Invasive Plant Species in North America

Disclosure: Some links may be affiliate links. If you buy an item via links on our site, we may earn a commission. Learn more.

It’s thought that there are around 17,000 species of native plants in North America but there’s also a worrying number of invasive species. In fact, it’s reported that there could be as many as 5,000 non-native plants growing in North American soil.

While many of these plants do not cause any problems, there are plenty that do, and they can harm the local ecosystem and environment. What’s more, with climate change showing no signs of slowing down, this could create more suitable environments for these invasive species to thrive.

What is an Invasive Plant?

When we talk about invasive plants, we are referring to plant species that are not native to an area. However, this doesn’t mean that all non-native plants are invasive. In fact, there are several species that live peacefully in North America, causing very few, if any problems.

An exotic plant becomes invasive when it has a negative impact on the local ecosystem, and this can occur in many ways. In order to determine whether a plant is considered to be invasive, there are certain characteristics that must apply, which include:

• The plant is native to a continent other than North America
• The plant must have been introduced by humans
• Family members of the plant are already known to be invasive
• The plant is adapted to various conditions
• Fruit and flowers are produced at a young age
• The plant produces a lot of seeds which have a high germination rate
• The plant grows rapidly
• The plant must be dominant in the new area
• The plant has few predators

As well as impacting the ecosystem directly, invasive plants may post other threats, such as those to humans and even the economy. The problem is that, for the most part, humans are responsible for introducing these plants to their new environment. This can be through global trade, gardening, and even during the transportation of animals.

Impact of Invasive Plant Species

Invasive plant species can have far-reaching consequences on ecosystems and economies alike.

Ecological Impact

One of the primary ways that invasive plants have a negative effect is on the local ecosystem. Ecosystems in any area should have a good balance, and this is often the case where only native species are found. The food chain remains intact, and there’s good biodiversity, but adding new plant species can throw these things out of whack.

And I’m not merely referring to visible impacts; the influence of invasive plants extends far beneath the surface. Remarkably, research has revealed that these plants can trigger increased carbon release from the soil due to their intricate interactions with soil-dwelling organisms.
Moreover, when non-native plants become aggressive, they can quickly take over the available resources, meaning that native species don’t get everything they need. In extreme cases, this can mean the total extinction of native plants in a specific area.

Economic Impact

While you might not imagine, at first thought, that an invasive plant could impact the economy, you would be mistaken. Imagine you’re running a farm, growing crops, and an invasive plant species finds its way in. The result could be that the new species takes over all of the resources, leaving your crops with a lackluster yield.

Of course, this directly translates into a loss of income, less produce for the consumer, and an overall hit on the agricultural industry in general.

What’s more, in order to attempt to control the spread of these plants, a lot of money, time, and man hours are required. These are costs that would otherwise be spared so again, this directly impacts the overall economy.

Furthermore, invasive species can damage infrastructure and cause problems like flooding, which again can mean extreme costs in terms of management for agriculture, fisheries, logging, and other industries.

With all that said, there are some benefits to invasive plant species. Research has shown that some, not all, invasive species have established themselves so effectively into their new environment that they’ve actually formed beneficial relationships with existing flora and fauna. One of the ways that this occurs is in native areas that cannot be restored, and exotic plants are able to provide habitat for local wildlife.

North American Invasive Plant Species

While not all non-native species in North America pose problems, it’s crucial to be vigilant. Certain ones can wreak havoc and necessitate expert intervention. Below, we’ve compiled a list of invasive plant species prevalent in North America.

1. Japanese Knotweed (Fallopia japonica)

If you were told to think of an invasive plant, I’d bet Japanese knotweed would be your first answer. It’s notoriously difficult to control species that quickly take over and can eventually become the dominant species in an area, interfering with local biodiversity.

Moreover, Japanese knotweed can damage infrastructure and, in the moist areas it grows, can prevent native species from taking root. Controlling Japanese knotweed can be very difficult, and while it is partially responsive to chemical weed killers, these products don’t stop it from spreading entirely.

This plant can grow to 5.9 feet (1.8 meters) in height and has bamboo-like stems. It was first introduced to North America in the late 1800s as an ornamental plant.

2. Kudzu (Pueraria montana var. lobata)

Kudzu refers to a group of climbing and trailing vines that are native to East Asia. Sometimes, they’re called Japanese or Chinese arrowroot. The plant was first introduced as a show plant back in 1876, and since then, it’s taken hold in 32 states as well as some very small areas in Canada.

