Like so many regions in New York, nearly every corner of Missouri has been hit hard with the invasive spread of Callery pear (Pyrus calleryana spp.). Callery pears are self-sterile, but it turns out they readily cross-pollinate with other cultivars. Also, the rootstock upon which a Bradford pear is grafted will sometimes sprout, eventually yielding flowers and viable pollen.
Fortunately, Missourians are often out in front with innovative approaches to urban forestry and invasive plant control. Here’s how they reduced the number of Callery pears and increased the use of native, non-invasive trees. Special thanks to Tina Casagrand of the Missouri Invasive Plant Task Force (MoIP) for her help with this post.
I moved from Rochester to the Hudson Valley in 2010. In the eight years since, I’ve noticed a steady proliferation of escaped Callery pears in the Valley. From one undeveloped bowl of land at a busy corner in my town emerges a cloud of white in the spring and some admittedly striking fall color come late October/early November. The problem is that not much else is growing there now, and many of these volunteer trees have reverted to thorniness, creating giant impenetrable thickets.
Callery pears have a mixed rating on wildlife value; on the one hand, bees and other insects visit the flowers in spring and a few species of songbirds eat the fruit after it softens in the winter. On the other hand, Callery pears do not support caterpillars in any significant numbers, so they do not provide adequate food for baby birds the way that oaks and other native trees do.
Why are self-sterile cultivars of Callery pear producing fruit? One way it happens is when fertile pear understock sprouts, flowers, and produces viable pollen. Another: by the late 1990s, the introduction of new Callery pear cultivars beyond ‘Bradford’, cultivars like ‘Aristocrat’ and ‘Chanticleer’, led to an unexpected dilemma: in areas where large numbers of Callery pears were planted, the self-sterile cultivars starting pollinating one another. Then came the fruit, then came bird dispersion of the fruit … and “Pyrus, We Have a Problem.”