A slew of invasive plant species have grown roots in Kentucky soil and more are migrating into the state, shouldering out native biodiversity.

Invasive plants are non-native, causing ecological or economic harm, and are usually introduced by human actions. Not all non-native plants are invasive. 

Agriculture, forestry and other prized Kentucky industries are at risk because of invasive plant species. They typically spread quickly, outcompeting and displacing important native species. 

“Biological invasions” in the U.S., spanning thousands of species of invasive plants, pests, mammals and other life, have incurred “at least” $1.22 trillion in costs from 1960-2020, according to a recent study. 

That estimate includes only “observed, highly reliable costs,” according to the study, and damages have risen in recent years — in the 2010s, invasive species cost the U.S. about $21 billion each year. 

As climate change makes Kentucky warmer and wetter, invasive species comfortable in warmer climates to the south could move north into Kentucky. 

“These milder winters fail to destroy imported insects and plants,” according to a government report on Kentucky’s emerging forest threats. “This may result in habitat destruction (or) fragmentation and loss of aesthetic value in recreational areas.” 

Here are 10 invasive plant species afflicting Kentucky’s forests and natural resources:

Tree of heaven

Deemed a “devilish invader” by The Nature Conservancy, tree of heaven grows rapidly and can take over a forest with its prolific seed production. 

“It forms these dense thickets that aren’t going to allow for the growth of the diversity of other tree species,” said Ellen Crocker, a professor in the University of Kentucky Department of Forestry and Natural Resources. 

A forest without a diversity of plant species cannot support a diversity of insects, birds and wildlife, causing ripple effects throughout an ecosystem. 

Tree of heaven is also “a favorite host” of the spotted lanternfly, Crocker said. 

The spotted lanternfly is an invasive pest ravaging the eastern U.S. and reported in Kentucky last year. Entomologists say its spread would cause extensive damage to the state’s forests and agricultural operations.

Tree of heaven can grow up to 80 feet, and is identifiable by its compound leaves, composed of many narrow leaflets, and “bark resembling the skin of a cantaloupe,” according to Penn State Extension. When crushed, the leaves produce a pungent odor comparable “to cat urine or burnt peanut butter.”

Bush honeysuckle

Bush honeysuckle has grown ubiquitously in Kentucky’s natural areas, identified by its opposite leaves, striped bark, shiny, round berries and sets of flowers that turn from white to yellow.

It blankets a forest’s understory, blotting out the sun for other, native plant life. Its leaves also come out earlier and stick around later in the year, shading native plants.

Since the plant’s understory dominance can prevent the growth of other species, it can contribute to erosion and water quality problems, Crocker said.

Bush honeysuckle can also release chemicals into the soil through a biological process called allelopathy, which can inhibit the growth of other plants.

The shrub is “found in abundance in Cherokee and Seneca Park,” according to the Olmsted Parks Conservancy, and “threatens native plants and park ecosystems.”

Research has also tied invasive honeysuckle to abundant tick populations, Crocker added, potentially helping the spread of tick-borne diseases.

Kudzu

Notorious as “the vine that ate the South,” kudzu has swallowed up wide swaths of rural Kentucky, snaking up telephone poles and carpeting hillsides.

It can grow up to 2 inches per day in the right conditions, according to the University of Kentucky Department of Forestry and Natural Resources. The vine’s alternating, compound leaves with three leaflets help identify it.

Kudzu is native to China, but was found to have useful applications in the U.S. as nutritious feed for livestock and as a soil stabilizer.

But it grew out of control across the South and beyond, and “its rapid growth can choke out trees and rapidly overrun fields and pastures,” according to the U.S. Forest Service. 

Consistent livestock grazing of cattle, goats and other ruminants is one method of controlling kudzu’s spread, research has shown.

Callery pear

Callery pears were planted as ornamental trees in Kentucky and across the U.S., noted for their blooming white flowers and pungent smell. 

Bradford pears, a cultivar of the Callery, are found across Louisville. All Callery pears are a “prohibited species” according to Metro ordinances, along with numerous other invasive species.

As the fruit of the Callery pear softens, birds eat it, helping their abundant spread, according to the Southern Group of State Foresters.

Callery pear “is not known for being a species that supports high biodiversity,” Crocker said, as the tree instead creates uniformity in a landscape and pushes out important native species.

To identify the tree, look for “alternately arranged teardrop shape rippled leaves with finely toothed margins,” according to Penn State Extension. “The bark is gray brown and becomes scaly with age.”

Autumn olive

This shrub, introduced to North America nearly 200 years ago from Asia, is comfortable in unforgiving soil, and it’s commonly found growing on former mine lands.

A single plant can create 200,000 seeds each year, according to The Nature Conservancy, helping it spread. Birds take the fruit, and the seeds are distributed “far and wide throughout pastures, along roadsides and near fences.” 

Like bush honeysuckle, autumn olive uses allelopathy to change soil chemistry, further displacing native plant species.

