California Native Plant Society

Conservation Program

Invasive Plants

Invasive plants are quickly becoming the premier threat to California's unique native plants and plant communities. They can  invade and quickly alter areas otherwise protected from impacts.  CNPS urges agencies and others involved in land management to  develop and implement invasive exotic control and eradication programs. CNPS chapter members participate in a wide variety of  local programs to help with this problem. 

European beachgrass (Ammophila arenaria) photo by Brother Alfred Brousseau

Issue Statement

Although the term 'exotic' somehow brings up the image of a plant holding a drink with a little umbrella, the reality is that exotic means 'out-of-place' in the California ecosystem. Although there has been debate about how long a plant has to have been resident in California to be considered native, a practical working definition employs pre-European-contact as the cut-off point, while those introduced since that time are considered exotic or non-native.

The problem develops with those exotic plants that spread into the surrounding ecosystems and displace the native plants. They do this either because they are free of their home-range diseases, more aggressive in their growth habits, or because they put out more seed that lasts longer in the soil, or because there is nothing to eat them or compete with them in the California ecosystem that is being invaded. These exotic plant properties may cause a crisis in the web of life of the invaded ecosystem, for the newcomer is not a food source and may support no life, while the displaced native plants take with them the pyramid of life that used the plant as the prime recycler of solar energy.

To the casual viewer there is seldom anything to see that would indicate an invasion of exotic plants is taking place. The green hills of spring are usually the green grasses of Europe that were introduced by the Spanish with their cattle. The feathery plumes of pampas grass on the hills of Big Sur might look like they have always been there. However, the sad fact is that the invasion is sometimes fast; so fast that within a decade the ecosystem of old has completely vanished.

CNPS has been working with the California Invasive Plant Council (Cal-IPC) and the California Department of Food and Agriculture to identify those plants which are putting California's flora at greatest risk, and to find methods to eradicate the menace. Agencies such as the Bureau of Land Management, the National Park Service, US Forest Service, and California state parks are also active participants in the fight against pest plants. Weed Management Areas are springing up all over the state, usually covering one or two counties. They are coalitions of private landowners, public agencies, non-profit organizations, and grassroots activists formed to combat a common enemy.

The need to protect natural ecosystems from invasion has run on a parallel and much less well-funded path than programs that are designed to control agricultural pest plants, although the two paths are beginning to converge. Agriculturists have actually opposed including non-crop-damaging exotic plants in state and federal eradication programs, as those programs may ban the shipment or interstate travel of pest-contaminated products, and obviously cause them some problems. However the swift occupation of wildlands and ranching lands by pests such as yellow starthistle, artichoke thistle, and whitetop have caused agriculturists and habitat conservationists to cooperate.

What are the solutions to some of these plant invasions? Plants can sometimes be removed by hand, especially where invasions are freshly started. The policy is to always work from the outside of the population, forcing the invader into a smaller and smaller perimeter. Small outlying populations should be removed early in the process. Piecemeal removal simply does not work. Sometimes the plants resist hand removal, due to a pervasive root structure or to their ability to reroot from small fragments. Two troublesome plants of the latter type are Cape (German) ivy and giant reed (arundo). Cape ivy, which was brought here from South Africa, is choking the summer-cool areas of the state, particularly the riparian corridors of coastal valleys; it may be capable of also invading inland riparian habitats. Giant reed, a bamboo-like grass that grows to the height of a house, invades riparian areas. Both the ivy and the reed can root from small segments, and therefore the worst thing you can do is hit them with a weed-whacker. If you pull the plant, bits and pieces break off, and the root remains to resprout. It can be removed by very intense manual labor, but this is not possible for multi-acre invasions. The only answer seems to be chemical, and there are some relatively benign herbicides (Roundup, for example) which seem to do the trick. Herbicide should be used with caution, but in many cases there are seldom viable alternatives, given the limited labor pools and finances of conservation organizations and public agencies.

Additional Information

CNPS Policy on Invasive Exotic Plants

California Invasive Plant Council (Cal-IPC)

Take a look at CalFlora's expanded weed photos and mapping

Notice of Intent to adopt a Negative Declaration for a proposal to introduce a non-native bumblebee, Bombus impatiens, into the agro-ecological system of the state of California for crop pollination (PDF 302K)

Internet Links on Exotic Plants

Citizens in Action: The Broom Education and Eradication Program in Forest Ranch


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