Invasive plant species are non-native plants that can cause harm to the environment, economy, or human health. These aggressive plants spread uncontrollably and displace native species. With their rapid growth and lack of natural predators, invasive plants can overwhelm natural habitats, reducing biodiversity. We discuss 16 of the most problematic invasive plant species that homeowners and gardeners should avoid planting or promoting.

What Makes a Plant Invasive?

For a non-native plant species to become invasive, it must have certain traits that allow it to spread aggressively outside of its natural range. Factors that contribute to invasiveness include:

Rapid growth: Invasive plants reproduce quickly and grow at faster rates than native species. Their vigorous growth allows them to crowd out native plants.
Prolific seed production: Invasive plants make lots of seeds that can spread far from the parent plant and persist in the soil for years. Even small amounts of seeds can lead to large infestations.
Early and extended flowering periods: Some invasive plants bloom earlier and longer than native plants, extending their reproductive period.
Adaptability: Invasive plants can thrive in challenging conditions beyond their native range, from poor soils to extreme temperatures. They flourish outside factors that naturally control them.
Lack of natural predators: Freed from diseases, insects, and herbivores that naturally keep their populations in check, invasive plant populations can grow uncontrolled.
Vegetative reproduction: Some invasive plants spread via roots, shoots, or plant fragments in addition to seeds. Pieces of the plant can break off and take root, forming new plants.

Impacts of Invasive Plants

Invasive plants don’t just crowd out native species in natural areas. They can cause widespread environmental and economic consequences, including:

Displacing native plants and reducing biodiversity.
Altering ecosystem functions like nutrient cycling, hydrology, and wildfire regimes.
Interfering with agriculture through competition and toxicity to crops.
Clogging waterways and increasing flood severity.
Spreading quickly and increasing the cost of control efforts.
Damaging infrastructure like buildings, roads, and power lines.
Harming human health through toxicity, allergies, and injuries from thorns.

Preventing invasive plants from establishing and managing existing infestations protects ecosystems, agriculture, and economies. Gaining awareness of the most problematic invasive plant species allows gardeners and land managers to avoid accidental introduction or spread.

16 Highly Invasive Plant Species to Avoid

Here are 16 of the most aggressive and harmful invasive plants found in the United States that all gardeners should refrain from planting or promoting:

1. Purple Loosestrife (Lythrum salicaria)

Native to Europe and Asia, purple loosestrife has extensively invaded wetlands across North America. It forms dense stands that crowd out native grasses, sedges, and other flowering plants important for wetland wildlife. A single mature plant can produce up to 2.7 million seeds per year. This prolific seeder spreads quickly and inhibits regeneration of native wetland species.

2. Japanese Knotweed (Fallopia japonica)

Brought from Asia as an ornamental plant, Japanese knotweed now causes extensive damage across the U.S. Capable of growing through cracks in concrete, this invasive weed creates costly damage to buildings, roads, and more. It forms dense thickets that displace native vegetation and alter riparian ecosystems. Pieces of its rhizomes can break off, wash downstream, and take root, spreading knotweed rapidly.

3. Parrotfeather (Myriophyllum aquaticum)

Native to South America, parrotfeather is an aquatic invasive plant that forms dense mats in lakes, rivers, and other waterways. It displaces native aquatic vegetation, reduces habitat for wildlife, and impedes water flow. Fragments of the plant detach and spread easily downstream, allowing parrotfeather to spread between waterbodies. It’s illegal to sell or transplant this aggressive invader in many states.

4. Eurasian Watermilfoil (Myriophyllum spicatum)

Like parrotfeather, Eurasian watermilfoil forms dense stands that crowd out native aquatic plants, reduce biodiversity, and alter aquatic habitats. Introduced for aquariums, it escaped cultivation and spread through lakes, ponds, and waterways. In addition to displacing native plants, it can interfere with fishing, swimming, and boating. Hybridization with the native northern watermilfoil also threatens native populations.

