Invasive Non-Native Plants

Kirke Wrench

 

Non-native plant species are those that are introduced to an area by humans either intentionally or unintentionally. These plants are also known as alien, exotic, introduced, and non-indigenous. They are an enormous concern for the National Park Service; recent information indicates that non-native plants are infesting 4600 new acres of federal land each day.

Nearly one in eight plant species in Sequoia and Kings Canyon National Parks is non-native. Many of these species appear to have a fairly small impact on the parks, but many others are drastically changing ecosystem structure and processes. The plants that have the biggest impact, such as yellow star thistle, periwinkle, reed canary grass, and Italian thistle, can take over large areas, completely excluding native species. Visit our invasive plant information page for details about these species, including identification tips and a history of projects to help manage outbreaks.

 

Yellow star thistle is highly invasive and has been widely dispersed throughout California as a result of human presence.

Photo by Jo-Ann Ordano, © California Academy of Sciences; inset photo by Jerry Asher, Bureau of Land Management

Non-native species reduce biodiversity, jeopardize endangered plants and animals and degrade habitats. Some species, such as giant reed, can completely dominate vast areas of land, excluding virtually all vegetation and dramatically altering water and fire cycles. Non-natives are also known to hybridize with native species, altering native genetic diversity and integrity.

Sequoia and Kings Canyon National Parks have a well-researched non-native plant management plan in place. Early efforts have already eradicated at least one highly invasive non-native plant species (yellow star thistle), and have identified several other infestations that are yet in their very early stages. The year 2002 marked a major increase in eradication efforts, with numerous non-native plants removed from the parks by the end of the year. These efforts continue and we look forward to healthy, less-impacted ecosystems in the years ahead.

 

What is a non-native?

Non-native plant species are those that occur outside their native ranges in a given place as a result of actions by humans. The term non-native can be used interchangeably with the terms alien, exotic, introduced, and non-indigenous (on this web site the term weed refers to non-native plants as well). Therefore, non-native plants can be thought of as those that accompany non-native peoples to a new land.


Native plant species are those that have not been introduced to a given area through the actions of humans. They are naturally adapted to their given area and they have generally existed there for much longer than have humans. The term native is often used interchangeably with indigenous. While some native species grow aggressively and can appear to invade natural areas, they are all in fact naturally adapted, and are valuable members of the ecosystem.

For conservation purposes, a non-native plant species cannot ever become native, even if it persists in an area for hundreds of years. By definition, a non-native plant did not arrive at its new location by natural means, and therefore it can never be considered a native species.

Most of the plant species that humans grow are not native to the areas in which they are grown;such plants include food crops, ornamental plants, roadside weeds, and garden plants. Most of these plant species do not reproduce without the aid of humans. Those that do reproduce and spread on their own are known as naturalized non-native plant species.

Innocuous Non-native Plants

While nearly one in eight plant species in Sequoia and Kings Canyon National Parks are non-native, a great number of them are considered innocuous. Innocuous non-native plants do not appear to threaten native species or ecosystems. For example, swine cress (Coronopus didymus) is a small, non-native, annual mustard that is known only from two locations in the parks, both of which are horse corrals. Swine cress is not known to be a problem species elsewhere and it is not considered to be a threat to Sequoia and Kings Canyon National Parks.

Invasive Non-native Plants

Invasive non-native plants are those that spread, often very quickly, from their original point of introduction into surrounding natural ecosystems. The term invasive non-natives can be used interchangeably with wildland weeds, noxious weeds, or pest plants.

There are numerous non-native plant species in Sequoia and Kings Canyon National Parks that are known to be highly invasive and are actively threatening park resources. These invasive species spread aggressively into pristine areas and displace or disrupt the natural plant communities. Himalayan blackberry (Rubus discolor) takes over moist areas such as stream banks and excludes vast numbers of understory plants. There are several well-entrenched populations of Himalayan blackberry in Sequoia National Park. We are actively controlling many of these species.

 

Why are we concerned?

Invasive non-native plants can cause enormous changes in park ecosystems by out-competing native plants, polluting the gene pool of native plants through hybridization, and dramatically altering fire, water, and nutrient cycles. Additionally, non-native plants are known to transmit non-native diseases to native plants;non-native diseases can be much more devastating than native diseases because native resistance to these diseases is often very low.