The problem with kudzu is that it spreads so quickly; around 60 feet (18 meters) per year. That might not be such a problem if it didn’t suffocate and shade native plants in the process. You can distinguish kudzu by its trailing vines with wide leaves and in summer, it produces pretty purple flowers.

Kudzu has proved difficult to get rid of using traditional plant control methods. So now, in many areas, especially the south, animal grazing and insects are being used in an attempt to quell its growth.

3. English Ivy (Hedera helix)

English ivy is another species of climbing plant with deep green leaves, woody stems, and can grow to 98 feet (30 meters) as an adult plant! Native to Europe and west Asia, English ivy was introduced to North America in around 1800 by colonial settlers.

These plants can be found all along the eastern portion of the USA as well as along the west coast from as far south as Arizona and as far north as Washington. Since this is a very aggressive species, English ivy can quickly take over an area and create a monoculture, preventing native species from being able to grow at all. What’s more, since it is able to grow in full shade, English ivy is often found in forests where it successfully competes for sunlight.

Controlling English ivy largely relies on hand pulling the plant from the soil or cutting it from trees. If you’re lucky enough to catch the first shoots, a herbicide treatment should be sufficient.

4. Purple Loosestrife (Lythrum salicaria)

Purple loosestrife is a perennial herb with dainty purple flowers that can grow up to 10 feet (3 meters) in height. Admittedly, these plants look pretty, but they’re also problematic. The plant has no natural predators and can thrive in many conditions where it alters the water content of the soil, making it hard for native species to survive.

While purple loosestrife can be found all over North America, it is most problematic in the northeastern parts of the United States and the south of Canada. There are several theories as to how the purple loosestrife made its way here from Eurasia, including accidentally through shipping or intentionally as an ornamental plant. In any case, the species has been here since around 1800.

Fortunately, purple loosestrife is relatively easy to get rid of. It grows in wetlands and moist soil and when it flowers, it’s very easy to recognize. You can hand pull younger plants or dig up mature plants using a shovel.

5. Norway Maple (Acer platanoides)

Norway maple is a large tree that can grow as high as 60 feet (18 meters) with a spread of 40 feet (12 meters). This species is native to eastern and central Europe and was first introduced to North America as a shade tree. What’s more, the tree could be used as an ornamental species in urban areas thanks to its ability to handle such conditions.

While it does effectively provide shade, the Norway maple also shades out native plants, depriving them of sunlight. This significantly affects biodiversity, but the trees’ roots are also known to destroy structures like sidewalks, which then require costly repairs.

It is possible to remove smaller Norway maples using a root shovel however, in most cases, you’ll likely need to hire the expertise of a tree surgeon to properly remove the tree.

6. Cheatgrass (Bromus tectorum)

Sometimes known as downy brome, cheatgrass is native to northern Africa, western Asia, and Europe. However, it is now a common sight on prairies, pastures, and fields in North America. While there is some dispute over when cheatgrass came to North America, the first specimen was discovered in 1861.

Cheatgrass is a winter grass species that is very fast-growing, and this is one of the main problems. However, the species only seems to be significantly problematic in certain areas, such as parts of Canada and the Intermountain West in the United States.

Downy brome grows to around 25 inches (64 cm) in height and the leaves have short, soft hairs. The plant typically flowers between May and June. While targeted grazing and mowing can help to control the grass, farmers are encouraged to find out where the cheatgrass is coming from to try to prevent further spread and reduce damage to crops.

7. Oriental Bittersweet (Celastrus orbiculatus)

While the red and gold fruits of this woody plant are a beautiful sight in the fall, looks can be deceiving as the oriental bittersweet is a very invasive plant species. For any native trees (even huge ones), oriental bittersweet is the stuff of nightmares.

The main problem with the plant is its rapid growth, but when you couple this with how it twines around trees and other plants, strangling them, it’s easy to see why it’s become a serious pest. If only we’d known that when humans introduced the plant in 1860 as an ornamental species.

It is possible to cut the vines and free any trees that they have wrapped around. However, the only real way to get rid of the problem is to attack oriental bittersweet at its roots.

8. Japanese Barberry (Berberis thunbergii)

Japanese barberry is a thorny shrub that produces long red berries containing seeds that have a very high germination rate. This wouldn’t necessarily be a problem on its own, but you have to also consider that these plants alter the pH level of the soil and affect the layer of leaf litter which makes it almost impossible for native plants to grow. Not to mention, these plants play host to ticks that can carry Lyme Disease, which can be very serious. 