5. Canada Thistle (Cirsium arvense)

Despite its name, Canada thistle is native to southeastern Europe and eastern Mediterranean regions, not Canada. It invades a wide range of habitats across North America, including prairies, forests, wetlands, and disturbed areas. Its complex root system allows Canada thistle to form dense colonies that push out native plants. The weed causes significant losses for the agriculture industry and is toxic to horses.

6. Giant Hogweed (Heracleum mantegazzianum)

This massive invasive plant from southwest Asia can reach over 15 feet tall with leaves up to 5 feet wide. Giant hogweed forms dense stands and shades out native vegetation, altering habitats. More seriously, its clear sap contains toxic chemicals that cause severe burns and blistering when exposed to sunlight. Contact with the sap and subsequent sunlight can lead to permanent scarring and blindness.

7. Garlic Mustard (Alliaria petiolata)

Brought to North America by early European colonists, garlic mustard has spread rampantly across woodland understories. A prolific seed producer, it forms dense stands that shade out native herbaceous plants and tree seedlings. Garlic mustard allelopathy also prevents other plant species from growing nearby. This invasion significantly alters forest ecology and regeneration.

8. Japanese Stiltgrass (Microstegium vimineum)

Extensively invading eastern forests, Japanese stiltgrass forms dense carpets that suppress native plants. It alters habitat for insects and wildlife, and changes soil composition through allelopathy. The prolific seeds spread easily on shoes, tires, and hiking gear. Stiltgrass also harbors plant nematodes that damage native plants. The cost of controlling its spread is estimated at over $100 million annually.

9. Chinese Privet (Ligustrum sinense)

Now extensively naturalized across southeastern woodlands, Chinese privet forms dense thickets that shade and outcompete native shrubs and trees. Alteration of natural habitats and plant diversity facilitates further invasion by other non-native plants. Fruits spread by birds expand its invasion. Chinese privet is also associated with mild symptoms of poisoning in humans.

10. Cogon Grass (Imperata cylindrica)

Native to Asia, cogon grass thrives in a wide range of habitats, from pine forests to prairies. It forms dense clonal colonies that displace native plants and create fuel for intense wildfires. The stiff leaves and sharp margins can cut humans and animals. Cogon grass contains allelopathic compounds that prevent other plant growth in invaded areas. It causes significant economic losses in forestry, agriculture, and ranching.

11. Hydrilla (Hydrilla verticillata)

Hydrilla is a submerged aquatic plant that forms dense mats in lakes, rivers, and ponds across North America. First introduced for aquariums, it escaped into the wild and spread prolifically. Hydrilla displace native aquatic plants, altering the habitat for fish and other wildlife. It impedes water flow, facilitates flooding, and obstructs recreational water activities. Hydrilla spreads between waterways via fragments and turions that float downstream.

12. Brazilian Peppertree (Schinus terebinthifolia)

This aggressive shrub from South America forms impenetrable thickets that take over open areas and displace native vegetation in the southeastern U.S. Its shallow root systems contribute to erosion. Brazilian peppertree inhibits regeneration of native species and alters natural fire regimes. Birds disperse its prolific fruits and expand its invasion. It also causes allergic reactions and skin irritation in some people.

13. Tamarisk (Tamarix spp.)

Several tamarisk or saltcedar species from Eurasia aggressively invade riparian habitats in the southwestern U.S. Tamarisk displaces native trees like cottonwoods and willows, altering ecosystems and reducing biodiversity. Dense stands obstruct water flow in rivers and increase soil salinity. Tamarisk thickets also increase wildfire frequency, putting ecosystems adapted to less frequent fires at risk.

14. Russian Olive (Elaeagnus angustifolia)

Brought to North America as an ornamental plant, Russian olive now aggressively invades riparian areas in the western states. It forms dense thickets that displace native trees and shrubs, alter habitat, and increase soil salinity. The long-lasting seeds spread by birds expand its invasion. Russian olive trees also consume large amounts of water, worsening the effects of drought on riverside habitats.

