March 21, 2005
Non-Native Invasive Plants
by Martha Proctor
The spread of invasive plants has been described as a “raging biological wildfire—out of control and spreading rapidly.” All over California, natural wildlands are under attack by invasive non-native plants. Non-native refers to species that were directly or indirectly introduced to a given region by human activity. Non-natives are not necessarily invasive. Non-natives can become invasive if introduced to an area with a similar climate to their native land and/or if the vast and complex array of natural controls present in their native lands are not present in their new location. The invasive plants that are most harmful to native biodiversity significantly change ecosystem processes, often so much so that many native plants and animals can barely survive in the area, if at all.
Non-native invasive species crowd out native species by abundant seed production, rapid growth, and better access to moisture during the dry summers. Invasives outcompete natives, displacing the native habitats of fish, insects, birds, plants, butterflies and other wildlife. Consequently, this spreading “biological wildfire” has resulted in the loss of many species and concomitantly has imposed tremendous cost on human communities.
Of the more than 900 plant species in Point Reyes National Seashore (PRNS), 321 are non-native. Of these, at least 30 are invasive enough to threaten the diversity of native plant communities in the Seashore. Much of California has been designated as a “global biodiversity hotspot”—one of 25 terrestrial regions in the world where biological diversity is most concentrated and the threat of loss most severe. Of the 25 highest priority invasive species at PRNS, 13 are escaped ornamental garden plants. Pampas grass and French broom, two of the park’s most difficult invasives, are still cultivated in home landscaping projects. Cape ivy and pampas grass populations have developed in Golden Gate National Recreation Area from garden waste dumped along the Fairfax Bolinas Road. Amazingly enough, the patches of Cape ivy in the park have increased 700% in the last 9 years.
PRNS (as well as the California Dept of Food and Agriculture, and the California Invasive Plant Council) has developed weed ranking lists, which rank plants as to their local invasiveness. A-1: most damaging (yellow star-thistle, Pampas grass, gorse, Cape ivy, capeweed, Distaff thistle); A-2: highly invasive, damaging and widespread (European beachgrass, French/Scotch broom, iceplant); B: less invasive but still a problem (purple star thistle, cotoneaster, fennel, Vinca major) and Red Alert: not as widespread but has the potential to move rapidly into natural areas (velvet grass).
Several of the plants listed above are still available from garden centers, garden catalogs, and websites. Before I realized the seriousness of the problem, I bought and planted Vinca major along the driveway—fortunately, it has not done well so has not become a problem. I will have to replace it with pachysandra, Serbian bellflower or ivory star jasmine as suggested in the helpful brochure entitled “Don’t Plant a Pest,” which is available from Cal-IPC (www.cal-ipc.org) or 510-525-1502.
So, what can you do to help stop the spread of non-native invasive plants? First, recognize the seriousness of the problem. Begin by learning which non-native species are invasive in your locality. Remove any invasive plants of local concern that you have planted in your garden . . . but don’t dump garden material where it can resprout and spread into wildlands. Avoid buying or planting invasive species in your garden; report any new invasive plant sightings to park authorities. Kim Cooper heads the weed management program at PRNS (464-5196). Report the sale of non-native invasive plants from garden centers, catalogs, or over the internet. Removing and controlling invasive plants is an expensive, labor-intensive job. Volunteer your time and money to help stem the tide wherever you live. As it says on the “Don’t Plant a Pest!” brochure, “Give them an inch and they‘ll take an acre . . .”