The species was first brought to North America in 1875 and was shipped from Russia to the Arnold Arboretum in Boston. Since then, it has thrived and is now found across the northeastern parts of the US.

So, how can we go about controlling this seriously invasive plant? The best option is to pull the plant up from the roots, although this is easiest in wet soil when the plant is still young. Alternatively, you can mow the plant several times per season which will eventually kill it, but this requires patience and dedication.

9. Chinese Tallow (Triadica sebifera)

Chinese tallow us a type of deciduous tree that produces greenish-yellow flowers in spring and can grow up to 60 feet (18 meters) in height. While they are beautiful to look at, these trees are not native to North America and, as such, are known to cause problems.

Originally found in China and some parts of Japan, the Chinese tallow was brought to Georgia and South Carolina by Benjamin Franklin back in the 1770s. It’s now common all over the southeast and, in the 1900s, was even praised for its seed oil. However, since a single tree can produce up to 100,000 seeds, it’s easy to see how the species could outcompete our native plants.

Chemicals are the best way to get rid of these trees, but if you want to take a more organic approach, it’s possible to cut the tree down. However, keep in mind that it will likely reshoot from the stump, so this will need to be removed as well.

10. Japanese Stilt Grass (Microstegium vimineum)

Sometimes known as Nepalese browntop, this plant is a type of fast-spreading grass which is native to parts of east Asia and India. The first recorded sighting in North America was in Tennessee back in 1900 in shipping packages but since then, Japanese stilt grass has been reported in other states, including Pennsylvania, Kentucky, Alabama, and South Carolina.

The problem with Japanese stilt grass is that it can spread so quickly. The reason for this is that each plant can produce up to 1000 seeds each year, and they can be spread by various methods so the plant has a good chance of reproducing. Moreover, since the plant doesn’t have many natural predators, it stands an even better chance of surviving and outcompeting native species.

To identify this species, keep an eye out for tapered leaves measuring between 2 to 4 inches (5 to 10 cm) in length. The grass can reach heights of up to 3 feet (0.9 meters), with stems reminiscent of young bamboo.

If you have issues with Japanese stilt grass, it is possible to hand pull the plant. However, for larger patches, consistent pruning is usually the best method of control as this helps to prevent the plant from flowering and therefore spreading.

11. Giant Hogweed (Heracleum mantegazzianum)

Giant hogweed was actually brought to the United States as a display plant in Victorian-style gardens. The first recorded plant was in 1917 in New York, but over the following 50 years, it managed to spread to southern Canada, Maryland, Virginia, North Carolina, and Washington. Although it’s unknown how this happened.

Listed as a noxious weed in North America, giant hogweed is native to Europe and Russia and is typically a coastal plant. Shockingly, a single giant hogweed can produce as many as 100,000 seeds, so it’s easy to see how the plant can spread and crowd slow-growing native plants.

But it’s not just the spread that’s the problem, giant hogweed can cause burns, which leave permanent scars, so contact with it should be avoided at all costs.

You’re looking for a tall plant with serrated leaves that can grow up to 9.8 feet (3 meters) in length. The underside of the leaves is hairy, and the stems are thick with a purple mottled coloration. In order to control the species, many states, including Michigan have banned it from being planted.

12. Japanese Honeysuckle (Lonicera japonica)

Japanese honeysuckle is an extremely large plant that can grow over smaller plants and even cover native trees. It’s possible for it to reach more than 23 feet (7 meters) in length. The plant features paired tubular flowers that come in white, cream, or pink, so there’s no denying it looks pretty. But you’ll want to avoid planting this woody vine lest it outcompetes native plants for sunlight.

Controlling Japanese honeysuckle isn’t always easy, as this semi-evergreen plant can be quite determined. However, herbicidal treatments have been known to be effective. Some people prefer to pull, cut or burn the plant, but this doesn’t have guaranteed success.

This species is, as its name suggests, native to Japan, but has become a serious problem in the US, particularly in states such as Michigan. It was first brought here in the 1750s as an ornamental plant, but people then didn’t know that this species of honeysuckle emits far more carbon that native honeysuckles.

13. Yellow Starthistle (Centaurea solstitialis)

Yellow starthistle is native to the Mediterranean but was introduced to North America after introduction to South America. It’s believed that it first came here during the California gold rush and appeared as a contaminant in fodder seeds shipped in from Chile.

