Exclamation! London planetree as a narrow crown that helps it fit in tight spaces.
When it comes to making leaves, American sycamore trees are a day late and a dollar short. First off, they tend to leaf out later than other trees in the spring. And when their leaves finally do emerge, often these trees are hit by a disfiguring disease called anthracnose.
The disease affects many hardwood trees, but anthracnose really goes to town on sycamores, blackening and distorting leaves, and attacking twigs, creating clusters of slender weak stems called "witch's brooms," according to Donna Danielson, an expert in the Morton Arboretum Plant Clinic. While anthracnose rarely kills an entire sycamore tree, it can certainly kill stems, detracting from these trees' lovely mottled white bark, large leaves and impressive height.
So what's a sycamore lover to do? Consider planting Exclamation! London planetree (Platanus x acerifolia 'Morton Circle') or Ovation London planetree (Platanus x acerifolia 'Morton Euclid'), two new disease-resistant hybrid trees that are on the market and available at nurseries.
These trees have all the beautiful attributes and hardiness of a sycamore, but are resistant to anthracnose disease, according to Dr. George Ware, Arboretum tree expert. He bred the new trees by crossing a winter-hardy American sycamore tree that showed good resistance to anthracnose for 10 years and an Oriental planetree, which has a higher degree of disease resistance than sycamore. A period of extensive testing and exposure of the new saplings to anthracnose showed consistent disease resistance.
These London planetrees tolerate a wide variety of conditions, according to Kunso Kim, curator and assistant director of collections at the Arboretum. "They are adapted to various soil pHs, including high alkaline soil. They grow well in urban soil conditions, where the ground is compacted clay and the air has a high amount of pollution," Kim says.
Both trees are pyramidal, with a dominant central trunk, a shape that is highly desirable for street trees in parkways and medians. Exclamation! London planetree has a narrow crown that helps it fit in tighter spaces. The leaves are similar to the large American sycamore leaves. Ovation London planetree can sport an upright oval shape and maple-like leaves. Their smaller size -- up to 60 feet tall -- is also more fitting for home landscapes such as typical city lots.
These London planetrees were bred and tested under the Chicagoland Grows Plant Introduction Program, a consortium of the Morton Arboretum, Chicago Botanic Garden and the Ornamental Growers Association of Northern Illinois, a network of nurseries.
"This program makes use of members' resources to collect, breed and test potential trees and other plants that meet criteria such as hardiness, ornamental qualities, adaptability and resistance to pests and diseases," says Kim, who chairs the woody plant committee. Find more information about London planetrees and other introductions at www.chicagolandgrows.org.
"We've developed organized and systematic ways to identify, test and market trees suitable for our environment," says Kim. "We want to release trees and other plants that are worthy, absolutely reliable and useful for multiple purposes."
Laurie Casey is a staff writer at the Morton Arboretum in Lisle.
