Invasive Species

Identifying & managing noxious weeds.

Scotch broom flowers
Photo courtesy of Eric Combs, ODA

Invasive or noxious weeds are plants that are not native to the local area which thrive and spread aggressively outside their natural range. Some invasive species have become serious problems that threaten water quality, drive away native wildlife, crowd out native plants and create fire hazards.

Among the most important noxious weeds found in southern Oregon are:

Identifying the Invaders

For help in identifying invasive species, The Silent Invasion Quick Guide has excellent pictures and descriptions of the most common invasive species. The Oregon State Noxious Weed List lists all the plants classed as noxious weeds (slightly different than invasive species, but still ones to look out for) with links to pictures and descriptions of each one. King County (Washington) also has an excellent source to identify invasive species with their weed photo index.

Removal of Invasive Species

If you are trying to get rid of existing invasive species, you have several options depending on what type of plant(s) you have and your philosophy on dealing with them.

Smaller plants can be “weeded” by manually pulling them out of the ground; larger plants and trees can also be manually removed with the right tools. It takes hard work, a good pair of gloves and persistence, but it will work. Do not put invasive species in your compost pile; composting will not kill the seeds of all invasive species and you might be giving them very fertile ground to sprout and grow! For tips and techniques, see Physical Methods to Manage Invasive Plants by the National Wildlife Refuge System.

Using chemical herbicides to kill invasive species is another option. Herbicide Advice for Homeowners has general information about different methods for using herbicides and general removal tips. For the complete guide, see the Pacific Northwest Weed Management Handbook, available in print or online.

For seed-bearing annuals, you can slow or stop the invasion by deadheading flowers before they have a chance to produce seeds for the next crop of plants. Burning and biological controls (such as the Cinnabar moth to control Tansy) are other options.

The Nature Conservancy’s Weed Control Methods for Natural Areas Handbook has detailed instructions for how to remove invasive weeds using a variety of methods, from mechanical to herbicides.

Repopulating with Native Plants

As your invasive species are removed, you can find native replacements at native plant nurseries. The Garden Smart Oregon; a guide to non-invasive plants brochure is an excellent resource for selecting non-invasive plants as replacements.

As your native plant population grows and any invasive plant species are controlled, the population of native wildlife is likely to increase as they will have the natural habitat they need.

Early Detection is Key

For some well-established noxious weeds, like yellow starthistle and Himalayan blackberry, eradication probably isn’t realistic. The best that can be hoped for is to contain existing populations and focus control efforts on the most important or sensitive areas.

For individual property owners, however, early detection and rapid response can help prevent invasive weeds from becoming established on their own properties. Early detection requires regular monitoring of the property to spot any weeds that show up but don’t yet have a firm toe-hold. It’s important to know what to look for. Critical locations to monitor for possible weed invasion include roadsides, ditches, and any areas where the soil has recently been disturbed.

Additional Resources

Oregon Public Broadcasting has an excellent program The Silent Invasion giving some history and explanation of the problem with invasive species.