Invasive Plants

Water hyacinth invasion
Water hyacinth invasion. Luís Cerqueira / Getty Images

Invasive species are non-native, alien species that once introduced to a new area colonize it heavily. Invasive plants are difficult to control, disrupt local ecosystems, and are costly in terms of damage to commercially important forests, pastures, hay fields, and many agricultural activities.

What Places Are Most at Risk?

The distribution of invasive plants is not random. Areas where the soil is disturbed give an opportunity for invasives to grow.

These plants are usually easily spotted on construction sites, eroded banks, overgrown lots, and abandoned agricultural fields. Most problematically, invasive plants flourish alongside roads, using them as corridors to reach new areas to conquer.

At a broader scale, places which naturally have a high level of plant diversity are more susceptible to non-native plant invasions. For example, California’s flora is exceptionally diverse, but the soil and climate characteristics that make it so are probably what facilities the establishment of so many newly arrived species.  

Islands are often severely affected by invasives. Their native ecosystems have been isolated for a long time in a state with minimal competition, and aggressive non-native species can easily displace the local plants. Hawaii has hundreds of non-native plant species, about 100 of which are heavy colonizers threatening not only native plants, but entire ecosystems.

What Makes a Good Invasive Plant?

Invasive plants come in all types and forms, but often have some characteristics in common:

  • It will come as no surprise that invasives can grow rapidly, taking a hold quickly and forming dense colonies against which native plants have a difficult time competing.
  • Invasives can mature fast, giving them the ability to set seeds earlier than the competition.
  • They also tend to produce a lot of seeds or fruit, sometimes thousands from a single plant.
  • These seeds or fruit are easily dispersed. The seeds may be light and carried by the wind, as is the case with dandelions. Or they may produce an abundance of small fruit which are eaten by birds. After the fruit is digested, the still viable seeds are defecated a good distance away, contributing to the plant’s spread. The bright red fruit of the bush honeysuckle are ripe in late summer, just in time for thrushes, sparrows, and many other birds to move the seeds along their migratory paths.
  • Successful invasive plants can tolerate a wide range of soil types and climate conditions. They tend to be generalists, instead of ecological specialists.

Notorious Invasive Plant Examples

  • Tamarisk or saltcedar (Tamarix spp.). A group of tall shrubs from Eurasia with scaled leaves, tamarisks take over the banks of streams and springs in the otherwise arid American Southwest. They created dense thickets that edge out other plants, and absorb most of the available water through a long tap root. As if it wasn’t enough, when their leaves fall off they create a very salty carpet of leaves on the ground, further preventing native plants from growing.
    • Water hyacinth (Eichhornia crassipes). This South American aquatic weed was originally introduced for its beautiful, showy flower and free-floating habit on ponds. Now established in the southeastern United States and parts of California, water hyacinth spreads very fast, clogging waterways, discouraging boating, swimming, and fishing. It creates a full mat covering the entire surface of the water, preventing sunlight and oxygen from entering. The presence of water hyacinth on a wetland or pond drastically reduces its biodiversity.
    • Cheatgrass (Bromus tectorum). This Eurasian grass is widespread in the West, dominating millions of acres where cattle have disturbed the fragile crust covering the soil. Cheatgrass creates a tight blanket of grass that grows quickly following seasonal rain events, excluding native flowering plants and bunchgrasses. It then dries once summer comes, turning into a tan-colored sea of highly flammable material. Wildfires burn hot as they speed through vast expanses of cheatgrass, killing native shrubs and trees like sagebrush and junipers.  
    • Common and Glossy Buckthorn (Rahmnus sp.). A plague of the northeast and Great Lakes states, buckthorn is a short tree thriving in the woods surrounding suburban areas and agricultural fields. It greens up early in the spring, shading out native plants and many of the delicate forest spring flowers. Along with abundant deer, buckthorn contributes to impoverishing forest flora. As with honeysuckle, common buckthorn produces bright fruit that are favored by birds, facilitating the plant’s dispersal.

    Why Is the Problem Worst Now?

    Many blame global climate change for most environmental problems, but in this case the driving cause is different. Most invasive plants arrive into new territory through economic trade. Sometimes the introduction is done on purpose, such as when plant species are brought in for their ornamental or landscaping qualities. Kudzu, a plant smothering landscapes all over the U.S. southeast, was initially cultivated to prevent erosion. The scratchy multiflora rose (one invasive I particularly dislike!) was propagated as a rootstock for ornamental roses. In other instances, the introduction was accidental. The western icon tumbleweed (or Russian thistle) is actually an invasive plant, having made its way to North America as a few seeds mixed in a bag of flaxseed carried over by immigrating European farmers in the late 1800s. Stowaway seeds can make it to foreign shores in gravel, soil, even on the soles of travelers.

    Global trade has skyrocketed in the last 50 years, with an increased use of shipping containers, one of the main vectors for invasive plants and insects. These boxes seal cargo far inland, travel across the globe on ships, and are delivered by road or rail far into the next continent to climate conditions which may be suitable to the stowaway seeds.

    What Can We Do?

    Invasive species control is an evolving science facing an immense task. Various mechanical, chemical, and biological methods of controlling invasive plants exist, but the most effective approach is to prevent them from entering in the first place.

    Regulations are slowly being put in place to limit the importation and sale of plants which show characteristics of being invasive. The federal Lacey Act is increasingly used to prevent importation of noxious weeds.

    There are a few personal steps you can take to minimize the spread of invasive plants:

    • Brush off equipment and pets to remove seeds before you enter a new area. This includes boots, clothes, off-road vehicles, camping gear, and gardening tools.
    • Learn about the invasive plants which may show up in your area. Once you can recognize an invasive plant, you are able to identify a new colony and can start removing it immediately or advise the property owner to do so.
    • When selecting ornamental and landscaping plants, you should choose native species. As an added benefit, they usually require less water and are hardier.