Noxious Weeds

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Oh, my aching muscles...

What makes a weed noxious?

By Bridget Simon and Mary Rabourn, King County, WA

A nightmare before weeding...

Giant hogweed is a 15-foot-tall plant with sap that burns your skin; tansy ragwort poisons and kills livestock; gorse is a dense shrub that helped burn down a town. The economic impacts of noxious weeds in agricultural areas or croplands are well known, but urban areas and natural resources are not immune to impacts from destructive, invasive plants.

What is a Noxious Weed?

Generally speaking, noxious weeds are non-native plants that are tough survivors, they are aggressive, very difficult to control and native plants can’t compete with them.  These invasive characteristics and their impacts are what get them on “the weed list.”  But, those characteristics alone do not make a weed noxious.  Noxious weeds are the plants adopted by the state noxious weed control board, and they are regulated for control by county noxious weed control boards.

Where Did These Plants Come From?

Most of our noxious weeds are native to Europe and Asia or the Mediterranean region.  Some were accidentally introduced when agriculture was brought to the New World.  Others were introduced in early trade. European ships carried soil or water as ballast to provide stability at sea.  The ballast was dumped on shore, and if often contained seeds or plant parts.  Purple loosestrife was first introduced to the Northeastern Seaboard in ballast.

Some garden plants lead a double life—they have a dark side with bad habits. Quite a few noxious weeds originate as garden plants, or as aquatic ornamentals for ponds or aquariums. English ivy, garlic mustard and yellow flag iris all escaped the confines of their garden homes to outcompete native and beneficial plants in their own homes.

Needless to say, not all exotic garden species are capable of being a noxious weed.  Many introduced plants behave themselves, and you only find them where you plant them, year after year.  The “escapees” quietly and steadily build a seed bank or an impressive root system.  The seeds sail along on the wind or plant fragments float downstream or they hitchhike on tires, shoes or animals to a more disturbed site – like a roadside or stream or wetland.   From there, the opportunities for impact of invasive plants are boundless. They overwhelm natural areas, agriculture areas or grazing lands, they choke waterways used for recreation, irrigation or salmon habitat.

What is the “Weed List?”

New plants infest about 4,600 acres per year in Washington State.  In order to deal with destructive plants that arrive and thrive in our state, the Washington State Noxious Weed List is updated annually.  A primary function of this list and of regulatory weed programs is to prevent the new establishment of invasive weeds, and to control the plants that are established.  Once plants make it on “the weed list” landowner control or eradication is required by law. 

The Washington State Department of Agriculture (WSDA) regulates the quarantine list. It is illegal to buy, sell or offer for sale, some known invasive plants.  It is illegal to distribute seed packets, flower seed blends or “wildflower mixes” of these plants.

Want to learn more? 

For more information on noxious weeds, or noxious weed laws, please contact the King County Noxious Weed Control Program (206) 296-0290 or visit

Natural Connections:

WA State Noxious Weed Control Board website:

Consult the noxious weed list in your municipality, so you don't inadvertently plant invasive species in your garden.

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