Recent estimates indicate that non-native plants are infesting 4,600 new acres of federal land each day. Out of the nearly 1,500 plant species in Sequoia and Kings Canyon National Parks, 183 are non-native, and new arrivals are discovered each year. While many of these plants are highly invasive, many of them also appear to be fairly innocuous. Even these innocuous species are a source of concern, however, as it is possible for a non-native plant species to reproduce slowly for decades and suddenly become highly invasive.

Giant reed is a prime example of how troublesome a non-native plant can be. It aggressively invades wet areas such as stream banks. It forms large continuous colonies, sometimes covering several acres and its root masses can become more than a meter (3 feet) thick. Giant reed sucks up enormous quantities of water and alters the very structure of stream banks. It is well adapted to fire and in fact appears to increase fire frequency in these delicate riparian habitats. Giant reed also contains many noxious chemicals such as cardiac glycosides and assorted alkaloids. Giant reed is restricted to a few small populations in Sequoia and Kings Canyon National Parks. The NPS is very concerned about this species and is taking steps to control it.

See our most invasive plants page for information about twelve of the most troublesome plants in Sequoia and Kings Canyon National Parks.

 

What is being done?

Prior to 2001, non-native plant management in Sequoia and Kings Canyon National Parks was conducted on an ad hoc basis by park staff and volunteers. In 2001, funds to begin a restoration and non-native plant management program became available.

Also in 2001, the Biological Resources Division of the U.S. Geological Survey published a report detailing the invasive potential and management priority for each non-native plant species in the parks. The park cannot feasibly manage all its invasive plants, so the most threatening and/or easy to control species must be prioritized. This report gave the parks the information necessary to begin management.

Prevention

Non-native plants are continuously introduced and spread around in the parks. Preventing this movement of non-native plants is the first line of defense in protecting ecosystems from degradation. It is also the most economical and efficient means of management.

Seeds of non-native plants travel wherever soil is moved--in the treads of car tires, bicycle tires, and shoes. Soil, sand, or gravel imported for construction or other management reasons can also contain non-native plant seeds. Many non-natives have hook-like seed coats and arrive in the park stuck to the fur of pets, wildlife, and pack stock or on people's clothing, shoelaces, and camping gear. Seeds can blow in from the gardens of neighboring private landowners or can wash downstream in rivers. Hay used to feed horses or straw used in revegetation projects can contain non-native plant seeds from the field in which the hay was grown.

The park is beginning to implement measures to prevent the introduction and spread of non-native plants. To successfully do this, many constituencies will need to become active and informed, including NPS staff, the visiting public, concessioners, permittees, contractors, and partners.

Detection

Early detection of invasions is essential to effective, low-cost control. Detection of new invasions in the parks is conducted by surveying recent disturbances for invasive weeds. These early detection surveys may be species based, such as surveying the recently disturbed Generals Highway corridor for the highly invasive yellow star thistle, or location based, such as surveying recent, high intensity fires in natural areas for weeds. A database of non-native plant observations has been developed and is continually updated with input from park staff, concessioners, and visitors. Observation forms are available at visitor centers for recording weed sightings.

Control

The species chosen for control are those that have the highest impact on the ecosystem and can be controlled with the least effort. (For more information about selection criteria, see a report of Exotic Species Threat Assessment in Sequoia, Kings Canyon and Yosemite National Parks). For more information on some of our top invaders, see our most invasive plants page. Certain plant communities are prioritized over others. Native, pristine communities have the highest priority for management, while altered but relatively naturally functioning communities such as foothills grasslands have lower priority. Sometimes the goal is eradication (complete removal of the species from the park), while other times the goal is containment (containing an infestation within its current boundaries and preventing further spread). Knowledge of a species' aerial extent is necessary for effective control and monitoring. Therefore, mapping of non-native species is carried out prior to and during control.

Most eradication efforts are done by hand or with tools such as weed wrenches and shovels. Herbicides are used only when other alternatives have been considered and eliminated, and when managers are confident that the proposed herbicide will do more ecological good than harm and will not endanger the health of the applicators or others in the area. Treatments are highly targeted, minimizing the drift of herbicides to non-target plants, and the principles of integrated pest management are followed.

