By Alan Branhagen, Executive Director, Natural Land Institute
Two ornamental trees have been added to the Illinois Noxious Weed List: Callery Pear (Pyrus calleryana) and Amur Corktree (Phellodendron amurense). I’ll admit, I specified planting Callery Pears way back in the early 1990’s before I moved from Rockford to Kansas City in late 1996.
Callery Pear in bloom at Greenwood Cemetery, Rockford, IL. Photo: Alan Branhagen
Callery Pear
In Kansas City I saw Callery Pears were one of the most commonly planted ornamentals and I observed how they were starting to widely escape. I had all of the few trees planted at Powell Gardens removed – ahead of the curve. When I left Missouri 21 years later for Minnesota, it was apparent Callery Pear had become one of the worst invasive plants in the Lower Midwest. Missouri banned the sale of the tree last year. When I returned to Rockford, I was shocked to see how widely this tree had been planted and how it had begun to escape widely in Northern Illinois too.
So how did this happen? Callery Pear is native to China and was brought to the United States for fruit tree research. Some Callery Pears have large, edible fruits – but a small-fruited variety was noted for its ornamental qualities – exceptional red fall color late in the season and a perfect popsicle form. This plant was cloned with the name ‘Bradford’ pear and rarely set any fruit because pears need cross pollination – clones are the same plant and cannot cross pollinate. As the trees grew old, the popsicle form turned out to be trouble – it was comprised of multiple leader branches that with age split apart, shattering the tree! Soon the tree was splitting apart all over the place so new clones were found with better branching habitat – bad news was, they could cross pollinate with ‘Bradford’ and soon they all started producing abundant little round pears. Starlings love these pearlets, eat the fruits, scarify the seeds in digestion, then disperse them far and wide complete with their dropping as fertilizer. Soon Callery pear trees were sprouting up everywhere!
Callery Pear fruit showing the “pearlets”. Photo: Alan Branhagen
Why is this bad? Because pears are not native to North America and have few insects tied to feeding on them – and insects are the building blocks of our web of life. They produce almost no caterpillars for native birds to feed their young. The trees also are very deep rooted and tenacious, cast dense shade and usually produce spines as seedlings revert back to more wild forms. They outcompete and shade out native plants and are very difficult to remove. Why are edible pears not a problem? Edible pears have big heavy fruit and birds do not spread the seeds far and wide, pears grown as fruit trees very rarely naturalize despite being planted here by the first Euro colonists.
What can you do? If you have a Callery Pear cultivar in your landscape, remove it! Replace it with a native American tree that has better ecosystem services! In Missouri, the Missouri Invasive Plant Council (MoInvasives.org) has a “Pear Buyback” and provide a free tree to anyone who shows they removed a Callery Pear.
Callery Pear leaves showing fall color. This tree escaped into a natural area. Photo: Alan Branhagen
This photo shows the classic corky bark of an Amur Corktree found at Kilbuck Bluffs Forest Preserve, Rockford, IL. Photo: Alan Branhagen
Amur Corktree
In Minnesota, I first met the Amur Corktree escaping – the Arboretum had removed all its female corktrees before I arrived because the tree had invaded all the woodlands around the collections. Corktree was obviously the most invasive tree found throughout the grounds and these were actively removed. I found occasional trees in woodlands all around the region.
Amur Corktree is native to Northeastern Asia – mainly in the region where Northern China and Eastern Russia meet – a climate quite similar to Northeastern North America. The tree is dioecious, meaning trees are usually male or female. The female trees, when pollinated by a nearby male, produce black, greasy berries that are dispersed by birds. The tree is named after its somewhat spongy, corky bark but is not related to the cork oak where true cork comes from. Corktree is in the citrus family so related to Prickly-Ash and Hoptree found locally, but unlike those native trees, supports no local insects.
Corktrees have been planted in the Rockford area for a long time. They were valued for their short, spreading crowns of horizontal limbs and clean, pest-free foliage. A magnificent tree occurs at the historic Rockford Country Club, a tree at Kilbuck Bluffs Forest Preserve was probably planted in the CCC (Civilian Conservation Corps) days, and the tree was widely planted in the old Rockford Nursery, now Klehm Arboretum where they persist.
Leaves from an escaped seedling of an Amur Corktree at Klehm Arboretum. Photo: Alan Branhagen
The sparse nature of the trees locally has not allowed them to produce abundant fruit except at Klehm Arboretum where male and female trees occur together. It is also the only place I’ve seen spontaneous plants in our immediate region. Southern Illinois reports significant infestations and that is also true farther north in the Minneapolis-St. Paul region.
What can you do? If you have a corktree with fruit (female), please have it removed. Again, replace it with an American native or non-invasive tree with higher ecosystem services. If we nip these in the bud now, we will not have a more expensive removal in the future.