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Unintended consequences of a tree

PHOTOS BY TOM BUTZLER All white flowering trees at this time are not Callery pear. Our native pin cherry (Prunus pensylvanica) left, looks very similar to invasive pear, right.

There are plenty of examples in which good intentions of plant introductions have led to problems. But the one that caught my eye this week was the flowering pear.

Traveling through Centre County this week, a field in the distance caught my eye. It was acres of white surrounded by much of the still-brown landscape. I drove to the area and walked around. It was almost blinding with the sun shining; a wall of white flowers.

Pears (Pyrus communis), a native to Asia, were first brought over to America by early colonists. But a bacterium wreaked havoc on pear production over time. Statistics show that over 86% of the crop was lost every year in the early 1900s. To solve this problem, scientists tried using a different pear species (Pyrus calleryana, Callery pear, also from Asia) as rootstock to impart disease resistance.

During the evaluation of the Callery pear, two items were noted by scientists: they could withstand a lot of harsh environmental conditions and were spectacular in bloom. The ornamental potential was recognized, and evaluations were conducted to determine if it could be another addition to our landscapes. A cultivar was selected and released as “Bradford” in the 1960s.

The other thing was the issue of fruit production. Fruits could lead to escapes in the natural environment, competing with native vegetation. But biology solved this problem as pears are self-incompatible and cannot produce fertile seed through self-pollination. In other words, the female flowers need pollen from a completely different variety for viable seed (within the fruit) to form. So, this fruiting issue was irrelevant because all flowering pears in the landscape industry were identical (clones of “Bradford”). Pollen moving from one tree to the next was the same, not from a different cultivar.

“Bradford” pears were workhorses in the landscape and planted extensively all over the U.S. But as these trees aged, an unknown foreseen problem emerged: weak branch structure. The narrow crotch angles of the branches caused them to split under their own weight or pressure from snow or wind. To solve this, other Callery pear cultivars with improved branching patterns were introduced. With all these new cultivars added to the landscapes, pollen was now available to complete flower fertilization and produce seeds that could produce a new generation of plants.

Now birds are included in the saga. Birds are often attracted to fruit for their sugars and often consume the seed. Once eaten, the seeds can be deposited anywhere birds fly. If the site is right, seeds germinate and take over. Most often, these can be seen in open sunny areas (pastures, roadside, abandoned lots). But these offspring are nasty. While they profusely flower, they also revert back to their Asian heritage and form thorny thickets. (Selected ornamental pears do not have thorns.)

And my experience this week is not isolated to the center of the state. Pennsylvania’s Department of Conservation and Natural Resources has Callery pear on its invasive plant list as a significant threat (one step below a severe threat). But we are not alone. It is becoming an issue around the country and appearing on invasive plant lists in states such as Alabama and Michigan.

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Tom Butzler is a horticulture educator with the Pennsylvania State University Cooperative Extension Service and may be reached at 570-726-0022.

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