Yesterday I noticed white flowering trees on the hills and swales near Robinson Town Centre (I-376 West). “How nice,” I thought, “Who planted those trees in the empty places?” No one. They’re invasive.
April is the perfect time to see the invasive extent of callery pears (Pyrus calleryana) because they bloom before our native white-flowering trees: chokecherry, downy serviceberry (shadbush), and hawthorn.
Originally imported from China in the early 1900s as root stock for pear orchards, USDA bred them as landscape trees in the 1950s and came up with a winning cultivar, the thorn-less sterile “Bradford pear.”
From 1960 to the 1990s callery pears were wildly popular as street trees in suburbia. They’re pretty in early spring, colorful in fall, and they grow well in the full sun and disturbed soil found in new subdivisions. The Bradford cultivar is also brittle so commercial plant breeders created other cultivars. That’s when the genie came out of the bottle.
In a single cultivar population the fruits are sterile but if two different cultivars are planted near each other, or even grafted together, insects cross-pollinate them and the trees produce fertile fruit. Birds eat the fruit and disperse the seeds. The trees escape to the wild.
Callery pears grow anywhere. A patch can start with a single tree that becomes a thicket in several years. Dense thickets push out all native species. To make matters worse, the wild trees can have 3-inch thorns! The best field-scale control measure is to brush-hog and then mow every year. They still come up!
Callery pears now grow wild from Texas to New York and Massachusetts. They’re listed as invasive in eight states including our own: Pennsylvania, Maryland, Virginia, South Carolina, Georgia, Alabama, Tennessee, and Illinois.
You’ll see them this month on open hillsides and fields along the interstate, near shopping centers, and at the edges of subdivisions.
Don’t plant callery (Bradford) pears. They are not nice flowering trees.
(photos by Richard Gardner and Britt Slattery via bugwood.org; click on the captions to see the originals)