Controls of Invasive Plants

By Thembi Borras

Preventing the spread of invasive plants is easier said than done given the vast expanses of invasive plants with which we are faced. Harding grass, which grows much taller than the native grasses it replaced increases the fire danger in many of the steep grasslands in Rancho Navarro where I live. Each year I notice more and more brooms and star thistle along our roadways. According to an article recently published in the May 2006 issue of the Mendocino County Farm Bureau's publication medusa head and barbed goat grass have become increasingly problematic to wildlife and livestock by reducing native populations of desirable feed and giant reed, also known as Arundo, and Himalayan blackberry have been charged with clogging local waterways.

Faced with a small backyard population of invasive plants well timed manual control can be effective. However, addressing large populations is a challenge because treatments may require long term commitments from stakeholders, can be expensive and can in themselves adversely impact the environment. The trick is for land managers to decide which control method is most effective, while being the least damaging to the ecosystem. Controls being used include public education, manual and mechanical, grazing, prescribed fire, biocontrol, herbicides and Integrated Pest Management (IPM) which incorporates a combination of these controls.

Manual techniques which include cutting, pulling or grubbing, which means pulling out all the roots, are very labor intensive. The advantage is that people can be trained to discriminate between the native plants and invasives. Mechanical techniques include mowing and mastication and can be cost-effective but are limited to gentle terrain and like grazing may cause unacceptable levels of ground disturbance and compaction. Also, machines and animals may not be able to discriminate between the invasive plants being controlled and the native plants being retained. Fire is also being used to control invasive plant species most often as a facet of some integrated pest management approaches. If the invasive plant species being targeted produces a prolific amount of seed, timing these controls to prevent annual seed production will maximize control efforts. In biocontrol, a natural enemy such as a parasite, predator or disease organism is introduced into the environment of the invasive plant or, if already present, is encouraged to multiply and become more effective in reducing the number of invasive plants. However, some biocontrol programs have resulted in significant, irreversible harm to untargeted species and to ecological processes and must be used with caution informed by the lifecycles and complex interactions of targeted and nontargeted species. Finally, although chemical control, which involves the use of an herbicide to reduce invasive plant populations, is controversial, Peter Warner, environmental scientist for the California Department of Parks and Recreation, considers "herbicides to be one of many essential tools in reducing the spread of invasive plants". The Nature Conservancy provides a checklist to be used when considering the use of herbicides entitled, To Spray or Not To Spray? and is contained in the Weed Control Methods Handbook, which is an excellent resource that covers in depth each of the control methods. It can be downloaded from the website http://tncweeds.ucdavis.edu/handbook.html.

You can get an Invasive Plants brochure at the Navarro River Resource Center, the number for which is 895-3230. You can also visit the Mendocino Coast Cooperative Weed Management Area website at www.mcwma.org and see how you can get involved locally or visit the website of the California Invasive Plant Council at cal-ipc.org.

A portion of this production was gleaned from The Nature Conservancy Weeds website at tncweeds.ucdavis.edu and communication with Peter Warner of the California Department of Parks and Recreation and Tara Athan, botanist for the Mendocino Coast Cooperative Weed Management Area.