A few weeks ago the Bradford pear trees were in full bloom, obvious with their white, puffball blooms scattered across the region. Quite often planted as front yard shade trees or in welcoming driveway rows, the Bradford pear is one of the first spring trees to bloom. But it poses an invasive threat to our ecosystem, as these sterile cultivars have found a way to breed.
The Bradford pear is one of several common landscape species that can contribute to the growing invasive plant problem in North Carolina and the Southeast. I’ll touch on some other shrubs and vines in a moment, but let me first explain the dilemma with the Bradford pear.
The Bradford pear is one of several sterile cultivars of Pyrus calleryana or Callery pear. The callery pear was first introduced to the U.S. in the early 1900s, imported from China as a way to combat fireblight, which was decimating the cultivated pear industry. Bred with the common pear, the callery pear proved to be the solution to the fireblight problem.
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As breeding continued over several decades, the Bradford cultivar was born in the 1950s, and by 1962 was being commercially grown as an ornamental tree.
Unfortunately, Bradford pears have a weak-branch structure, a genetic flaw. It causes the tree to split in half over time. In response, stronger callery pear cultivars followed the Bradford, such as Aristocrat and Chanticleer. Self-sterile like the Bradford pear, they also have the ability to reproduce by co-mingling with one another.
“The Bradford pear is a self-incompatible callery pear cultivar,” said Johnny Randall, director of conservation programs at NC Botanical Garden. “But if the Bradford pear cross-breeds with a different callery pear cultivar it produces fruit. Furthermore, if any root sprouts come from the base of a Bradford pear, these arise from the callery pear rootstock, which are self-compatible and can cross-pollinate with the Bradford pear flowers on the same plant.”
Perhaps by now, you’re wondering why it matters that an ornamental tree like the Bradford pear can reproduce? Why does it matter that this tree has the capability to spread?
Try to remember back a few weeks when the pears were in bloom. All of those stately white puffballs doting front yards and highways had dozens of offspring blooming as well, whether you noticed or not. Open fields, vacant lots, stream banks, tree lines and power-line easements were covered in a sea of small, white trees, which is prime real estate for wild pears to grow. All of these wild callery pears spread fast, at an alarming rate.
The North Carolina Native Plant Society classifies Pyrus calleryana as an exotic plant species that poses a severe threat to the ecosystem. A “severe threat” classification means that it “has invasive characteristics and spreads readily into native-plant communities, displacing native vegetation.”
“Callery pear grows very fast and can spread by rhizomes, which allows it to colonize areas very fast and exclude native plants,” Randall said. “It is also an incredibly hardy tree that can grow in a wide variety of soils and moisture conditions. I have witnessed acres of callery pears in fallow agriculture fields and miles of callery pears along roadsides.”
The spread of these wild pears chokes out native species, plants that local wildlife depend on for survival. Not only do many native plants produce food for birds and other animals, but they draw a wealth of insects that the birds rely on to feed their young. With callery pears choking out these native species, the ecosystem suffers.
There are also other social and economic consequences of the spread of callery pears.
“It’s difficult to place an economic impact on the effects of Bradford and callery pear,” Randall said. “But when you add its effects to the dozens of other invasive plants, the sum must be considerable in terms of lost natural function of ecosystem services, expense in control, potential negative herbicide effects and loss to agriculture where pears have invaded.”
Keep in mind that just because you didn’t see a seedling come up beside the Bradford pear in your front yard, doesn’t mean that it didn’t reproduce. Fruit produced by callery pears is carried for miles by wildlife, resulting in a great number of wild seedlings sprouting alongside tree lines and under power lines.
There are many other commonly planted landscape shrubs that are considered invasive. Nandina, wisteria, burning bush, privet and English ivy are all examples of commonplace landscape staples.
Although not commonly grown and sold, Asian wisteria is one of the most invasive plants to which I’ve ever been witness. This aggressive vine can quickly take over swaths of wooded areas, spreading through underground shoots. Japanese and Chinese wisteria will climb anything it encounters, strangling trees over time — literally squeezing the life out of a tree.
And we see English ivy everywhere, as it’s one of the most hardy and tolerant ground covers. Used in a certain manner, English ivy can serve a great landscape purpose without getting out of control.
“My own feeling about English ivy is that if you want to grow it, never let it go vertical,” said Sheilah Lombardo, a Forsyth County Extension master gardener. “Never let it climb a tree or a building. Then it turns into an adult form and it starts setting fruit and the fruit is spread by the birds.”
“Ivy on the ground and in a woodland can create an ivy desert. There’s nothing there but ivy, and nothing growing in it because nothing can coexist with it, aside from a large tree.”
Lombardo also pointed out the difference between an invasive plant and an aggressive plant, an important distinction to make when choosing landscape plants.
The Native Invasive Species Council defines an invasive as “a species that is non-native (or alien) to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health.”
There are many common landscape plants that are simply aggressive and spread easily, such as rudbeckia.
“There are plants that are garden aggressive,” Lombardo said. “For instance, there is a native plant called obidient plant, physostegia. It is an extraordinarily aggressive little plant in the garden. It just takes over real estate like nobody’s business. It’s not technically invasive. It’s important to make a distinction between something that is truly an invasive exotic plant and something that’s just a thug in the garden.”
Randall, Lombardo and the N.C. Native Plant Society all encourage gardeners to be informed of invasive-plant species, as the consumer is at the first line of defense for commercial-plant production. There are no laws prohibiting nurseries from growing invasive plants, so the laws of supply and demand are at the heart of the invasive plant problem in North Carolina.
“The public can and does play a huge role in preventing invasive-plant issues,” Randall said. “The public drives demand and the nursery industry responds. The home gardener, the landscape industry, landscape architects, and others can easily determine what plants are known (as) potential invasives and steer away from these. The public can also demand of the nursery industry that the plants they sell are not invasive.”
So the next time you’re shopping for new landscape additions, keep in mind that you don’t want to contribute to the invasive-plant problem. There are so many plant choices these days, ones that won’t spread their seeds across the neighborhood. Be sure to plant wisely. Here is a website listing plants that you should avoid: http://ncbg.unc.edu/plants-to-avoid.
If you have a gardening question or story idea, write to Amy Dixon in care of Features, Winston-Salem Journal, P.O. Box 3159, Winston-Salem, NC 27101-3159 or send an email to her attention to gardening@wsjournal.com. Put gardening in the subject line. Find Amy Dixon on Facebook at https://www.facebook.com/WSJAmyDixon.