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What makes a plant invasive?

  • September 30, 2017
  • James Campbell
  • At first I loved them. I marveled at their ability to bloom profusely with only the seasonal rains. They painted broad strokes of yellow across Marin County. Turns out broom, along with a host of other beautiful invasive plants, are wreaking economic and environmental havoc right in our backyards.

    When plants native to one area of the world are moved to another area they can sometimes flourish because they are no longer controlled by the insects, diseases, foraging animals or weather that kept them in check in their native range. The important biological difference between invasive plants and weeds is the ability of invasive plants to disperse, establish and spread without human assistance or disturbance. Some naturalized plants do not spread away from where they were introduced and are not generally a significant problem either in the garden or in natural habitats. However, naturalized species that do spread and survive in new areas are called invasive plants. In California, 3 percent of the plant species are considered invasive but they occupy a much larger, and growing, proportion of our landscape.


    Landscape transformations happen when invasive plants win the competition against native species for resources like sunlight, water and soil nutrients. Invasive plants form monocultures (dense stands of one plant) that can change habitats by reducing the food and shelter needed by native wildlife. French broom, for instance, was found to have reduced the arthropod populations by one third in the Golden Gate National Recreation Area. The dense stems of a broom monoculture make regeneration of most other species difficult or impossible. Since it grows more rapidly than most trees, French broom shades out tree seedlings.

    Broom also creates a dangerous fire hazard; as broom plants grow, the inner stems die back, providing a highly flammable fuel. Broom burns readily and carries fire to the tree canopy, increasing both the frequency and intensity of wildfires. The flowers and seeds of brooms are toxic or unpalatable to humans, livestock and native wildlife. French broom also decreases visibility along roadways. The California State Department of Food and Agriculture has listed brooms as a class C pest species, that is troublesome, aggressive, intrusive, detrimental or destructive to agriculture, the cultivation of trees and important native species. The California Invasive Plant Council has placed Scotch, French and Spanish brooms on their high alert list, species that have severe ecological impacts on physical processes, plant and animal communities, and vegetation structure.


    The most effective method of controlling invasives is to prevent their introduction and establishment in the first place. The economic costs of preventing invasive plant infestation is far lower than the costs for eradication once established. Invasive weeds have become an extremely serious problem in Marin County. Several species of invasives have become established here rendering thousands of acres of pastureland, rangeland and natural areas unusable. If nothing is done to slow and stop these invaders, it will become unfeasible to attempt to control or manage them. When invasive weeds are found, there are a variety of methods to control them, including hand pulling, weed whipping, hoeing, shoveling, mowing, biological control, over seeding with native grasses, herbicide treatments, prescribed burning, mulching and the use of goats.

    To learn more about identifying and removing invasive weeds, go to the University of California Weed Research and Information Center at wric.ucdavis.edu. Be a part of the solution for our county — if you are not controlling broom, thistle and other invasives on your property, you are a part of the problem.