This plant has had huge success in the California region since the climate is very similar to its native climate. What’s more, humans have contributed to the spread through mowing and other activities. What’s more, these plants have extremely long root systems which make them resistant to many biological methods of control.

Growing up to 6 feet (1.8 meters) in height and with a wide spread, this plant will often outcompete native species. However, one of the main threats is to livestock, particularly horses whose health can suffer as a result of eating the plant. This comes in the form of chewing disease which, if left untreated can result in the death of the animal.

14. Butterfly Bush (Buddleja davidii)

I won’t lie, for the longest time, I believed that the butterfly bush was a native species. After all, we’re always told they’re excellent choices for pollinator gardens. But it’s true, the butterfly bush is invasive and is actually native to China. It’s thought that the plant was first brought to the US in 1900, and it’s now found in more than 20 states!

While the bush does provide nectar for butterflies, it provides no benefits for their caterpillars. What’s more worrying is that these bushes are extremely fast growers and can quickly outcompete native plants. Because of this, controlling these plants in your garden can be time-consuming and sometimes not even possible at all.

You’ll know if you have a butterfly bush as you’ll see the showy purple flower spikes at the ends of the stems; these bushes can grow to a height of 15 feet (4.6 meters) but often don’t stay in your garden, spreading far and wide.

15. Black Locust (Robinia pseudoacacia)

The black locust is a species of legume tree that can grow to around 98 feet (30 meters). It has a thorny trunk and showy white flowers that fall in pretty strings contrasting with the dark leaves. But while it might look beautiful, the black locust is, in fact, an invasive species.

The tree is found throughout the southeastern portion of the United States, where it was first noted in the late 1880s. It was originally brought here as a food source for bees and for the hardwood lumber industry. However, it’s a very fast-growing species, and that could be problematic.

Black locusts require full sun to thrive, so they’re usually found in young woodlands. But it does have the ability to fix nitrogen into the soil, which prevents native plants from growing, and the tree will quickly take over. What’s more, it’s very easily able to survive in dry or poor soils.

These trees are very difficult to get rid of and cutting or plowing them down only seems to exacerbate growth. However, during trials, cattle grazing seemed to quell the growth and eventually eradicate the tree after around four seasons.

16. Common Buckthorn (Rhamnus cathartica)

The common buckthorn is a species of shrub that is one of the biggest problematic invasive plants in North America. There are particular spreads of common buckthorn in Ontario, Canada where the plant is one of the first to grow new leaves in spring and the last to drop leaves in the fall.

The issue with this is that the plant has a major advantage over native species and can easily outcompete them. What’s more, since common buckthorn piles a lot of nitrogen into the soil, this can be damaging to local plants.

Originally brought to North America in the 1800s from Europe as an ornamental shrub, the plant is now a common sight both in the wild and in gardens. However, they aren’t usually sold in nurseries because of their invasive characteristics.

Common buckthorn produces dark berries and has dark gray bark; it can grow up to 25 feet (7.6 meters)! Removal of this plant requires the removal of the root ball.

17. Salt Cedar (Tamarix spp.)

Sometimes referred to as tamarisk, the salt cedar is a spreading shrub or tree with light pink to purple flowers. Native to Eurasia, the salt cedar is not fussy about where it grows, which means it has been known to take over native species like willows and cottonwoods.

Another issue with the salt cedar, which was introduced in 1823 as an ornamental plant is that it puts a lot of salt into the soil. This can make it difficult for native plants to thrive. Not to mention that these trees consume up to 200 gallons (757 liters) of water a day, leaving nothing for local species.

Salt cedars are an incredibly fast-growing species that are notoriously difficult to control. They can also grow back from shoots and roots, so cutting them isn’t usually effective. However, cutting the tree to the stump and covering the area with herbicides can be a good way to get rid of them.

18. Water Hyacinth (Eichhornia crassipes)

The water hyacinth is native to South America, and while it has been naturalized across the world, it is often considered to be invasive. In the US, the plant has been reported in more than 25 states. These plants are free floating and have wide, leathery, dark green leaves and small, light purple flowers.

There are several problems with water hyacinth, most notably how rapidly it grows. The species can quickly cover a body of water, blocking sunlight from entering and outcompeting native plants both above and below the surface.

Additionally, these plants have the ability to drain bodies of water, such as reservoirs, owing to its high evapotranspiration rate. What’s more, water hyacinth has a tendency to absorb harmful substances so, when the plant dies, it sinks to the bottom of the water, causing contamination. The plant also has the ability to interfere with hydroelectric power and boat traffic, making it problematic to humans.

