CINCINNATI (AP) — It's finally spring and the callery pear trees — white and beautiful — along highways and suburban cul-de-sacs are flowering again.

Once called a "marvel" by those who first brought them to America, they are now described as a scourge on the environment.

This January, the trees were placed on Ohio's invasive species list, meaning in-state nurseries and landscapers must phase out selling the trees over the next five years, said local scientist and University of Cincinnati biological science professor Theresa Culley.

Culley has helped lead the charge to stop the spread of these trees.

After first being introduced in the end of 19th century, Callery pear trees from Asia became one of the most popular decorative trees in America by the mid-20th century — until they began reproducing and spreading uncontrollably.

Originally, they were introduced to save a valuable crop of pear trees that suffered from a destructive disease known as fire blight.

American horticulturists liked them so much that they started doing selective breeding (in scientific lingo, creating cultivars) to make the branches stronger and to get the prettiest shapes out of the trees.

That's where we got the Bradford pear, Cleveland Select, Aristocrat and other breeds of ornamental pear trees.

"Not only were they beautiful, fast growing, and inexpensive, but they were also extremely tolerant of different growing conditions," Culley wrote in an article last year for "Arnoldia," the magazine for Harvard University's Arnold Arboretum. "As each successive year revealed more and more wild pears blooming, public alarm began to sound."

The original commercially-available tree, the Bradford, was sterile, unable to reproduce when they were planted en masse in America.

To the shock of many, though, the trees were appearing along forest edges, wetlands and within forests.

The different cultivars "are capable of cross-pollinating with any other tree of the same species if they are genetically different," Culley said.

And that's what happened, the cultivars begun cross-pollinating and bearing fruit, which leads to reproduction.

Now, Callery pears and their cultivars are present throughout the Midwest and East Coast of the United States and spreading north. They can now be found growing wild on the outskirts of Madison, Wisconsin, where the trees were initially never expected to survive.

No. 1: They are one of the first trees to bloom and last to drop their leaves, and so they crowd out other native plants.

No. 2: Their branches tend to become structurally unsound and potentially dangerous after about 15 years.

No. 3: The white flowers stink and the fruit, after they fall, leave a slippery mess on sidewalks and roads.

Culley said the new rule in Ohio allows a 5-year phase out to soften the blow to businesses that sell the pear trees.

"More and more states are starting to enact regulation," said Culley, who is a former board member and president of the Ohio Invasive Species Council. "Ohio is becoming a leader."

It's a classic case of man's best intentions going horribly wrong, Culley said, but communities are working to change that — like Lebanon in Southwest Ohio.

The city plans to proactively cut down all of the city-owned Callery pear trees and is encouraging its residents to do the same.

"If you have a Callery pear on your property, it is recommended to consider replacing it with a good native alternative," Lebanon Town Hall News' April edition reads.

A few alternatives for trees that grow from small to medium size and provide nice spring blooms?

"Red buckeye, serviceberry, redbud, yellowbud, blackhaw viburnum and flowering dogwood."

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Online: https://cin.ci/2r3806O

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Information from: The Cincinnati Enquirer, http://www.enquirer.com