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Marin IJ Articles

Escaped Exotics, Invasive Plants

February 13, 2012
Katie Martin

Some years ago, I was the “Escaped Exotics” chairperson for the Marin chapter of the California Native Plant Society.  What does that mean? One friend said it sounded like a bunch of plants in black-and-white striped pajamas running loose in the countryside! Other terms that are used to describe these plants include non-natives, naturalized aliens, pest plants and invasive plants.

Why are they bad? Some of these plants do have pretty flowers. Consider the graceful plumes of pampas grass or the large pea-shaped flowers of Scotch broom.  These aggressive plants can crowd out our native plants, and decrease biodiversity. They are a threat to the rare and endangered plants of Marin County.

Exotic plant species were brought by Father Junipero Serra to San Diego Bay as early as 1769. According to “California’s Changing Landscapes,” evidence of at least 16 exotic weed species (dated to 1824) was found in the string of missions along California’s coast. Other introduced species were brought in ship ballast, grain contaminants or for ornamental purposes. Many came from parts of Europe with a similar Mediterranean climate as ours.

According to the “Marin Flora,” California has 5.867 plant species, 1,025 of which are introduced. Marin County has 1679 different plant species. This is a rich and diverse flora in a small area of 520 square miles. Of these, 583 are introduced species. You may be familiar with some of them: Pampas grass, Scotch or French broom and yellow star thistle.

Pampas grass, Cortaderia jubata, is native to Bolivia, Peru and Ecuador. Each plume contains thousands of seeds. It was introduced into California for landscaping and erosion control. Cal Trans used it on Devil’s Slide, Highway 1 south of San Francisco. It was common in West Marin, but local people have worked hard to decrease the populations.

There are several species of broom that have been introduced from Europe. French broom, Genista monspessulana, was brought in for landscaping 30-40 years ago. They are members of the pea family with bright yellow flowers, like sweet peas. On a warm summer day you can hear the pods explode and see the seeds scattered widely. A medium-sized shrub can produce 8000 seeds per year! The seed is long lived in the soil, making control difficult. In 1994 a prescribed burn was conducted by Mt. Tamalpais State Park along Panoramic Highway to control broom.

Yellow star thistle, Centaurea solstitialis, has solitary, yellow, composite flowers and stout spines on the buds. It is native to dry open habitats of southern Europe. It now infests 12 million acres in California. It can survive on as little as 10 inches of rainfall per year. China Camp State Park has tried to control yellow star thistle on their lands.

These plants have common characteristics that enable them to invade, establish and outcompete our native plants. First, we must remember that all plants compete for space to grow in, for light, water and soil nutrients. Exotics can outcompete and crowd out natives because they have:

  1. Large numbers of seeds
  2. Often seeds can be dispersed a long distance from the parent plant
  3. Seeds are long-lived in the soil, forming a seed bank for future generations
  4. Free from their native predators, they have a competitive advantage
  5. They may have fast growth rates
  6. They have wide tolerances for environmental factors, e.g., heat/cold, wet/dry, good soil/poor soil.

The California Invasive Plant Council (Cal-IPC) is a non-profit organization committed to our native plants and natural landscapes. Cal-IPC's mission is to “protect California's lands and waters from ecologically-damaging invasive plants through science, education and policy.” The inventory of invasive plants that they have developed lists and categorizes non-native invasive plants that threaten the state's wildlands. The state of California, realizing the importance of the problem passed the noxious weed bill AB1108 in 1993. A brochure published by the Marin/Sonoma Weed Management Area  (MSWMA) describes other invasive species: cape ivy, gorse, distaff thistle, medusahead, oblong spurge, Italian thistle, giant reed, barbed goatgrass and purple star thistle.

If you want to help remove invasive plants, you can volunteer with the MMWD watershed or Point Reyes National Seashore. The volunteer coordinator for MMWD is Suzanne Whelan (415 945-1128). Contact her about the broom pull every third Saturday, 9 a.m. - 12 noon. At Point Reyes National Seashore you can call Volunteer Program Manager Doug Hee (415 464-5145).

Invasive non-natives are a serious and significant problem in Marin County, California, and the Pacific Northwest. But as the past chairperson for escaped exotics, I don’t want to leave you without hope. I believe that with a multi-faceted approach we can find solutions to this problem. Practical methods for removal and revegetation must be used. Public awareness through education efforts is also necessary. Further scientific research into the nature of these invasive plants will aid in their removal. And there must be cooperation between government agencies, the horticultural industry and concerned citizens.

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