What does it mean for a plant to be invasive?

Scotch broom (Cytisus scoparius) displaces native plants and makes reforestation difficult. It burns readily and carries fire to the tree canopy, increasing the frequency and intensity of fires. Photo by Joe DiTomaso

Most of the plants used in gardens and landscaping do not invade or harm wildland areas. But a few vigorous species can — and do — escape into open landscapes and can cause a variety of ecological problems.

There are three things that make a plant invasive:

  1. Not native to the area of interest
  2. Can survive, reproduce, and spread without help from humans
  3. Causes or is likely to cause environmental harm, economic harm, or harm to human health

These are the hallmarks of an invasive plant. Stopping the spread of these plants lets California’s natural ecosystem thrive, helps wildlife survive, and allows people to use and enjoy the beautiful and diverse landscapes of our state.


1. Not native to the area of interest

Robinia pseudoacacia. Photo by Flickr user Kristine Paulus

What’s a native plant anyway? The simple answer is that a plant is native to a particular area if it “either currently or in the historical past, are said to occur there” (U.S. DOT). The area of interest may be a nation, state, region, ecosystem or habitat. A plant can be native in one part of the U.S. and invasive in another. For example, black locust (Robinia pseudoacacia) is native to the central Appalachians and Ozarks but invasive in California.

Native plants in California grew here prior to European contact. We know which plants are native here because of specimens, seeds and drawings of “new world” plants that were brought back to Europe by early explorers. Paleobotany – studying plants in the fossil record – also helps understand which plants are native to an area.

Native plants evolved over a very long time and co-evolved with animals, fungi, and microbes. Native animals depend on these native plants for food, habitat, and places to raise their young. Native plants also provide secondary compounds that nectivores, herbivores and others need to combat pathogens.

Many non-native plants can be grown in California without causing any problems. These plants often need human help (watering and fertilizer) to grow and tend to stay within landscaped areas. They don’t self-seed or have seeds that can be transported to wildlands by animals, water, or wind. Some plants can also grow a whole new plant from just a piece of themselves. These plants can start a new invasion when discarded somewhere or washed down a waterway.

2. Can survive, reproduce, and spread without help from humans

Arundo donax. Photo by Jason Giessow

“Invasive” and “naturalized” are often used interchangeably, but don’t mean the same thing. A plant that has naturalized is a non-native plant that can survive and reproduce without human help. However, it does not spread away from where it has been introduced, and naturalized plants are generally not a significant problem either in a garden or natural setting. A naturalized species can become an invasive species if and when they spread to new areas where the conditions allow them to outcompete native plants and disrupt natural ecosystems. Many species that we consider garden or agricultural weeds depend on human disturbance of soil and our addition of water and fertilizers to survive and reproduce. Plants that tend to become invasive are plants that thrive in disturbed landscapes, can grow rapidly, produce a lot of seeds, and can reproduce vegetatively as well.

3. Causes or is likely to cause environmental harm, economic harm, or harm to human health

Many non-invasive plants in California, even ones that are naturalized, are fairly harmless. Some invasive plants cause more harm than others, depending on how widespread and aggressive they are.

How do invasive plants cause harm?

Invasive plants are different from other non-native plants because they have negative effects on the ecology and economy of California in various ways. The economic cost is as significant as the ecological cost: in California. At least $82 million goes to fighting invasive plants every year.

Ecological impacts

California is incredibly geographically diverse, which led to the evolution of the diverse flora we have, 24% of which are found no where else on earth. Invasive plants harm native plants by outcompeting native plants and becoming the dominant vegetation. Some invasive plants like English ivy or Algerian ivy can spread along the ground (and up trees), covering the ground and blocking the light that native plants need to germinate. Other invasive plants cover the ground with a thick layer of leaf litter that prevents the germination of native plants.