19. Garlic Mustard (Alliaria petiolata)

A species of herbaceous broadleaf weed, garlic mustard was first introduced to North America (Long Island, NY)  from Europe in 1868. One of the main problems with this plant is that it can survive in a very general habitat, including roadsides, woodlands, and agricultural land, and it spreads very quickly.

Garlic mustard features toothed triangular leaves and can grow up to around 3 feet (0.9 meters). The species is known to take over ground cover in forests which means that native plants struggle to survive. Such affected species include sugar and red maple, as well as trilliums. What’s more, it can destroy important fungi within the ecosystem that native woody plants rely on.

While hand-pulling these invasive weeds can be effective while the plant is young, it’s important that this continues for several years because, being a biennial, the seeds remain viable for a long time.

20. Tree-of-Heaven (Ailanthus altissima)

With a name like the tree of heaven, you could be fooled into thinking that this was a beneficial species, but it’s anything but. First introduced to North America in Philadelphia in the 1700s, the tree was thought to be a good host for the silk moth.

The tree has fern-like leaves and can grow to 100 feet (30 meters) in height. It’s a fast growing species that can take over native plants. However, the most concerning thing about it is that it produces a toxic chemical in its bark, which can seriously threaten nearby flora.

Native to China and Taiwan, the tree of heaven produces clusters of seeds which can spread as far as 2 miles (3.2 km)! So successful is the spread of the tree that it’s now found in 42 of the 50 states. The spread is further exacerbated by the fact that these trees are able to thrive in adverse conditions (although they do need full sun.)

To get rid of these trees can be difficult, but targeting the roots with herbicide treatments seems to be the most effective method.

21. Chinese Privet (Ligustrum sinense)

Chinese privet, just like all other species of privet, is able to adapt to various light conditions. This means that, no matter where it is growing, it’ll quickly outcompete native plant species. What’s more, it’s able to produce both sexually and asexually, and since it doesn’t have many shrub competitors, it’s free to take over.

Originating from China, Laos, and Vietnam, the Chinese Privet was first brought to the United States in the mid-1800s as an ornamental plant. It’s prominent in the southeast and is listed as a noxious weed in some states, including Florida.

The species has small opposite elliptical leaves that grow to around 2 inches (5 cm) and is an evergreen species. If you want to get rid of this plant, it’s important to fully remove it before it flowers as this will prevent the spread. However, for large scale removal, the use of herbicides is generally preferred and is considered safer than mechanical methods.

22. Canada Thistle (Cirsium arvense)

Listed as a noxious weed in 43 states, the Canada Thistle has been in the United States since the 1600s. Despite its name, the plant is not actually native to Canada but instead comes from parts of Europe and North Africa.

Since the rhizomatous root system of the Canada thistle has the ability to take over, one of the main causes of concern is that the plant interferes with crop production. But agriculture aside, it’s also  a threat in the wild since its wide spread can shade and overtake that of native plant species.

With erect stems up to 4 feet (1.2 meters) in height topped with purple to white flowers, the Canada thistle is easy to recognize. However, it’s known to be hard to remove and repeated use of herbicide treatments is often required.

Impact of Climate Change on Invasive Plant Species

Climate change has a lot to answer for, but one of the things that I don’t see getting enough attention is the impact of climate change on invasive plant species. So, how and why is this a problem?

Before the effects of climate change took hold, many exotic plants would have struggled to thrive outside of their native environment. But now, the planet’s conditions are changing and this makes it easier for plants to survive in areas other than where they are native.

Consider that the seasons are becoming longer, the temperatures are rising, and there’s often more humidity in the air and it’s easy to see why exotic plants may be able to thrive in North American soil. What’s more worrying is that many of these plant species play host to parasites. For example, the hemlock wooly adelgid.

Additionally, climate change also impacts natural habitats, making them less resistant to invasive plants. As a result of this, a significant amount of money is being lost annually in damage control (an estimated $1.4 trillion), and there are serious threats to our food security because of this.

In areas like the Sierra Meadows, there is a delicate balance in the ecosystem, with native plants providing resources for local wildlife. But now that climate change has altered the conditions and made it easier for invasive species to thrive, this could all be thrown out of whack.

Since many non-native species like things hotter, they’re able to take over in a warming world. In studies, it was demonstrated that non-native plants were able to more quickly adapt to changing conditions compared to native species. The concern here is that this gives them the upper hand in terms of having access to resources.