Noxious weeds and invasive non-native plants are plants that have a tendency to invade and become serious pests, threatening ecological and economic systems.

These invasive plants generally also have high reproductive and growth rates, long-lived seeds, and deep roots that compete with native plants for water, light and nutrients.

These plants arrived in this country without the suite of natural enemies that control their abundance in their country of origin.

In some cases, this contributes significantly to their invasiveness by giving them a strong competitive advantage.

Weeds like yellow starthistle and spotted knapweed can sweep across landscapes within just a few years, leaving a monoculture of undesirable plants in place of a healthy ecosystem.
 

   What are the impacts of weeds?

Noxious weeds can -

• Crowd out native plants by competing with them for water, light and nutrients.
• Cause injury and discomfort to hikers, anglers, hunters and whitewater rafters because of painful spines.
• Decrease the quality and quanitity of fish, wildlife and native plants, including rare and endangered species.Increase erosion and runoff.
• Lower property values.
• Crowd out desirable forage plants for livestock and wildlife, including game species.
• Cause illness, injury and sometimes death in wildlife and livestock
• Reduce the success of reforestation in productive timberlands.

Weeds are easily transported to new areas. They can be spread to new areas by vehicles, heavy equipment like bulldozers and graders, hikers, bicyclists, pack and saddle stock, livestock and pets.

Weeds establish and reproduce swiftly: A few weed seeds accidentally moved into a new area may not seem alarming, but when the biological potential of these few seeds is considered, there is cause for great concern. For example, a robust yellow starthistle plant can produce up to 150,000 viable seeds. Even at more typical production rates of a few hundred seeds per plant, a flush of seedlings is produced that coats the ground within the first year, edging out the native plants. Seedling densities of 2000 seedlings per square foot are not uncommon in a heavy infestation.

     What can you do?

Avoid planting invasive plants in garden settings
.
Take care not to inadvertently plant invasive weeds in your garden. Many species of invasive weeds are still sold in nurseries and seed catalogues: examples are Scotch broom, French broom, Spanish broom, bachelor's buttons, rose campion, pampas grass, and dalmatian toadflax. When planted in rural gardens, these attractive but highly invasive plants may spread to adjacent wildlands. Contact the California Native Plant Society at www.cnps.org for a list of substitute plants that will not spread.

Avoid carrying weed seeds in your shoes, gear,
pet's fur, stock, etc.
.
Learn to recognize problem weeds in your area so that you can be aware when seeds or contaminated soil might spread to them to a new site.  Many seeds of native and weedy plants are adapted to spread by "hitching a ride" on fur or any other vector.  But by being aware of the weeds pictured in this brochure, you can be sure not to spread the most invasive weeds inadvertently.

Prevention and control

Prevention is far cheaper and less damaging than waiting until a weed problem has developed.  The question of which control method to use can be avoided in many cases by preventing the spread of weeds or by manually pulling the first few that arrive.  Many people are uneasy with the use of chemicals for weed control; good prevention minimizes the necessity for herbicides.  See the website at www.cdfa.ca.gov/weedhome and click on EncycloWeedia for more information about weed control methods.

Restoration

Restoration is just as important as choosing the right control methods and timing.  It involves enhancing or maintaining the native plants at a site so the site can resist re-invasion by noxious weeds.

     Resources and Contacts

For resources and additional information on noxious weeds in San Francisco, please contact:

San Francisco Recreation and Park Department,
Natural Areas Program 415/753-7267

California Department of Food and Agriculture,
Agricultural Biologists 559/445-5031

United States Department of Agriculture, Forest Service
559/877-2218 (North Fork Office)
559/855-5355 (Prather Office)

United States Department of the Interior,
National Park Service 209/379-1217

United States Department of the Interior,
Bureau of Land Management 916/978-4474

Good websites for more information
about weeds:

www.invasivespecies.gov

www.cdfa.ca.gov/weedhome

www.caleppc.org