Callery Pear: Perfect Street Tree or Walmart Indicator Species?
by Mary Jasch
Sometimes the ordinary is really the most complex. Take Callery pear, a.k.a. ‘Bradford’ Pear or Pyrus calleryana. Its existence in the U.S. since the early 1900s is quite evolutionary.
This Chinese import became the new favorite street tree in the 1960s – white flowers in spring, cookie cutter shape, and inexpensive, fast grower that handled drought, pavement, disease, pollution, heat, wood-boring beetles and herbivory. Back then, its fruits were hard little nuggets – hardly anything you’d call “fertile” (although they were). Now, Callery pear fruits are bigger and very fertile and the tree is reproducing like mad.
“’Bradford’ pear is highly self-incompatible, but will set numerous fruits when cross-pollinated,” says Richard T. Olsen, US National Arboretum. “No USDA person ever said they were sterile.”
Nowadays some say Callery pear is invasive; others say there’s no need to worry because, as an early successional tree, it is short-lived and will eventually be shaded out and die anyway. But as of 2005, Callery pear has spread to 26 states, with hybrids found wild in 11.
Callery pear fruits were first brought to the U.S. around 1917 for fire blight resistance research. At that time, fire blight had decimated over 86% of the annual crop of the common pear, Pyrus communis, so the government paid plant explorers to bring back fruit of a closely-related species, P. calleryana.
With demonstrated vigor and resistance, Callery pear proved to be the perfect root stock for the P. communis scion. By 1952, its ornamental virtues were discovered and its development begun. Eight years later, the ‘Bradford’ pear, P. calleryana’s first cultivar, was introduced.
All ‘Bradford’ trees have been developed from that single, first ‘Bradford’, which died not long ago to make way for a parking lot in Glenn Dale, Maryland.
“So they’re all clones of the same exact tree and they’re just grafted onto different rootstock, which normally is Pyrus calleryana. All the ‘Bradfords’ that we have today are genetically identical,” says Theresa Culley, University of Cincinnati plant ecologist who does population genetics. She is funded by the USDA to study the spread of Callery pear.
Since its welcome into suburbia, ‘Bradford’ cultivars selected by the USDA, universities and nurseries have been cloned, grafted, sold and planted (300,000 ‘Bradford’ by 1981). All trees in each cultivar are genetically identical. Some cultivars with different names are also genetically identical: ‘Chanticleer’, ‘Stone Hill’, ‘Select’, ‘Glenn’s Form’ and ‘Cleveland Select,’ meaning they are really all the same cultivar, just coincidentally selected by different people around the same time from a fine-looking tree they spotted in Cleveland. Since ‘Bradford’ remains the most fire blight-resistant, most cultivars are grafted onto its rootstock, although other cultivars are used too.
Because P. calleryana and its cultivars cannot self-pollinate, they reproduce by cross-pollinating. This is where a good tree goes bad.
Callery pear movement mechanisms
1. When someone replaces a dying Bradford with a different cultivar, it can cross-pollinate remaining Bradfords and all other cultivars within mating range.
“We found out in my lab, using hand-pollination and genetic techniques, that the pears are spreading because different cultivars planted in the area are cross-pollinating with one another and now all the trees can produce fruits,” Culley says.
2. They root sprout and these sprouts mate with the cultivar scion, producing viable fruit and seed spread by wind and birds along roadsides and in natural areas. (Is it spreading into wooded areas or just “disturbed” areas, which can hardly be called “natural”? And what is a natural area?)
“The root stock, if it sprouts and flowers, is genetically different from the top of the tree because it’s been grafted. Now the root stock can cross-pollinate with the top of the tree. Now you can potentially get an invasion from a single cultivated tree that someone doesn’t take care of. We have gone into wild populations and done parentage analysis and with most of them, the parents are cultivars. Sometimes you start to see the root stock showing up.”
3. Mowing a lawn too low around Callery pear and nicking its roots can induce root sprouts. Poor property management is another culprit. Culley has seen bunches of wild pear seedlings in restored prairie where management bushwhacked everything to the ground. “That’s good for everything but the pear. That just promotes sucker growth. Then they ended up with shrubby thickets.”
So why is the spread of P. calleryana a problem? What are the ecological impacts?
