NEW ORLEANS (AP) — Stinky but beautiful and wildly popular landscape trees have spawned aggressive invaders, creating thickets that overwhelm native plants and sport nasty four-inch spikes.
Bradford pears and 24 other ornamental trees were developed from Callery pears – a species introduced to America a century ago to save ravaged pear orchards. Now their invasive descendants have been reported in over 30 states.
“Worse than killing hornets!” was the tongue-in-cheek title of a 2020 U.S. Department of Agriculture webinar on Callery pears, including the two dozen thornless ornamental varieties sold since the 1960s.
“They’re a real threat,” said Jerrod Carlisle, who found four trees in his yard and one at a neighbor’s had spawned thousands on the 50 acres (20 hectares) he was turning cropland into timber in Otwell, a community of approximately 400 in southern Indiana.
Indiana is among 12 states in the Midwest and West that have reported invasions, though most are in the south and northeast.
Until 2015, Carlisle leased his field to a farmer. Then he enrolled it in a USDA crop reduction program that funded the planting of 29,000 trees as wildlife habitat.
Carlisle realized prickly-flowered pears were a problem in 2019. When he cut or mowed them, new shoots appeared. Trees sprayed with herbicide sheets grew back. Cutting the bark in a circle around the trunk kills most trees. Not these.
He and his 17-year-old son felled about 1,400 Callery pears, applying herbicide to the stumps. But he estimates there are about 1,000 left to do.
Without regular maintenance, fields near seed-bearing trees can be covered with sprouts within a few years, said James “JT” Vogt, a scientist at the U.S. Forest Service’s Southern Research Station in Athens, Georgia.
“If you mow it, it germinates and you get a thicket,” he said. “If you burn it, it also germinates.”
According to David R. Coyle, an assistant professor in the Department of Forestry and Environmental Conservation at Clemson University, seedlings as young as a few months old have spurs that can puncture tractor tires.
The stench from the tree’s streams of white blossoms has been compared to perfume gone bad, rotting fish, chlorine and a cheese sandwich left in a car for a week. The trunks branch out in a deep V, so after 15-20 years they tend to break during storms.
But Frank N. Meyer, an agricultural explorer who brought 2,500 plant species, including his namesake Meyer lemon, to the USDA in the early 1900s, called the Callery pear wonderful, noting that it survived the drought and poor soil.
At the time, a fungus called fireblight was devastating American pear orchards, University of Cincinnati researchers Theresa M. Culley and Nicole A. Hardiman wrote in a 2007 BioScience article on American history. of the plant.
And, as the researchers had hoped, grafting edible pears onto Callery roots produced blight-resistant fruit trees.
In 1952, USDA workers noticed a spikeless mutant growing among Callery pears from seed. By grafting its cuttings onto the roots of other Callery pears, they cloned an ornamental line they named Bradford pears. This variety was commercially available in 1962, Culley and Hardiman wrote.
Other seedlings grew into 24 more ornamental varieties. All are so beautiful, hardy and insect resistant that they have been planted all over the country.
Bradford and other Callery ornamentals are the third most common trees of 132 species planted along New York streets – more than 58,000 out of 650,000 in 2015, the most recent count, the spokesperson said. of the city’s parks department, Dan Kastanis.
But the city no longer plants them, Kastanis said. Nor Newport News, Va., which got rid of its Bradford pears in 2005. South Carolina, Ohio and cities like South Bend, Indiana have banned or are banning all commercial varieties of Callery pears.
Some states, including Missouri and Alabama, are asking homeowners and landowners to stop planting them or cut existing ones and apply herbicide to the stumps. Several, like North Carolina, offer free native trees to landowners who provide photos proving they have cut Callery pears on their property.
For the USDA, which ordered Meyer to send Callery pear seeds from China, the nasty spurs and inedible marble-sized fruits were irrelevant. What mattered was that the plant was resistant to fire blight.
Genetically identical pears do not produce seeds, so botanists thought cloned varieties were safe for ornamental use.
In 1971, the USDA even published a pamphlet on their care, presenting them as trees that bloom multiple times from spring through fall, thrive in many climates and soils, and do not attract plant pests.
Now the USDA describes Callery pears as nearly ubiquitous and has studied the best way to kill them.
Their adaptability is one of the reasons they are so invasive. And their insect-resistant waxy leaves keep insect-eating birds away.
“They’re kind of a food wasteland for a bird,” said Coyle, who runs Clemson’s annual “Bradford Pear Bounty,” providing native saplings to landowners who have cut down their Callery ornamentals.
It turned out that while trees of the same variety can’t produce seed with each other, two different varieties within a pollinator’s range can produce fruit that crashes into pavements and nourishes starlings and robins, which spread the seeds widely.
In addition, the rootstock can send sprouts. If these are not regularly pruned to prevent them from flowering, they can pollinate with the grafted tree to produce fertile seeds, noted Culley of the University of Cincinnati.
“A wild population can potentially come from a single landscape tree that someone plants in their yard,” she said in an email.
Carlisle, the Indiana landowner, thinks he eventually got ahead of his invasion because native trees planted for reforestation, specifically six species of oak, cast enough shade to inhibit Callery seedlings.
“I really believe I’m in eradication mode now,” he said.
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