Autumn olive grows up to 20 feet tall, and can be identified by the silver-gray color in its foliage, small, alternating leaves and cream-colored flowers.

Burning bush

Named for its bright red fall foliage, burning bush is an invasive shrub popular for ornamental and landscape uses. 

“Despite its demonstrated invasiveness, it remains one of the most popular landscaping shrubs sold by nurseries and retailers throughout many parts of the U.S.,” according to Purdue Extension. 

Burning bush has been widely reported across the eastern U.S., and is known to leave its original cultivation area and invade natural areas, where it can spread quickly and crowd out native species.

To identify burning bush, look for “football-shaped, oppositely arranged, finely toothed or serrated leaves that are between one and two inches long,” according to Penn State Extension. “The leaves turn a brilliant purplish red to scarlet before senescing, or dying, in the fall.”

Wintercreeper

An evergreen vine used as an ornamental ground cover, wintercreeper is considered invasive for its ability to carpet an area, impeding native wildflowers, shrubs and trees.

It may also climb dozens of feet up into trees, Crocker said, and can make the trees more vulnerable to damage from strong winds or storms in the winter.

The vine can be identified by its “slightly toothed” and “glossy evergreen leaves with prominent silvery-white” veins, according to the Missouri Department of Natural Conservation.

Wintercreeper produces berries and fruit only when growing upward as a vine, making it more likely to spread widely in this form, Crocker said, since birds will eat the berries and carry the seeds into other areas. 

It’s also one of several vines found in Louisville’s Olmsted parks, and takes advantage of the winter months to spread outward and up into trees.

Left unmanaged, wintercreeper could crowd out native spring ephemerals in parks and natural areas, including Dutchman’s breeches and wood poppy, according to the Olmsted Parks Conservancy.

Paulownia

Paulownia, also known as princess tree, is another species considered invasive for its fast growth and ability to outcompete native species. 

“It tolerates high soil acidity, drought, and low soil fertility enabling it to survive, grow, and reproduce on harsh, exposed sites,” according to the Southeast Exotic Pest Plant Council. “… Its ability to sprout prolifically from adventitious buds on stems and roots allows it to survive fire, cutting, and even bulldozing in construction areas.”

Despite its overall resilience, the tree is not very cold tolerant, Crocker said, and warmer conditions in Kentucky could make the species more problematic.

Paulownia trees can be identified by their large, heart-shaped leaves, rough, gray-brown bark and fuzzy leaves that grow opposite each other along the stem.

Paulownias are not to be confused with the native northern catalpa, a similar-looking species whose leaves grow in a whorled pattern instead of opposite pairs, and lacks the same fuzz on its leaves as the Paulownia.

Porcelain berry

Porcelain berry is another invasive vine in Kentucky, somewhat resembling wild grapevine. It’s known to overtake native vegetation and can wrap around trees, stealing resources and strangling them.

The vine grows well in many different environments and soil types, and birds and other animals eat its berries, helping it spread.

It’s also among numerous invasive species threatening Louisville’s parks.

“Like all invasive plants, Porcelain-Berry grows rapidly because it did not evolve here with the checks and balances of competition from the plants and animals that live here,” according to the Olmsted Parks Conservancy. “Instead it takes the place of the plants that supply the resources our native wildlife needs.”

The plant has small flowers and alternating leaves, and its “distinctive speckled berry” comes in different colors — including blue, pink, purple, and cream, according to the Minnesota Department of Agriculture. Porcelain berry can be distinguished from wild grape by its white pith (the center of its stem) and small spots on its bark.

Garlic mustard

First introduced to the U.S. from Europe in the 19th century as a medicinal herb, garlic mustard quickly spread throughout American fields and forests. 

Like some other invasives, garlic mustard emerges earlier in the spring, blocking native plants from sunlight, water and nutrients.

One plant’s thousands of seeds may be transported by the wind or wildlife, and they can grow “almost anywhere,” according to the Kentucky Department of Fish and Wildlife Resources.

Garlic mustard typically grows to 2 to 3.5 feet tall, and produces four-petaled white flowers in the spring and summer.

Unsurprisingly, garlic mustard tastes like garlic and mustard — and “all parts of the plant are edible and nutritious,” according to Philadelphia Parks and Recreation, although the plant should be cooked to remove trace amounts of toxins.

How to combat Kentucky’s invasive plant scourge 

A vast majority of Kentucky’s forests and natural areas are privately owned, meaning it’s mostly private landowners who are faced with the incursion of invasive plants.

The easiest opportunity to prevent the spread of invasive species is before they’ve gained a foothold, Crocker said, and there are a variety of resources available to landowners around the state looking for help in managing natural areas and combating invasive species.

Some resources include:

“There’s a lot that we can be doing to promote the health of natural areas, to help our native communities deal with these invasive issues,” Crocker said.

“I don’t want people to feel discouraged. I want them to feel empowered that they can play a big role in promoting the health of natural areas.”

Connor Giffin is an environmental reporter for The Courier Journal. Reach him directly at cgiffin@gannett.com or on X @byconnorgiffin.