Ten Plants that Don't Belong in Your Landscape

Invasive Plants

The gardener in the know quickly learns to hate all invasive species. Plants like this show up in a new neighborhood and, like the plant equivalent of a Mongol horde, quickly drive away the native plants by out-competing them. An invasive plant is a non-native species, often originally imported as an ornamental plant, or accidentally brought over from its natural home. Pretty or not, invasive plants swoop in and co-opt large areas of habitat, as a rule because no one bothered to import its natural enemies and predators at the same time. 

Not all invasive species are plants. For instance, through though native to southernmost Texas, nine-banded armadillos can be now found across the southeastern states. Every armadillo in Florida and nearby states is believed to have descended from abandoned or escaped pets and zoo specimens that multiply like rabbits without natural predators like wolves and coyotes.

The following plants are ten common invasive plant species that can be found in large areas of the United States. Gardeners can do their part to control these species: learn to identify invaders found in your state from your natural resources department, local master gardeners and other resources.

Cajeput Tree

A Cajeput tree (Melaleuca quinquenervia) is a native of Australia first imported to the U S in the early 20th century for ornamental planting. In Florida and Louisiana wetlands, this tree - also called paperbark, bottlebrush or broadleaf teatree - grows wild, crowding out native species. The tree’s rapid growth combined with a lack of natural enemies allows it to displace native species, reducing the food and shelter available to native plants, insects, birds and animals. Dense stands of Cajeput create a fire hazard, allowing hot crown fires in environments that would normally be sawgrass prairie.

Cogongrass 

Cogongrass or Japanese Blood Grass (Imperata cylindrica) is originally a native of Africa and East Asia. This plant was introduced to the U S Gulf Coast sometime between 1910 and 1930 as forage for livestock and for the control of erosion. Like many invasive species, Cogongrass forms monospecies stands that crowd the native grasses and other plants out, thereby reducing available food and shelter for the native plants and animals, as well as ground-nesting birds. 
       
The species can be found all along the Gulf Coast from Florida to Texas plus Georgia and South Carolina, with some pockets in Oregon. Cogongrass has displaced a variety of native species because it forms an almost impenetrable mat of roots and leaves. It’s especially fast-spreading where the ground has been disturbed.

Japanese Honeysuckle

Originally found in east Asia, Japanese Honeysuckle (Lonicera japonica) was introduced as an ornamental in New York back in 1862. Just as aggressive as it is attractive, through more than a century and a half it’s spread like wildfire from its original plantings at farmsteads and in fencerows. It’s considered an invasive species by agriculture departments in every state east of the Mississippi except Maine, Alabama, and Mississippi; and it’s also listed in Kansas, Oklahoma, Texas and Hawaii. 

Honeysuckle’s tough, woody vines grow rapidly, reaching to thirty feet long. The vines can strangle young and even mature trees in dense stands. Surprisingly, honeysuckle can still be found sold as an ornamental ground cover; but in the wild the plant can crowd out native species with its dense canopies that block the light necessary to understory plants.

Kudzu

The modern scourge of Atlanta, kudzu (Pueraria montana v. lobata) was once considered an excellent ground cover to help control erosion. Native to Asia, kudzu has now overrun large areas of the southeast, choking growth wherever it gains a foothold. The vines, which are known to reach to 100 feet in length and four inches in diameter, can grow up to a foot per day in the warm season. Kudzu kills native plants by choking them, and often breaks or uproots trees under the weight of the dense foliage. 

Originally introduced for forage, erosion control and basket-making, kudzu has no enemies in North America. Where native, the vine is held in check by a combination of insects and fungi that weren’t imported with the plants. Kudzu infestation can be found in every state east of the Mississippi except the far northeast, along with several Great Plains states and Oregon.

Leafy Spurge

Most widespread in the northern Great Plains states and the Mountain West, Leafy Spurge (Euphorbia esula) came over from Europe in the early 19th century. Research suggests that it was an accidental invader, arriving as a contaminant in imported seeds. It has since spread from Michigan to Washington and as far south as Colorado, as well as California, Virginia and North Carolina. 

