers, with the help of bees, caused the “sterile” street trees to set viable fruit. Creech and Ackerman minimized the problem, saying it was highly localized.

Clues to the callery pear’s invasiveness are buried in a journal paper written by Ackerman in 1977, when he announced the introduction of another variety named Whitehouse, chosen because it was more upright than Bradford and better suited to small gardens. But this selection was a fugitive from Glenn Dale, growing as a seed deposited by a bird on a neighboring property; Bradford was one of its parents. The callery pear had escaped the reservation.

Four years later, the National Arboretum introduced an even more columnar variety named Capital. Meanwhile, commercial plant breeders were bringing to market other varieties of the callery pear, and by the mid-1980s, a dozen or so were available for planting. Many of these were introduced to get around the Bradford’s poor branch structure and propensity to break. Sterile by themselves, any two of these varieties together would produce a heavy fruit set for winter bird dispersal. The stage was set for the callery pear’s quiet occupation of the countryside.

Since 2012, Bergmann has led efforts to contain the callery pear on 232 acres, though it has been mapped on 425 acres within the Montgomery park system and may occupy countless more areas, she said. Crews have employed a number of tactics, including felling trees with chain saws, girdling their trunks with a blade and spraying the wound with a systemic herbicide. But the trees are mostly beaten back with an annual cutting by a Bush Hog mower or in woodier areas a beefier machine called a forestry mulcher.

“It’s not a fix like building a bridge or putting down a road,” she said. “Once you cut down a field of Bradford pears, they’re going to try to come back.”

It seems that the callery pear has been a curse to those who tried to master it. Wild trees of Pyrus calleryana in China are not invasive or particularly common. It’s what we did to the pear that has turned it against us: We brought it to an alien environment, selected one for unnatural propagation and then fused genetically different individuals together. We planted it intensively across the entire continental United States, seeding its eventual spread. Today it is a roving, free-range freak.

In Mary Shelley’s epic tale, Victor Frankenstein laments the monster he created. “I must pursue and destroy the being to whom I gave existence,” he tells Captain Walton.

If Creech had regrets about introducing the Bradford pear, he kept them to himself. In economic terms for nursery growers, the tree was manna from heaven. By the 1990s, he knew of its structural ailments and the problem with self-seeding. But when these were raised, his response was always the same. “He would say, ‘Yes, the tree has had its problems, but it put a lot of kids through college,’ ” said Lynn Batdorf, the arboretum’s retired boxwood expert, who was a young horticulturist under Creech in the 1970s.

A decorated World War II Army veteran who ended the war in a German POW camp, Creech was beloved by the horticulturists who worked for him. He organized scientifically important plant-collecting trips to Japan and presided over a golden period of ornamental tree and shrub development at Glenn Dale and later at the National Arboretum, where he was director from 1973 to 1980. He leveraged his contacts in Japan to establish the world-famous bonsai collection at the arboretum.

I sat down with the current arboretum director, Richard T. Olsen, and asked him to explain how Creech could have gotten the Bradford pear so wrong. Olsen says you can’t judge what happened without understanding the historical context. The mission of scientists like Creech and Fairchild was to find and manipulate plants in a way that solved a problem, met an unmet need or simply offered an attractive new plant for the American nursery industry and consumers.

Olsen recites Thomas Jefferson’s line: “The greatest service which can be rendered any country is to add a useful plant to its culture.”

For Creech and his peers working in the 1950s, the potential environmental effects were not part of the decision-making. “These people were pretty smart,” Olsen said. “Our values have changed.”

In addition, he said, invasive plants have been enabled in their spread by centuries of environmental disturbance. “If we think in our forests we are dealing with a pristine habitat, we are deluding ourselves,” he said. Peter Del Tredici, a retired senior research scientist at Harvard’s Arnold Arboretum, said the concept of exotic invasive species didn’t emerge until 30 years ago, even though European settlers recorded escaped plants as early as 1672.

Only a fraction of nonnative ornamental plants become invasive, but those that do have the capacity to completely transform natural areas. Bergmann has established an army of weed-control volunteers to work in county parks, but the idea of eradicating almost 40 species of key invasives is unrealistic.

“Without thinking much about it, we have globalized our environment in much the same way we have globalized our economy,” Del Tredici has written. He lives in the Boston suburb of Watertown, where he is seeing the first wave of callery pear invasion, young plants “in highly disturbed habitats where no maintenance has occurred,” he told me.

In the South, the march of the callery is much more evident. “It’s definitely starting to really become a prominent invasive in Alabama,” said Nancy Loewenstein, an extension specialist at Auburn University. She sees it in disturbed industrialized areas, in the understory of pine plantations and along the banks of the Alabama River and other riparian areas. “There are some areas that are just white in the spring,” she said.

Bergmann, who is 67 and looking at the end of her career, says she owes it to future generations to try to contain the Bradford pear and other invasives in the hope that one day scientists can find a way to eradicate them.

It is a problem that Fairchild could not have envisioned a century ago, but the Bradford pear persists as a case study in how even the brightest scientific minds can be blind to what they might be creating long after they have gone. In November 1917, Fairchild wrote to Meyer thanking him for the first batch of callery pear seed. “I believe,” he predicted, “these will hardly fail to have in them something of extreme value for this country.”