Public gardens rally against a common foe

UC College of Arts and Sciences Professor Theresa Culley is an expert on invasive species who advises federal agencies and states such as Ohio about the economic threats posed by nonnative plants.

You don’t have to look far to find examples of problem plants.

Callery pear trees were introduced to the United States as a fast-growing ornamental tree with pretty spring blooms. The first cloned variety was sterile, but once other varieties were created with sturdier trunks that were less likely to split, they began to cross-pollinate and produce fruit. Soon, these trees escaped cultivation and began showing up in forests and highway edges across the Midwest.

Amur honeysuckle was a favorite landscaping bush for yards across the United States and to prevent erosion along roadsides. But this bush from China leafs out earlier in the spring than other plants and stays green through the early winter. As a result, its leaves often shade out other ground covers. And birds spread its red berries far and wide.

There has been a bigger push to address the spread of invasive species from state to state as well. States remind park visitors not to remove or introduce firewood from place to place to prevent the spread of invasive insects such as the emerald ash borer.

The new UC collaboration is already sharing useful information. 

Some public gardens are reporting that Amur corktree, an import from East Asia, is starting to show up in more wild places. Its roots have allelopathic properties, or can suppress the growth of other plants to wage chemical warfare that gives it a competitive advantage.

As a result of the public garden collaboration, they shared an alert with landscapers and nurseries urging them to sell only non-fruit-bearing Amur corktree in a bid to stem the wild expansion.

UC’s Culley said other examples include a common ground cover called wintercreeper and golden rain tree, which was introduced to North American nurseries from China and North and South Korea in the 1950s and now is growing wild in at least three states. As many as eight public gardens in the network reported that these two plants posed a potential problem as an invasive species.

“The data are important because we can prioritize strategies to deal with them,” Culley said.

Other introduced nonnative plants popping up include European buckthorn, white mulberry and Siberian elm.

Kurt Dreisilker, head of natural resources and collections horticulture at the Morton Arboretum in Illinois, has helped to organize the gardens network. It's useful for public gardens to know what other gardens are seeing across the country, he said.

“Sometimes those species may not be known as invasive so the network can get the word out ahead of time before plants become invasive,” Dreisilker said.

Gardens might be especially vigilant when they see potentially problem species pop up nearby or at multiple gardens, he said.

“I absolutely do think this network holds promise to help prevent plants from becoming invasive.”

While some public gardens focus exclusively on native plants, others cultivate a diversity of nonnative plants to introduce the public to exotic trees, flowers and bushes they might never get to see anywhere else. The gardens also help conserve rare or declining species from around the world, Culley said.

“Many of these gardens bring in plants from around the world for education,” she said. “And they’re becoming deeply involved in conservation. But there is still an inherent fear that they will be blamed for problems with invasive species that were not known to be a problem when they were planted.”

Some gardens choose to share data on invasive species as long as they are not publicly identified, she said.

Culley said what makes public gardens good sentinels is the botanical expertise they offer along with typically extensive recordkeeping.

“The gardens are called living collections. They know where each plant is from and whether it is producing fruit,” she said. “And many have their own plant collections called an herbarium dating back decades or more.”

UC, too, maintains an herbarium containing more than 125,000 specimens.

Public gardens also can be a bellwether for the gradual creep of some species that are taking advantage of a warming climate to move north into states where winters historically have been too cold for them, she said.

Culley said she is optimistic that public gardens will be a useful source of information for park and refuge managers nationwide.

“There is a lot of hope, but it requires everyone working together,” she said.

Featured image at top: Colorful tulips are a highlight of spring at the Cincinnati Zoo & Botanical Garden. Photo/Colleen Kelley/UC Marketing + Brand