Known for its showy yellow flowers, this plant is adapted to most environments but is most invasive where it encounters dry, disturbed soil; hence its abundance in the Mountain West. While difficult to control by hand or chemical means, this invader may eventually be controlled by insect enemies imported from its native habitat.

Mile-a-Minute

As is suggested by the common name, mile-a-minute (Persicaria perfoliata) grows rapidly and indiscriminately, latching onto trees, structures, and even rocks with vines armed with sharp, hooked barbs. Once it reaches a forest’s upper story, mile-a-Minute spreads rapidly through the treetops. The large leaves blocks sunlight from reaching the trees beneath, weakening them and leaving them subject to damage and disease.

The plant was originally introduced as a contaminant to ornamentals in the late 1930s. It has since spread through southern New England and the Mid-Atlantic states, and has footholds in the Midwest and Oregon.

Purple Loosestrife

Lythrum salicaria, common name Purple Loosestrife, is also a European native that arrived early in the 19th century, possibly as seeds lurking in a ship’s ballast. Since arrival, it’s spread across almost all of North America except the southernmost states and Mexico. 

Purple Loosestrife flourishes in riparian environments where, like most invasives, the plant out-competes native species. It’s a classic example of an aggressive invader, creating vast quantities of seeds that spread both by wind and water. In its native habitat, Loosestrife is kept in check by insects that eat the flowers or destroy its seeds. Several species of European insects are being imported by the USDA in hopes of controlling the plant.

Russian Olive

Russian Olive (Elaeagnus angustifolia) is in reality neither an olive nor Russian. The Eurasian plant came late to North America, arriving sometime in the early 20th century for use as windbreaks and ornamentals. In the past century, it has spread to nearly every state and to all the Canadian provinces. It’s considered an invasive species from coast to coast and from the Gulf of Mexico to the 49th parallel.

Like all invasive species, Russian Olive crowds out native plants by growing quickly and across multiple habitats. Wherever it thrives, it interrupts normal forest succession and places a burden on water of water and aquifers. 

Tamarisk

First imported in the early 1800s from Eurasia as an ornamental, Tamarisk (Tamarix ramosissima) or Salt Cedar is common across the entire western U S plus part of the Midwest and east coast. In the past two hundred years, Tamarisk invades watercourses across the desert southwest and beyond. The plant is highly adaptable, and can thrive in a wide range of conditions including saline and alkaline water. 

Besides out-competing native species where it grows, Tamarisk also depletes water bodies, lowering water tables and destabilizing stream channels where abundant. Tamarisk eradication methods are either manual or chemical means, and there have been tests of insect predators.

Tree-of-Heaven

The widely-dispersed Tree-of-Heaven (Ailanthus altissima) is definitely not heavenly. This fast-growing invader grows almost anywhere, including gutters and cracks in sidewalks. It is especially well-adapted to growing in urban areas, where it frequently damages structures by forcing its root system into cracks and other small spaces. 

Tree-of-Heaven came to North America in the 18th century and has since spread far and wide. It’s present in almost every state and is considered invasive over most of the west, from Texas to North Dakota and west to the coast. Tree-of-Heaven’s invasive nature is partially because of its rapid growth and prolific seed production, and partially because it secretes a toxic chemical that suppresses growth of other plants nearby. The wood and sap produce a distinctly unpleasant odor. 


The agriculture or natural resources departments of almost every state can provide lists of plants considered invasive in your area, plus animals and insects. Check the U S Department of Agriculture for nationwide information, and for links to state agencies. An invasive species has an unfair advantage when competing with native species that have natural checks and balances. Out-competing native plants plants means more than just reducing the number of plant species: native plants provide food and shelter for fauna that have evolved in parallel with the plants, and also act as “nursemaids” for other plant species. Invasive species almost invariably reduce biodiversity in the areas they overrun. 


More information:

The United States National Arboretum | Invasive Plants  
U S Department of Agriculture | National Invasive Species Information Center