Although there is no documented case of an invasive plant causing the extinction of a native plant, by the time we have realized that there are only a few plants or seeds of that native plant left, it may be too late to save the species. This “extinction trajectory” was outlined by scientists at the University of Canberra and Stellenbosch University, South Africa (Scientific American).

Least Bell’s Vireo. Photo by Steve Maslowski/USFWS.

Many insects can only feed or reproduce on native plants, so when those plants are replaced by non-native, their numbers can dramatically decrease, causing a decrease in the animals that feed on them. An example would be the affect of Arundo donax on riparian birds. Where Arundo has become the dominant riparian vegetation, birds like the Southwestern Willow Flycatcher (Epidomax traillii extimus) and Least Bell’s Vireo (Vireo belii pusillus) don’t have enough food to survive. Removal of Arundo in the Santa Ana River Basin in southern California has dramatically increased the number of nesting pairs of least Bell’s vireo.

Changing Soil Composition

Species of broom such as French broom (Genista monspessulana) and Scotch broom (Cytisus scoparius) are members of the pea family (Fabaceae). Like many legumes, they are able to fix nitrogen in the soil, which makes them able to grow in low-fertility soils and displace native plants to form a dense monospecific stand (area with mostly one species). Tamarix species increase salinity in the soil by exuding salt through glands on the leaves. The salt in the leaf litter inhibits the growth of native riparian species.

Hydrological impacts

Riparian habitats are plant communities of woody vegetation that are found along rivers, creeks, streams, and lakes. They are one of our most important, and most neglected habitats in California, and are a place invasive plants can invade because they have water, a precious resource that is scarce in most of the State. As much as 95% of California’s historic riparian habitats have been removed, degraded, and disturbed (WCB). Riparian habitats are important for water quality and quantity, and the survival of wildlife from steelhead trout, to birds, amphibians, and mammals that use this habitat corridor to migrate, reproduce, and use as refuge. These important, rare, and degraded habitats are further degraded by invasive plants like Arundo donax, Tamarix, water hyacinth, and others.

Arundo donax (giant reed) is one of the worst invasive plants for riparian habitats. It outcompetes native riparian plants, changing the structure of the riparian vegetation and increasing flood risk. New Arundo can grow from pieces of the plant that wash downstream.

When Tamarix species grow along a waterway, the trees trap sediments, narrowing the stream channel and resulting in more frequent flooding.

Eichhornia crassipes (Water hyacinth). Photo by Forest and Kim Starr.

Eichhornia crassipes (water hyacinth) may look harmless in a pond, but when released into waterways it causes immense damage because of its ability to grow so quickly and so densely. In addition to impacts to water flow, boating and recreation, water hyacinth increases water loss from lakes and rivers because of its high evapotranspiration rate. Decomposition of  the plants lowers the oxygen level in the water and can lower the water’s pH, which is harmful to fish. It displaces native aquatic plants like pennywort (Hydrocotyle umbellata) that are important habitat for juvenile fish, while creating habitat for mosquitoes in the pockets of water on top of its leaves.

Increased fire frequency and intensity

Grasses like Pennisetum setaceum (green fountain grass) and Cortaderia selloana (pampas grass)shrubs including species of broom, and fire-adapted trees like Eucalyptus globulus (blue gum eucalyptus) can create a fire hazard and make wildfires more frequent and more intense. These plants add more carbon to the fuel load, endangering native plant communities that aren’t as fire tolerant. Broom burns readily and carries fire up to the tree canopy, and the stringy bark of eucalyptus can be carried away while burning, spreading spot fires that can endanger homes. Leaf litter from Tamarix (saltcedar) increases the frequency of fire, with the saltcedar sprouting vigorously after the fire. Arundo donax is another riparian invader. Its dense growth presents a fire hazard and often grows near urban areas.

Economic impacts

Invasive plants can degrade rangelands, timberland, and agricultural fields, increase fire potential, reduce water availability, accelerate erosion and flooding, and diminish outdoor recreation opportunities. The economic cost of invasive plants in California is at least $82 million every year.