15. Yellow Floating Heart (Nymphoides peltata)

Yellow floating heart is an aquatic plant from Asia that invades ponds, lakes, and slow-moving waterways. Forming dense floating mats, it crowds out native aquatic plants that provide food and habitat for wildlife. The fast-growing invader impedes water flow, recreation, and shoreline access. Fragments detach and spread easily to new waters. Several states now prohibit its sale and transport due to its rapid spread.

16. mile-a-minute weed (Persicaria perfoliata)

Introduced from Asia as a ornamental vine, mile-a-minute weed now engulfs other vegetation across the eastern U.S. Growing up to six inches per day, its vines spread rapidly and form dense blankets that shade and kill native plants. Triangle-shaped seeds spread prolifically, allowing new infestations to form easily. Mile-a-minute weed also harbors the fungal pathogen that causes deadly anthracnose disease in native plants.

Preventing the Introduction and Spread of Invasives

While these plants may seem attractive or harmless at first glance, their introduction often leads to unforeseen and severe environmental and economic consequences. Here are some tips all gardeners should follow to avoid accidental spread:

Choose native species when landscaping instead of exotic imports that can potentially escape.
Check your state’s invasive plant list and regulations before purchasing new plants. Avoid buying known invasives.
Carefully contain exotic ornamental plants that have invasive tendencies in enclosed areas. Avoid planting near natural areas.
Ask for native plants at your local nursery to increase demand and availability of safer options.
Learn to identify key invasive plants in your region. Monitor your property frequently and remove invasives early before infestations spread.
Support early detection and rapid response efforts to spot and control new invasive plants before they become widely established.
Clean equipment, boots, and tires after visiting infested areas to remove seeds and plant parts that can spread invasives to new locations.
Dispose of invasive plants properly. Do not compost invasive plant material as seeds can still spread and emerge. Bag plant parts and dispose of in landfill-bound waste.
Report new invasive plant infestations on private or public lands to state agencies and local weed management organizations. Early notification improves rapid response efforts.
Volunteer for local invasive plant removal efforts. Help restore native plant communities in parks and protected areas.
Spread awareness in your community about impacts of invasive plants and how to stop their spread. A little knowledge goes a long way.

Conclusion

Invasive plants can seem deceptively harmless at first, but small infestations often turn into major problems down the road. Taking some simple prevention measures and avoiding the use of known invasive plants allows gardeners and landscapers to enjoy the beauty of ornamental plants responsibly, without contributing to ecological damage and financial burdens caused by invasives. Check state regulations, know which exotic plants to avoid, and learn to identify the major invaders in your region. With vigilance and care in plant choices, we can keep invasive species from overwhelming native habitats and protected natural areas.

Frequently Asked Questions

What makes an invasive plant so harmful?

Invasive plants are able to spread rapidly and displace native species outside of their natural ranges. Traits like prolific seed production, rapid growth, lack of predators, and adaptability to diverse conditions allow them to overwhelm habitats and crowd out native plant communities that support local ecosystems.

What effects do invasive plants have?

Invasive plants reduce biodiversity, alter habitats, hinder agriculture, damage infrastructure, and cost billions in control efforts each year. Specific effects include displacing native plants, changing soil nutrients and chemistry, altering hydrology, shifting wildfire regimes, hybridizing with native species, and harming human health through toxicity or injury.

How do invasive plant seeds spread to new areas?

Invasive plant seeds spread near and far through wind, water, animals, humans, vehicles, and equipment. Long distance spread occurs via contaminated agricultural products, machinery, commercial goods, clothing, and vehicles traveling between regions. Local spread happens through bird droppings, adhesion to tires and shoes, and flow downstream in waterways.

What can gardeners do to avoid using invasive plants?

Choose native plants, research unknown ornamentals for invasive traits, plant non-natives carefully away from natural areas, ask nurseries for safer alternatives, learn to ID regional invasives, monitor for and remove new infestations promptly, clean equipment after use in infested areas, and dispose of plants and seeds properly.

How can you tell if a plant is invasive or not?

