Negative Impacts of Non-Native Weeds

Non-native plants and animals present very real threats to our natural resources and cost our economy billions of dollars each year. Invasive species compete with native species and are able to spread aggressively because they do not have predators and disease susceptibility in the new area. Non-native weeds in particular impact agricultural productivity and the health of our natural resources.

Non-native weeds can harm wildlife and livestock through toxicity, by disrupting the food cycle, and displacing native food sources. Tansy ragwort is an example of a non-native plant that invades pastures and is toxic to livestock.

As non-native weeds spread, they can degrade habitat for endangered species and disrupt natural ecosystem processes. For example, knotweed growing along a stream will prevent native tree seedlings from establishing and growing. The native trees would provide shade to keep the stream cool and deep roots to stabilize soil; knotweed does neither. Reed canary grass is another example of a non-native weed that can increase flooding and reduce fish habitat quality when growing along a stream.

Non-native weeds are a problem for humans too: weeds can adversely impact recreational areas, create erosion problems, and reduce agricultural profits. Non-native thistles and scotch broom spread aggressively in pastures and reduce forage yield; aquatic weeds can choke lakes and reduce swimming and boating opportunities; and English ivy can worsen and hide excessive erosion on slopes.

Tansy in horse pasture

Tansy taking over horse pasture

Fragrant water lily and Eurasian milfoil on Lake Sammamish

Fragrant water lily and Eurasian milfoil

knotweed and erosion

Knotweed and erosion

scotch broom in field

Scotch broom in field