CINCINNATI, Ohio – Of the approximately 2,300 vascular plant species growing in the wild in Ohio, about 78 percent are native. The other 22 percent – more than 500 species – are not native to Ohio, having been introduced from other states or countries. Most non-native plants have been introduced for erosion control, horticulture, forage crops, culinary and medicinal use, wildlife foods or by accident.

These species become very invasive and displace native plants in woodlands, wetlands and prairies, and oftentimes onto farms. Invasive plants are a major threat to the health of the ecosystems and to the viability of rare species.

University of Cincinnati botanist Denis Conover has studied the problems associated with invasive plants. Conover, along with his students, examined the impact that nonnative, invasive plants are having. His latest study was published in the journal Ecological Restoration.

“The nonnative, invasive species are very detrimental to native ecosystems,” Conover said. “Once they get going in the natural areas, they can take over and exclude the native plants and the animals that depend on them.”

In this study, Conover used the arboretum at Cincinnati’s Spring Grove Cemetery, which has recorded more than 1,200 species of trees, shrubs, vines and other plants over its 178-year history. The arboretum is home to 26 noteworthy, mature trees known as “champions” for being the largest or best representation of their species, including a national champion September elm tree and an Ohio champion American yellowwood.

Conover said the arboretum is home to many beautiful native plants, including an enormous white oak that was a sapling when the Mayflower dropped anchor in Plymouth Colony. But Conover said some exotic plants at the arboretum are bearing fruit and seeds that are being carried by birds or the wind to neighboring woods.

Some exotic plants introduced to people’s yards and gardens will never pose a problem, Conover said, while others can become invasive.

“You don’t know. It may take decades before they show any signs of being invasive, like the Callery pear tree,” Conover said. 

Horticulturists introduced most of the nonnative plants from Europe and Asia as ornamentals. Their seeds eventually spread in the wild. The biggest culprit? Amur honeysuckle, a woody shrub that has taken over many eastern forests.

“It has escaped into the wild and is propagating on its own,” Conover said. “Amur honeysuckle is now the most abundant woody plant in Hamilton County. One bush can produce thousands of seeds that get dispersed by birds and mammals. In some woodlands, the Amur honeysuckle layer is so dense that the only native species remaining are older trees whose canopy is already growing above the shrub layer.”

Some invasive plants are successful because they produce chemicals that hinder the growth or germination of nearby competitors, an insidious weapon called allelopathy, Conover said. He said where these introduced plants are found, there is often far less biodiversity to support wildlife and the food chain. One they take hold, eradicating plants like Amur honeysuckle is labor-intensive, expensive and time consuming.

“Native plants just don’t have a chance. Everything that depends on the native plants – insects, birds – can be lost,” Conover said. “When they introduce nonnative plants to the United States, they can also import fungal diseases that can wipe out native trees, which is what happened with the American chestnut.”

Callery pear trees, with their pretty spring flowers and quick growing times, were a favorite tree to plant in front yards of new subdivisions. Today, they grow wild along highways and forests. Ohio lawmakers plan to phase in a ban on the sale of Callery pear trees.

Conover’s survey found dozens of other examples of foreign species that have taken root in southwest Ohio’s woods, ones that even farmers should be keen to. Winter creeper and English ivy are ground cover commonly planted in gardens, college campuses and cemeteries. White mulberry was introduced to the United States in colonial times to feed silkworms. Today, it takes over disturbed forest and road edges. Multiflora rose produces beautiful white flowers and a tangle of woody thorns that make removal particularly treacherous.

Other invasive plants Conover included in his survey included porcelain berry, tree of heaven, winged euonymus, European buckthorn, Oriental bittersweet, common privet and lesser periwinkle. It also found Norway maple, Amur cork tree and white poplar along with herbaceous species such as lesser celandine, garlic mustard, Japanese knotweed and Japanese stilt grass.

Conover said Ohio’s biggest threats are garlic mustard, multiflora rose, shrub honeysuckles, Asiatic bittersweet and buckthorns.

The most invasive plants in Indiana are purple loosestrife, Japanese honeysuckle, autumn olive, glossy buckthorn and garlic mustard. In Kentucky, the list has tree-of-heaven, garlic mustard, musk thistle, oriental bittersweet, poison hemlock, crown vetch and Chinese yam.

Illinois: glossy buckthorn, bush honeysuckles, garlic mustard, multiflora rose, autumn olive, burning bush and Bradford pear. Tennessee: kuzdu, ailanthus, paulownia, bush honeysuckle, Japanese stilt grass, bittersweet, honeysuckle vine and privet.

Michigan: Japanese stilt grass, autumn olive, tree-of-heaven, oriental bittersweet and spotted knapweed.