Once a favorite plant of landscapers, the Callery pear tree is now being banned by states, urged by homeowners to cut it down.

When people think of spring, they often imagine blooming flowers and trees. And if you live in the Northeast, Midwest, or Southern United States, you’ve probably seen a medium-sized tree with long branches covered in miniature white flowers—the Callery pear (Pyrus hop tree).

For decades, the Callery pear—which comes in many varieties, including ‘Bradford,’ ‘Aristocrat,’ and ‘Cleveland Select’—was one of the most popular ornamental trees in the U.S. But it is now widely considered invasive species. Land managers and plant ecologists like me are working to eradicate it to preserve biodiversity in natural habitats.

From 2023 it is illegal sell, plant or grow Callery pear tree in Ohio and Pennsylvaniaand it will become illegal in South Carolina October 1, 2024 North Carolina AND Missouri will give residents free native trees if they cut down Callery pear trees on their property.

How did this tree, once in high demand, end up being designated by the United States Forest Service as a “Weed of the week”? The devil is in the biological details.

A quasi-perfect tree

Botanists brought the Callery pear to the US from Asia in the early 1900s. They deliberately bred a horticultural variety to enhance its ornamental value. In doing so, they created an arboricultural wonder child. As The Recent York Times reports observed in 1964:

“Few trees possess all the desirable characteristics, but the Bradford ornamental pear comes remarkably close to the ideal.”

State-of-the-art Callery pear varieties produce an explosion of white flowers in spring, followed by murky green summer leaves that turn deep red and maroon in autumn. They are also very tolerant of urban soils that can be very dense and hard for roots to penetrate. The trees grow quickly and have a round shape, making them suitable for planting in rows along driveways and roadsides.

During the suburban development boom after World War II, Callery pear trees became extremely popular in residential settings. In 2005, the Society of Urban Arborists named the variety ‘Chanticleer’ city ​​street tree of the year. However, the breeding process that created this and other Callery pear varieties produced unexpected results.

Cloning to produce an American original

To ensure that each Callery pear tree would have radiant flowers, red leaves and other desirable features, gardeners created identical clones a process known as grafting:creating seedlings from tree cuttings with desired characteristics.

This approach eliminated the messy complexity of mixing genes during sexual reproduction and ensured that when each tree matured, it would have the characteristics desired by the homeowners. Each tree of a particular variety was a genetically identical clone.

Grafting also meant that Callery’s pear trees could not bear fruit. Some fruit trees, such as peaches and tart cherries, can fertilize your flowers with your own pollen. In contrast, the Callery pear is self-incompatible: pollen on an individual tree cannot fertilize flowers on that tree. And since all Callery pear trees of a particular variety planted in a neighborhood would be identical clones, they would essentially be the same tree.

If the tree cannot bear fruit, it cannot spread into its natural habitat. Gardeners and landscapers used to think that planting Callery pears near natural habitats, such as prairies, was completely sheltered, because the species was trapped in place by its reproductive biology. However, the tree would break free from its isolation and spread its seeds far and wide.

The Great Escape

A botanist from the University of Cincinnati Teresa Culley and colleagues found that gardeners who tinkered with Callery pears to breed up-to-date varieties made the individual plants differ enough escape from the fertilization barrierIf only ‘Bradford’ pear trees were grown in the area, no fruit could be produced – but if someone added an ‘Aristocrat’ pear to their garden, the two varieties could cross-fertilise and produce fruit.

When Callery pear trees in gardens and parks began to seed in nearby areas, wild populations of trees were established. These wild trees could pollinate each other as well as neighboring trees.

In today’s landscape, the Callery pear tree is surprisingly prolific. The copious flowering that gardeners have deliberately bred into these varieties now produces a huge harvest of pears every year. Although these miniature pears are not generally edible to humans, birds feed on the fruit, then fly away and excrete the seeds into their natural habitats. The Callery pear tree has become one of the most problematic invasive species in the eastern part of the United States.

A hard problem

Like other invasive species, Callery pears displace native species. After Callery pear seedlings spread from the edges of habitats to grasslandshave advantages that allow them to dominate a given area.

IN my research laboratorywe have discovered that the Callery pear tree puts out leaves very early in the spring and sheds them in tardy autumn. This can absorb more sun than native speciesWe also found that during the invasion these trees change the soil and release chemicals that inhibit the germination of native plants.

The Callery pear is very resistant to natural disturbances. In fact, when my student Meg Maloney he tried to kill trees using controlled fires or applying liquid nitrogen directly to the trunks after the trees were cut down, her efforts were unsuccessful. Instead, the trees sprouted aggressively and he apparently gained strength.

When the Callery pear tree enters natural areas, its seedlings produce very sharp, stiff thorns which can puncture shoes and even tires. This makes these trees a hazard to people working in the area as well as to native plants. Another nuisance factor is that when Callery pear trees flower, they produce strong smell which many people find unpleasant.

Currently, direct application of herbicides is the only known method of controlling Callery pear infestations. However, these trees are so effective at spreading that poisoning their seedlings may simply create space for other Callery pear seedlings to establish. It is not clear how habitat managers can avoid the confusing ecological cycle of invasion, herbicide application, and reinvasion.

Forbidden, but not gone

In response to the work Ohio Invasive Plant Council and other experts, Ohio took an unusual step Callery pear ban to prevent its ecological invasion of natural habitats. However, these trees are common in residential areas throughout the state and have established sturdy populations in natural habitats. Ecologists will work in the future to maintain openness and biodiversity in areas where Callery pear invasion is realized.

In the meantime, homeowners can lend a hand. Gardeners recommend that people who have a Callery pear on their property remove it and replace it with something that is not an invasive species. Few trees have all the desirable characteristics, but many native trees have attractive visual features and do not pose a threat to ecosystems in your region.

This article has been updated to reflect the March 2024 bans on Callery pear cultivation.