Q. What makes a plant invasive? And why should we be concerned? — George, Los Osos
A. When plants that have evolved in one area of the world are introduced to another area, some of them outgrow native plants that are important sources of food and habitat for wildlife. The introduced plants have a competitive advantage because their growth is no longer limited by natural predators found in their own environment.
Many of these weeds are agricultural pests and many impact wildland areas as well. It is estimated that invasive plants cost $82 million a year in control, monitoring and outreach expenses in California alone.
Some of these plants were introduced for practical reasons. For instance, ice plant was installed along highway berms to prevent soil from washing onto roadways. Pampas grass was planted on hillsides in the Big Sur area to retain the buffs. Both of these proved to be ineffective. And both are invasive and difficult to eliminate.
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Other introduced plants used as landscape ornaments, such as periwinkle (Vinca major), can run amuck because of rampant growth. Any part of it that touches water will root and form a dense mat, crowding out any plants in its way.
Brooms — Scotch, French and Portuguese — on the other hand, favor dry habitats and wreak havoc in pastureland and increase the potential for wildfires. In farms and rangeland, brooms can crowd out crops and forage, lowering land values. Since they are not palatable to livestock, grazing does not reduce their numbers.
What can the home gardener do to help? Practice responsible landscaping. Learn more about invasive plants and avoid planting them.
Instead of ice plant, use wild strawberry. For a colorful groundcover, use hardy geranium or star jasmine to replace periwinkle. Substitute Mexican deer grass (Mulenbergia rigens) for pampas grass. Forsythia is a yellow flowered, non-invasive shrub that can stand in for brooms. Educate others about invasive plants. Discourage your local nurseries from selling invasive plants.
For more information on invasive plants and alternative plants for landscaping, visit the websites of the California Invasive Plant Council, http://cal-ipc.org and Plant Right at http://plantright.org.