They have given rise to aggressive invaders who create thickets that overpower native plants and display nasty four-inch spikes.

Callery pear was the source of 24 ornamental trees, including Bradford pears. This species was brought to America in the early 1900s to help save devastated pear orchards. Their invasive descendants are now reported in over 30 states.

"Worse that murder hornets!" This was the tongue-in cheek title of a U.S. Department of Agriculture webinar about Callery pear varieties, including two dozen ornamental varieties that are thornless since the 1960s.

Jerrod Carlisle said, "They're really a menace." He discovered that the four trees in his yard, and one at the neighbor's, had produced thousands of eggs on 50 acres he was turning into woods in Otwell (a small community of 400 people in southern Indiana).

Indiana is one of 12 states in the Midwest and West that have reported invasions. However, most of these are in the Northeast and South.

Carlisle rented out his field to farmers until 2015. He then enrolled the field in a USDA crop-reduction program, which paid for 29,000 trees to be planted as wildlife habitat.

Carlisle realized that the spiky flowering pear pears were a problem for 2019. New sprouts appeared when he cut them or mowed them. The leaves of trees that were sprayed with herbicides grew back. Most trees are killed by cutting off the bark around the trunk. These are not the best.

His 17-year-old son and he have cut down approximately 1,400 Callery pears. They also applied herbicide to the stumps. He estimates that there are still about 1,000 to go.

James "J.T." said that fields close to seed-producing trees could be covered in sprouts within a few years. Vogt is a scientist at U.S. Forest Service’s Southern Research Station, Athens, Georgia.

He said, "If you mow it it sprouts and you get thicket." It also sprouts if you burn it.

David R. Coyle is an assistant professor at Clemson University's Department of Forestry and Environmental Conservation. He said that seedlings less than a month old can bear spurs that can push through tractor tires.

The tree's white blossom billows emit a stench that can be compared to perfume gone sour, rotting fish, chlorine and a cheese sandwich left in the car for a week. They tend to split in storms after about 15 to 20 years of being branched in V-shaped shapes.

Frank N. Meyer was an agricultural explorer who brought more than 2,500 plants, including the Meyer lemon, to the USDA in early 1900s. He called the Callery pear amazing, noting its ability to withstand drought and poor soil.

In a 2007 BioScience article, University of Cincinnati researchers Theresa M. Culley (and Nicole A. Hardiman) wrote that a fungus called "fire blight" was ravaging U.S. pear orchards at the time.

Researchers had hoped that grafting edible pear trees onto Callery roots would produce fruit trees resistant to blight, as it did.

USDA workers discovered a spikeless mutant among Callery pears in 1952. It was a seedling. They cloned a Bradford pears ornamental line by grafting the cuttings onto the roots of other Callery pear trees. Culley and Hardiman reported that the variety was commercially available in 1962.

Another 24 ornamental varieties were created from seedlings. They are all hardy, beautiful and resistant to insects.

Callery ornamentals such as Bradford and Callery are the third most commonly planted trees among the 132 species along New York City streets. They account for more than 58,000 of the 650,000, according to Dan Kastanis, a spokesperson for the city parks department.

Kastanis stated that the city has stopped planting them. Newport News, Virginia also has stopped planting Bradford pears. It was forced to do so in 2005. All commercial Callery pears are banned in South Carolina, Ohio, and other cities, including South Bend, Indiana.

Missouri and Alabama ask homeowners and landowners to either stop planting them, or to remove existing plants and apply herbicide. Many states, including North Carolina, offer native trees for free to landowners who submit photos that prove they have removed Callery pears from their property.

The USDA ordered Meyer to ship Callery pear seeds from China. It didn't care about the ugly spurs or the inedible, marble-sized fruit. The important thing was that the plant was resistant fire blight.

Because genetically identical pears do not produce seeds, botanists decided that cloned varieties could be used for ornamental purposes.

The USDA published a brochure in 1971 about their care. It stated that they are trees that can bloom multiple times each year, thrive in all climates, and don’t attract pests.

The USDA now describes Callery pear as nearly ubiquitous, and has been researching the best way of killing them.

They are invasive because of their adaptability. Their insect-resistant waxy leaves keep insects away from them.

Coyle said, "They're kinda a food desert to a bird," and he leads Clemson’s annual "Bradford Pear Bounty," which provides native saplings for landowners who have felled Callery ornamentals.

It was discovered that even though trees of the same species cannot produce seeds together, pollinators can have two varieties that produce fruit. This allows starlings and other birds to spread the seeds.

The root stock can also produce sprouts. Culley, University of Cincinnati, said that if they are not regularly pruned to stop them from blooming, they can cross-pollinate to produce fertile seeds.

She said that a single tree in a yard can be enough to create a wild population.

Carlisle, an Indiana landowner thinks he is finally ahead of his invasion. He believes that native trees planted for reforestation (especially six oak species) are providing enough shade to prevent Callery seedlings from growing.

He stated, "I believe that I am in eradication mode now."