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'Noxious Weed' is the traditional, legal term for nonnative plants that are proven to have a negative impact on the environment, and are highly destructive, competitive, and/or difficult to control or eliminate.

These invasive species arrive in the Northwest in a variety of ways; as ornamental garden plants, wildflower seed mixes, agricultural crops, contaminated hay or seed, aquariums, or water gardens.


Once established, noxious weeds aggressively displace desirable vegetation, reduce agricultural productivity, compromise public health and safety, lower property values, increase flood hazards, and destroy native plant and animal habitat.



The Top Ten Noxious Weeds That Threaten Pierce County.


Common Fennel
  • Licorice scented perennial 4-10 ft. tall.
  • Leaves are dark green and feathery.
  • Bears umbrella shaped clusters of small yellow flowers.
  • Tap roots can reach depths of 10 feet.
  • Out competes native plants and reduces native wildlife habitat.
  • Once establshed it is difficult to control, due to its strong competitive abilities and persistent seed bank.

Dalmation Toadflax
  • Perennial herb 2.5 to 5 feet tall.
  • Waxy, heart shaped, light green leaves.
  • Bright yellow flowers with a tinge of orange in the center, looks like a snapdragon.
  • A persistent, aggressive invader, pushing out native grasses and other perennials.
  • Rapidly colonizes open sites.
  • Contains poisonous alkaloid, toxic to livestock.

Wild Chervil
  • Biennial plant grows from 1to 4 ft. tall.
  • Has hollow ridged stemswith fern-like leaves.
  • Bears small white umbrella shaped flower clusters.
  • Poses a serious threat to native plants and agriculture .
  • Is highly adaptable and will grow in almost any type of soil.
  • It has an aggressive growth habit, quickly creating monocultures.
  • Most of Pierce County is highly susceptible to Wild Chervil.

Tansy Ragwort

  • Invasive, persistent, toxic weed
  • 1st year: rosette stage; darkgreen basal leaves, ruffled in appearance
  • 2nd year: one or more flowering stems bolt up to 6 ft. tall
  • Produces numerous yellow flowering heads which are daisy-like in appearance with 13 petals
  • Contains toxins that cause chronic irreversible liver damage; a threat to livestock and agriculture.

Poison Hemlock

  • Toxic biennial plant up to 8 ft. tall.
  • 1st year: basal rosette with fern like leaves.
  • 2nd year: produces tall stem with distinctive purplish blotches and dark green, glossy, leaves.
  • Flowers are flat-topped clusters of small, white, flowers.
  • All plant parts are extremely poisonous to humans and livestock.
  • Wear gloves and protective clothing when handling this plant, can cause contact dermatitis.
  • Do not burn or compost this plant.


  • There are 3 species of knapweed that threaten Pierce County: spotted, diffuse, and meadow knapweed.
  • 1st year: rosette form.
  • 2nd year: flowering stalk elongates.
  • Easily identified by its bracts (the leaf-like structure at the base of the bloom) arranged in an imbricate pattern, like shingles on a roof.
  • Bracts are spine tipped or scaly. Flowers are lavender to pink and sometimes white.
  • Very aggressive species, one of the most dominant weeds of the west, can infest large areas very quickly.
  • Contains phytotoxins which inhibit the growth of surrounding native plants, allowing knapweed to outcompete and form monocultures.
  • Has very long tap root.

  • A dense, spiny, evergreen shrub, 3-10 ft. tall, closely resembles scotch broom.
  • Aggressive, invasive plant crowds out other vegetation, forming impenetrable stands of dense thorny growth.
  • Has sharp thorn-like leaves up to 3 in. long.
  • Clusters of bright yellow, pea-like flowers form into inch-long pods containing 1 – 4 seeds.
  • Plants form a center of dry, dead vegetation, that coupled with its high oil content creates a serious fire hazard.

Perennial Pepperweed
  •  Bushy plant about 3 ft. tall.
  • Waxy leaves & rounded clusters of small white flower at branch tips.
  • Aggressive, non-native plant displaces native species.
  • Poses serious threat to natural areas.

Giant Hogweed
  • Huge perennial plant from15-20 ft. tall.
  • Hollow stems 2-4 in. thick with dark reddish, bumpy blotches.
  • Its enormous compound leaves (up to 5ft. wide) are deeply divided and edges are jagged.
  • White umbrella-like, flat topped flower-heads can reach up to 2.5 ft. wide.
  • The toxic sap from this plant causes severe burning and scarring which may last years.
  • Do not handle without gloves, goggles and protective clothing.
  • Watery sap can squirt up to 5 ft when stem is cut or broken.

Dyers Woad
  •  An aggressive weed, grows from 1-4 feet tall.
  • 1st year: rosette stage: bluish green succulent leaves with fine hairs.
  • 2nd year: numerous stiff stalks bolt up bearing small yellow flowers.
  • Contains a germination inhibitor chemical, which limits the growth of nearby plants, giving Dyers Woad seedlings a growth advantage over desirable native plants.