Culley, who is working on the issue, says in eastern and southern states Callery pear forms huge thickets and physically out-competes other plant species. Some are easily a half- to several acres and about a third of them have thorns. The tree may also be allelopathic like other invaders (barberry, garlic mustard, honeysuckle) and inhibit growth of native species. “It’s a whole other world when you go beneath the soil,” she says.
P. calleryana grows in Zones 5 to 9, but cold tolerant cultivars are being developed. “If those get out and cross, I can see the whole range of the species shifting northward,” says Culley who found a patch growing in downtown Madison, Wisconsin, due to heat island effect.
East Coast scientists are working on developing sterile cultivars. One is ‘New Bradford’ reportedly a sterile, seedless variety. However, Culley says she has collected fruits from it. “The truth is these trees are so popular that people have different cultivars all over the place. It is likely that a whole group of ‘New Bradford’ trees is going to attract pollen. And who knows what kind of root stock they’re on,” she says.
Enter Thomas G. Ranney, Ph.D., Professor Horticultural Science, Mountain Horticultural Crops Research & Extension Center, North Carolina State University, on what, exactly, is an invasive species. “I find it hard to put plants strictly in categories of being invasive or not. I adopt the federal definition of invasiveness. It indicates for a plant to be invasive it has to be non-native and it has to have done harm to the environment or human health in excess of any benefits.”
Ranney sees Callery pear naturalizing in old fields as a transient, pioneer species, shade-intolerant.
“As somebody told me in an audience, once, it tends to naturalize in areas where they’re going to build the next Walmart. It’s rare to find it in true natural areas – woodlands or forest settings. It’s usually in areas where somebody’s come in and cut down all the trees and abandoned the field or pasture. As succession starts going it might have some Callery pear. The big question is: is it causing significant environmental harm and I’m not sure that it is.
“The real estate it’s taking up is in heavily disturbed areas where humans have come in and done major disturbance. They’re the ones who have really done the harm. You can’t come around and then point your finger at a Callery pear like, ‘We just cut down the forest so we’re going to blame you for the environmental impact.’
“You can show people a picture of a field where humans have done massive environmental harm and Callery pear is growing there and people will go, ‘Oh, my gosh, there’s Callery pear.’ And they’re completely oblivious to the fact that somebody has cut down the forest. So let’s keep this in perspective. If we’re talking about conservation and biodiversity, you can’t blame the Callery pear for that one.”
Ranney admits, though, that Callery pear, growing along roadsides and major interstates, is weedier than he likes. He is working on developing seedless forms of Callery pear along with other nursery crops. His main approach is to hybridize a tetraploid with a diploid and get a triploid, which has problems reproducing but is indistinguishable from its diploid counterparts.
Over the past seven years, he has produced large populations of triploid Callery pears that are now being evaluated for sterility, disease resistance and ornamental merit such as cut leaf foliage, different flower colors and cold hardiness. “It’s part of a broader breeding program. Seedlessness is just one of the objectives. We’re probably two or three years away from making final selections for introduction.”
Ranney on thickets: “How much of the real estate does it occupy? What is the biodiversity in the area? Is it a 100% Callery Pear and absolutely nothing else? Or is Callery pear incorporated into the biodiversity there? A lot of times ecosystems are quite tolerant of additions of more species. I don’t think you can assume that it displaced everything else. I have seen areas – not acres – where a few hundred trees sporadically grow. How much harm is it doing and how does it influence biodiversity are the questions to ask. Callery pear is relatively short-lived – around 30 years. I would be interested to come back in another 15 years to a site like that and see how well they persist. They’re not very shade tolerant and will start to die off after they get significant shading.”
So, what’s a gardener to do?
Responsible homeowners who like their Callery pear and want to keep it too should diligently remove root sprouts. Or spray the tree during peak bloom with the chemical ethephon, 95% effective at preventing fruits while minimizing blossom drop, Culley recommends. And don’t mow the roots.
Culley, president of the Ohio Invasive Plant Council, recommends alternatives:
American fringe tree (Chionanthus virginicus)
Serviceberry (Amelancier laevis ‘Cumulus’)
Yellowwood (Cladrastis kentukea)
Redbud (Cercis canadensis)
Green hawthorn (Crataegus viridis ‘Winter King’)
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published April 12, 2011