Too smart for our own good: the Callery Pear and the Bradford Pear

I have been looking at these amazingly thorny young trees that are growing on some of the berms (levees/roadways) that run between the ponds of the constructed wetlands. What amazing texture – and what horrible thorns. It would be hard to imagine pushing my way through a thicket of these.

Callery pear hybrid web

Frankenstein meets the Bradford pear tree

Ruth Mead (Senior Environmental Educator at the Phinizy Center) pointed out to me that these thorny trees are related to Bradford pear trees. You know Bradford pear trees – they are everywhere, in parking lots and suburban yards. For a long time, these were considered perfect landscaping trees: they grow fast, they are oblivious to poor growing conditions, and they are pretty most of the year (flowers in the spring, glossy green leaves in summer, dramatic leaf colors in the fall). Later on, people started recognizing that mature Bradford pear trees crack and break limbs easily because of the growth pattern of their branch structure. But other than that, this seemed like an ideal, easy tree to grow in yards and on city streets. Many people still love and grow them.

It turns out that the truth is much more complex than that. Remember the promise of kudzu? It was supposed to be this great, fast-growing plant that controls erosion on steep hillsides in the South. It was imported from Asia. We all know that kudzu turned out to be a disaster, the green monster that Eats The South. In other words, kudzu is a poster child for the concept of an invasive species. How did a good thing become a bad thing? Sometimes human decisions go wrong.

Callery pear closer web

“Franken-Bradford” closer up

Bradford pear trees were developed in the U.S. as a variant of Pyrus Calleryana, or Callery pear trees. Seeds from Callery pear trees were brought from China beginning around 1918 as part of a possible solution to a pear tree disease that was devastating American pear fruit crops. The Callery pear tree is highly resistant to various diseases, and it grows well in a variety of environmental conditions. Over decades of experimentation and research, botanical scientists in the U.S. developed cultivars of this plant with promising qualities. The most famous cultivar was the one named Bradford. And for decades, Bradford pear trees were planted seemingly everywhere, with few problems. A useful feature of the Bradford pear was that it seemed to be NOT invasive – it was basically sterile. Bradford pear trees produce lots of pretty flowers, but usually no fruit or seeds. What’s not to love about that? If you need another tree, you can buy them at all the major garden centers, because horticultural businesses can make more trees through grafting and other methods.

However, scientists did not stop experimenting with the Callery pear tree when they discovered the Bradford cultivar in the 1960s. In the decades since then, more variants have been brought onto the market – pear trees with various desirable qualities that are a little different from the Bradford. Within the last ten years, there has been a sudden explosion of spontaneously appearing trees like the one in my photo. Many of these unwanted Callery pear trees grow extremely rapidly and have very tightly arranged branches studded with long thorns. They appear in thick stands at the edges of cleared land, along fence lines, and along roadways. Cutting the trees down makes them grow even faster and thicker from the roots. They are resistant to diseases, too painful for deer to munch on because of the thorns, and they crowd out the plants that would naturally grow in these spots. Yow! I think I could write a horror movie script about these trees. How did this situation come to pass?

Callery pear closest web

Scary!

It turns out that, for specific genetic reasons, Callery pear varieties are often sterile in terms of being able to pollinate themselves or other trees of the same variety. So, a tree that is sterile isn’t going to become invasive, right? But as more varieties came on the market, the different varieties were sterile in relation to their own kind – but often fertile when pollinated by a slightly different variety. And the different varieties back-crossed with each other, often bringing out extremely undesirable qualities (such as lots of thorns) in trees that then grow freely in locations that are not mowed regularly. Another kudzu-monster has been born. And did you know that the southern U.S. is on the same latitude as the part of China where the original Callery pear tree is native and grows most easily? American Callery pear hybrids will be that much harder to eradicate in our region because of this.

So was there ever a point when reasonable scientists, administrators, government officials, etc. could have reasonably suspected that their efforts to improve American pear trees might take a dark turn? And what do we do now about our invasion of self-hybridizing Asian pear tree descendants?

For further history of the science involved with Callery pear research, this is a good article:
http://bioscience.oxfordjournals.org/content/57/11/956.full

-Priscilla Hollingsworth