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Battling Invasive

Timothy W. Miler  
Battling Invasive Streamside Weeds

Tim Miller inspecting Butterfly BushSome of the most important non-native plant species frequently invading Pacific Northwest riverbanks and lakeshores are the knotweeds: Japanese, Bohemian, Himalayan, and giant. Knotweeds may reach heights of 15 feet or more, forming dense colonies that kill nearly all other vegetation in their path. Dr. Tim Miller is a weed scientist working to devise strategies to combat these brazen invaders, starting with identifying their weaknesses

Miller has studied the ability of these knotweeds to grow from pieces of cut stems and rhizomes. He has also assessed their susceptibility to various herbicides at different stages of growth, as well as considered innovative ways to apply those herbicides. For knotweeds, these control strategies include using foliar sprays, applying herbicide onto the knotweed stems with a wiper, injecting herbicide into the hollow stems, and bending and/or cutting the weeds prior to herbicide treatment. Miller’s work has greatly enhanced the ability of land managers to successfully manage these knotweeds.

Reed canarygrass is another non-native species very commonly found in riparian areas. This rhizomatous perennial grass rapidly colonizes stream banks and seasonally wet areas throughout the Pacific Northwest. Once established, it crowds out most other plants. In order to re-establish native plants along the water’s edge, Miller determined that two years of chemically or mechanically suppressing the weed’s growth allows enough time for fast-growing native trees such as red alder, black cottonwood, and willow to establish themselves and begin to shade out the sun-loving grass. Thereafter, continued weed control is generally unnecessary. He is currently working to improve the success of western red cedar re-establishment on these sites by transplanting shade-tolerant trees in combination with cottonwood or willow plantings.

Miller has also studied other riparian weeds including butterfly bush, yellow flag iris, hairy willow-herb, indigo bush, wild chervil, meadow knapweed, and giant hogweed. All studies have been cooperative efforts with county weed control coordinators, Washington State Department of Agriculture, Ecology, and State Parks personnel, or representatives from the U.S. Fish and Wildlife and National Park Services.


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Contact Information
Timothy W. Miller, Ph.D.
Associate Scientist / Extension Specialist
Crop and Soil Sciences

Washington State University
Mount Vernon Northwestern Washington Research & Extension Center
16650 State Route 536
Mount Vernon, WA 98273-9761

Telephone: 360-848-6138
Fax: 360-848-6159

Miller earned his B.S. degree in Range Management in 1981, and his M.S. and Ph.D. degrees in plant science in 1987 and 1995, respectively, all at the University of Idaho. He joined the faculty in the Department of Crop and Soil Sciences at WSU in 1997 and is stationed at the Northwestern Washington Research and ExtensionCenter in Mount Vernon. His extension duties involve conducting trials for improving weed management in western Washington crops, such as vegetable seed (spinach, beet, cabbage, and others), ornamental bulbs (tulip, daffodil, and iris), small fruit (blueberry, raspberry, and strawberry), cucurbits (cucumber, squash, and pumpkin), and potato. His research has focused on primocane management of red raspberry and how those programs affect yield and growth of raspberry. Since 1997, Miller and his staff have conducted 621 trials including nearly 14,000 individual treatments. He has given over 500 presentations to over 40,000 people on a host of weed, herbicide, agricultural, and horticultural topics. Other recent work includes evaluation of biodiesel crops for western Washington production as well as control trials on various regional noxious weeds.