Many “weedy” plants are essentially harmless; they are simply plants that are growing in an area that isn’t ideal from one person’s point of view. However, other weeds have larger impacts on the environment. Once they’ve escaped cultivation, they can crowd out native plants and fundamentally alter the environment where they grow. These changes not only help insure that the native plants cannot re-establish in these areas, they can often have other more far reaching consequences. Weeds that can invade and alter natural areas are often referred to as “invasive exotics”.
For example, many broom species (Genista monspessulana, Cytisus scoparius, Cytisus striatus, and others) crowd out native vegetation on roadsides and other disturbed sites, and then use water (when it’s available) at prodigious rates, leaving a dry, oily, highly flammable thicket of brush behind. This brush is not palatable to native herbivores, so it builds up along the very routes firefighters depend on for firebreaks and the movement of equipment, increasing the risk and severity of wildfires. When these brooms do catch on fire, they burn with such intensity that most, if not all, plant matter is eliminated from the site. The soil in these areas then becomes subject to erosion, polluting our water and potentially eliminating other species, like salmon, from local streams. That’s a lot of consequences for one small, pretty plant that was once widely sold by nurseries.
Hybrid spartina (Spartina foliosa x alterniflora) grows so thickly that it eliminates competion from native spartina, and also effectively eliminates the breeding habitat for several species of native shore birds from the San Francisco bay. The introduction of scarlet wisteria (Sesbania punicea) has had a similar effect on hundreds of miles of the Sacramento river shoreline.
Elimination of habitat, including the removal or reduction of native vegetation, is the major driving force behind localized extinction of native species. While large and noticeable events, such as the die-off of our native oaks caused by sudden oak death, justifiably garner major media attention, the invasion of the North Bay by a large number of exotic weeds is likely to have equal or greater long term ecological impacts. Weeds like these are changing California for the worse, but it’s happening slowly, so that it rarely makes the headlines.
There are literally hundreds of invasive exotics listed by the California Invasive Plant Council, which rates the risk and potential ecological impacts of each plant, for various regions of California. The Marin Sonoma Weed Management Area is a group that monitors which weeds occur in the north bay area, and what risks are posed by each species. The Bay Area Early Detection Network works to eradicate small infestations of invasives before they become too extensive to remove economically.
California Invasive Plant Council (Cal-IPC)
The Bay Area Early Detection Network - http://baedn.org/
Marin Sonoma Weed Management Area - http://marinsonomaweedmanagement.org/
The Weed Research and Information Center - http://wric.ucdavis.edu/