Research your state and region’s invasive plant lists, look for warning labels on plant tags at nurseries, and learn about the specific traits that aid invasiveness like prolific seeds, rapid spread, lack of predators, and adaptability to diverse habitats. If an unknown plant spreads aggressively in your yard, it likely has invasive tendencies. Seek identification and control advice early on.

Why are aquatic invasive plants like hydrilla and water hyacinth so problematic?

Aquatic invasive plants form dense surface mats or underwater stands that displace diverse native aquatic plant communities important for habitat and waterfowl food. Reduced plant biodiversity negatively impacts water quality. Dense stands of invasive aquatic plants also impede water flow, worsen flooding, obstruct recreation, hinder fishing, and spread easily between waterways via floating fragments. Preventing their introduction and controlling existing infestations is crucial for protecting waterbodies.

16 Types of Invasive Plants You Should Avoid

Introduction

Invasive plant species pose a major threat to ecosystems across the United States. When non-native plants are introduced to new areas, they can proliferate uncontrollably. In the absence of natural controls like herbivores, diseases, and parasites from their native ranges, invasive plants crowd out native plant communities. They alter habitat, reduce biodiversity, harm ecosystems, and cause costly damage.

This article profiles 16 highly invasive plant species that all gardeners and land managers should avoid using or spreading. Learn to identify these harmful plants, understand why they are so problematic, and get tips for preventing their introduction and spread.

Notorious Invasive Plant List

Kudzu (Pueraria montana)

The fast-growing “vine that ate the South” smothers native vegetation from forest understories to abandoned fields with its extensive runners and leafy biomass. Kudzu grows up to a foot per day. Severe shading kills mature trees and shrubs, devastating ecosystems.

Cheatgrass (Bromus tectorum)

Occupying over 100 million acres across western states from forest to rangeland, cheatgrass alters natural fire regimes and habitat for native wildlife. Its fine fuels increase fire frequency and severity. Cheatgrass also reduces forage value of rangelands.

Common Reed (Phragmites australis)

Edging out native grasses, sedges, and other wetland plants, the tall and dense monocultures of common reed decrease biodiversity in marshes and shorelines across North America. It alters hydrology and biogeochemical cycles in invaded wetlands.

Japanese Honeysuckle (Lonicera japonica)

Japanese honeysuckle blankets shrubs and small trees with its vines, shading out native vegetation across eastern and southern woodlands. Toxic to wildlife and humans, its nectar also provides little nutritional value to pollinators.

Tree of Heaven (Ailanthus altissima)

All parts of this rapidly growing Asian tree are toxic to native species. It produces chemicals that prevent the establishment of other plant species nearby. Prolific wind-dispersed seeds spread tree of heaven quickly.

Spotted Knapweed (Centaurea stoebe)

Toxic to native plants and animals, spotted knapweed reduces biodiversity as it invades prairies, fields, and grasslands across North America. It degrades wildlife habitat and forage value of rangelands. A single plant can produce over 1,000 seeds annually.

Leafy Spurge (Euphorbia esula)

A prolific seed producer, each leafy spurge plant can generate up to 130,000 seeds annually. It forms dense colonies that displace native grasses and forbs, completely altering prairie habitats used by wildlife and cattle. The toxic sap deters grazing.

Purple Loosestrife (Lythrum salicaria)

Escaping ornamental gardens, each purple loosestrife plant grows into a tall spike of thousands of magenta flowers that produce prolific seeds. It crowds out native wetland vegetation vital to wildlife habitat and waterfowl food sources.

Saltcedar (Tamarix spp.)

Multiple Tamarix species aggressively displace native riparian trees like cottonwoods and willows across the southwestern U.S. Altering habitat, increasing soil salinity, and using large amounts of groundwater, saltcedar creates unfavorable conditions for native plants.

Yellow Iris (Iris pseudacorus)

Both submerged and emergent forms of yellow iris take over shallow wetlands, crowding out native plants. Toxic to livestock when ingested, it provides little food value for wildlife. Each flower stalk can produce thousands of long-lived seeds.

Eurasian Watermilfoil (Myriophyllum spicatum)

Now found across most of North America, this feathery aquatic plant