Gardeners share blame for noxious weeds

Marianne Ophardt, Washington State University Benton County ExtensionNovember 14, 2013 

Gardeners always seem to want something new or different for their landscapes and gardens. That is one reason we have some troublesome weedy plants in our country that are considered noxious and invasive.

What is a noxious weed? The Washington State Noxious Weed Control Board says it is an invasive, non-native plant that threatens agricultural crops, local ecosystems or fish and wildlife habitat. This includes non-native woody trees and shrubs, grasses, flowering and aquatic plants. The board says "half of all invasive, noxious weeds are escapees from gardens; the rest are plants accidentally introduced to Washington through human travel and trade."

One example of a noxious weed is Scotch broom (Cytisus scoparia). It is covered with bright yellow flowers when it blooms in late spring, making it obvious along our state's highways. It creates a triple threat because it displace the native plants of that area, it encourages other non-natives by increasing the available nitrogen in the soil (as a legume, it fixes nitrogen and releases it into the soil), and the plants provide fuel for wildfires. More than $100 million in losses by the agricultural and forestry industries in Washington and Oregon are attributed to the expense of controlling this noxious weed.

Native to Europe and North Africa, Scotch broom was introduced into Washington in the early 1800s as an ornamental plant. Later, it was planted by the Soil Conservation Department of the U.S. Department of Agriculture for helping prevent soil erosion. It may be an "easy-care" yellow flowering shrub, but it has become a nightmare.

Purple loosestrife (Lythrum salicaria) is another attractive plant that is also a noxious weed. It will easily grow in a garden with plenty of moisture. It causes problems in wetlands and waterways, where it forms dense stands that crowd out native plants and wildlife habitat.

Purple loosestrife is native to Europe and is believed to have been introduced to North America in the early 1800s, perhaps as a contaminant of ships' ballast, a medicinal herb garden plant or an ornamental for gardens. Whatever manner it arrived, it didn't take long to become a problem. It was found in Lake Washington in 1929 and in the 1940s in Spokane County. In the 1970s, it was found in the Grant County's Winchester and Frenchman Hills Wasteways. Within 20 years, it dominated 23,000 acres of this desert wetland, crowding out all native vegetation.

Purple loosestrife is a prolific seed producer. One plant can produce more than 2 million seeds a year. Plus, the plants are perennials. If allowed to grow in wetlands, it doesn't take it long to completely takeover if not controlled.

It's hard to believe purple loosestrife seeds can still be bought.

Gardeners may chafe at regulations designed to reduce the threat of invasive plants, insects and plant diseases. However, they need to be informed and responsible, not trying to find a way around these regulations.

-- Alternatives to noxious weeds: Some noxious weeds are pretty and may seem desirable to gardeners, but there are alternative plants that don't pose a threat in our state. The Washington Noxious Weed Control Board has a publication called Garden Wise: Non-invasive Plants for Your Garden for Eastern Washington that provides alternatives to noxious weeds, including purple loosestrife and Scotch broom at They also have a other publications pertaining to noxious weeds.

-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.

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