Education

For prevention or management efforts to be successful, the public must: (1) understand the threats posed by invasive plants, (2) change behavior that results in the spread of invasive plants, and (3) provide support for management efforts.

A newsletter, invasive plant observation cards, and this web site have been developed to provide outreach materials to the public. Park staff is being trained to identify priority invasive plants and to inform the public about invasive plant issues.

We are working with local communities, especially private landowners within the larger boundary of the park, to encourage the use of native plants to replace invasive landscaping. A plant exchange project is currently underway, in which the park provides showy native plants of local genetic stock in exchange for the residents' removal of their invasive plants.

The park has developed a policy to prevent the introduction and spread of non-native plants and is beginning to implement these Best Management Practices. To successfully do this, many constituencies are becoming active and informed, including NPS staff, the visiting public, concessioners, permittees, contractors, and partners.

 

What can you do?

Non-native plants are transported to new lands by humans. Therefore the most important way to combat the problem of non-native plants is to be sure you are not dispersing them. Many of these plants produce seeds that cling with amazing tenacity to shoes, clothes, car tires, pets etc.

Learn the most invasive plants in Sequoia and Kings Canyon National Parks. Call us with your observation or ask for an Exotic Plant Observation Card at any of our visitor centers. Remember, never pick or pull any plants in the park; contact us and we will handle the control efforts (this is true for plants on all public lands).

Vehicles often disperse non-native plants. Therefore it is important to wash vehicles, tires, and vehicle undercarriages before entering the Park.

If you use pack animals, we recommend bringing only certified weed-free feed into the parks. Please maintain this diet for your animals four days prior to entering the parks as well. Thoroughly brush all pack animals and clean their hooves and gear before transporting them to the parks.

If you’re backpacking into the wilderness, be extra vigilant about cleaning your boots and gear at home or at the trailhead before you begin hiking.

Do not camp in or hike through weed-infested areas. Please be sure to stay on designated trails and roads. Clean your gear, clothes, vehicles and tires between outings and camping trips. Please ensure that all seeds are safely disposed of in the trash.

Non-native plants can disperse widely and quickly. Therefore even if you do not live in a natural area, it is important not to cultivate invasive non-native plants around your home. Also, do not pick flowers or plants in natural areas. Many wildflowers are actually invasive weeds, and a handpicked bouquet may transport seeds to a new area.

For identification and information on non-native plants in your area, contact your local county/university extension office, nature centers or garden clubs. Nature centers and land trusts often organize weed-eradication projects, and these are excellent opportunities to learn about the biology and eradication of non-native plants.

Spread the word about non-native plant invasions. Prevention is our most powerful tool, and by following a few simple conventions we can ensure the preservation and health of our natural lands.

 

Reports and Species Lists

Exotic Species Threat Assessment and Management Prioritization for Sequoia-Kings Canyon and Yosemite National Parks
An extensive report by the U.S. Geological Survey published April 2001.

Exotic species threat assessment in Sequoia, Kings Canyon and Yosemite National Parks
This research paper was presented at the 2001 George Wright Society Biennial Conference.

Non-native Plant Species of SEKI (XLS format)
This Excel spreadsheet lists all non-native plant species currently or previously found in the parks. For each species it includes common name, scientific name, and family.

Non-native Plant Species of SEKI EXPANDED (XLS format)
This Excel spreadsheet lists all non-native plant species currently or previously found in the parks. For each species it includes common name, scientific name, and family. It also includes Park and international taxon codes, data on life forms, and categorization codes demonstrating invasive potential (these last were developed by the Biological Resources Division of the U.S. Geological Survey, and the California Exotic Pest Plant Council).

List of SEKI Non-native Plant Species organized by family
List of SEKI Non-native Plant Species organized by type and common name
List of SEKI Non-native Plant Species organized by management category

 

Links

California Department of Food and Agriculture's Noxious Weed Information
California Department of Food and Agriculture's Integrated Pest Control Information
California Invasive Plant Council
Sierra Nevada Network Inventory and Monitoring Program
California Native Plant Society
The Nature Conservancy's Global Invasive Species Team