Weeds to Watch out for—August 2017

As we roll into August, plenty of noxious weeds across the county are blooming and going to seed, and those of us in the weed world are doing our best to keep up. Here’s what to watch out for this month:

Tansy ragwort (Senecio jacobaea) often grows in pastures, where it poses a serious threat to horses and cattle.

Starting off with regulated noxious weeds, first up is the infamous tansy ragwort (Senecio jacobaea). This Class B noxious weed is flowering and getting ready to seed right now. Tansy ragwort is especially worrisome because it’s toxic to horses and cattle, who are most likely to eat it when it’s dry and mixed with hay. Add to that the 150,000 seeds each plant produces, all of which can remain viable in the soil for up to 15 years, and you have one nasty plant.

You can identify tansy ragwort by:

  • First-year plants in low rosettes with ruffled leaves and often reddish stems
  • Second-year plants 2 to 4 feet (up to 6 feet) tall
  • Leaves dark green on top, whitish-green below
  • Mature ruffled leaves have deeply cut, blunt-toothed lobes
  • Clusters of small daisy-like flowers with orange-yellow centers and usually 13 yellow ray petals atop stems

For more information on tansy ragwort identification and control, visit these pages:

Tansy Ragwort Noxious Weed Alert

Tansy Ragwort Best Management Practices

You can also check out our July “Weed of the Month” post on tansy ragwort, and our recent post on noxious weeds and their look-alikes.

Centaurea stoebe
Spotted knapweed (Centaurea stoebe) in bloom. Photo by Matt Lavin / CC BY.

Next, spotted knapweed (Centaurea stoebe). This Class B noxious weed will also be blooming and starting to seed this month. A lover of well-drained soils in full sun, the short-lived perennial thrives in dry, disturbed sites throughout King County.

You can identify spotted knapweed by:

  • First-year plants in a basal rosette with deeply lobed, grayish-green leaves
  • Mature plants up to 5 feet tall with up to 20 upright branched stems
  • Stout taproot
  • Small, oval flower heads with pink or light purple flowers
  • Bracts that have distinct black spots with vertical lines below (bracts are those scale-like structures growing around the base of the flower head)

For more information on spotted knapweed identification and control, visit these pages:

Spotted Knapweed Noxious Weed Alert

Spotted Knapweed Best Management Practices

You can also check out our June “Weed of the Month” post on spotted knapweed, as well as a number of other blog posts.

Garden loosestrife (Lysimachia vulgaris) infestation at Marymoor Park on Lake Sammamish.

A number of our riparian and aquatic weeds are also flowering and seeding right now. For instance, the resilient garden loosestrife (Lysimachia vulgaris), a Class B noxious weed, is currently blooming along King County’s shores. You can identify garden loosestrife by:

  • Round erect stems covered in soft hairs, growing 3-6 (sometimes 10) feet tall
  • 3-5 inch long, ovate leaves in whorls of 3 (sometimes 2 or 4) with hairy undersides
  • Showy, yellow, 5-petaled primrose-like flowers in clusters at stem ends
  • Flower bases ringed by green sepals with distinct orange-brown edges
  • Dry, egg-shaped capsules containing a few seeds each
  • Creeping red rhizomes up to 15 feet long

For more information on garden loosestrife identification and control, visit these pages:

Garden Loosestrife Noxious Weed Alert

Garden Loosestrife Best Management Practices

You can also check out our August “Weed of the Month” post on garden loosestrife, as well as a number of other blog posts.

Purple loosestrife (Lythrum salicaria) has magenta flowers clustered in tall, dense spikes.

The somewhat similar purple loosestrife (Lythrum salicaria), another Class B noxious weed, is also flowering. You can identify this wetland perennial by:

  • Stiff, 4-sided stems growing usually 6-10 feet tall
  • Simple, smooth-edged leaves opposite or whorled attached directly to the stem
  • Numerous small magenta flowers with 5-7 petals in tightly-packed narrow spikes
  • Woody tap root, fibrous roots and rhizomes

For more information on purple loosestrife identification and control, visit this page:

Purple Loosestrife Noxious Weed Alert

Purple Loosestrife Best Management Practices

You can also check out our other blog posts on the plant.

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Phragmites (Phragmites australis). Photo courtesy of Washington State Noxious Weed Control Board.

Phragmites (Phragmites australis), or common reed, is a tall wetland grass that’s a Class B noxious weed. Washington State has both native and non-native strains of phragmites, so be sure to get expert identification before controlling this plant. Its seed head plumes are forming right now, though it spreads primarily through rhizomes. Here’s how to identify phragmites:

  • Perennial grass with tan, woody, rough and dull, rigid, hollow stems up to 15 feet tall, forming dense stands
  • Leaves flat, stiff and lanceolate, 8-16 inches long with bases 0.5-2.0 inches wide, gray-green when growing
  • Vigorous rhizomes more than 60 feet long, growing more than 6 feet each year
  • Purple-brown-silver seed head plumes at end of stalk, 6-20 inches long and up to 8 inches wide

For more information on phragmites identification and control, visit this page:

Phragmites Best Management Practices

Brazilian elodea (Egeria densa) in Lake Sammamish. The weed spreads through plant fragments in North America, making disturbances such as motorboats especially troublesome.

There are also a couple of regulated aquatic weeds to watch out for this month. Brazilian elodea (Egeria densa), a Class B noxious weed, is flowering right now, though it isn’t known to seed in North America and instead reproduces vegetatively. Here’s how to identify Brazilian elodea:

  • Perennial growing in up to 20 feet of water
  • Usually submergent, occasionally forming floating mats
  • Leaves appear smooth-edged, densely packed in whorls of 4 (up to 6)
  • Showy white flowers with 3 petals and yellow centers, fragrant, floating on water surface

For more information on Brazilian elodea identification and control, visit this page:

Brazilian Elodea Best Management Practices

parrot boat 2
Parrotfeather (Myriophyllum aquaticum) forms mats up to a foot above the water’s surface, looking like a miniature forest of pine trees.

Parrotfeather (Myriophyllum aquaticum), a species of milfoil that’s another Class B noxious weed, also reproduces vegetatively in North America. You can identify parrotfeather by:

  • Spikes of feather leaves growing up to a foot above the water’s surface, looking like miniature pine trees
  • Leaves on stems in whorls of 4-6
  • Underwater leaves less stiff
  • Robust stems with roots forming along them
  • Tiny, inconspicuous white flowers where leaves attach to stems above water

For more information on parrotfeather identification and control, visit this page:

Parrotfeather Weed Identification

Invasive knotweeds (Polygonum spp.)

Moving into the semi-regulated weeds, we have the invasive knotweeds (Polygonum x bohemicum, P. cuspidatum, P. sachalinense, P. polystachyum). These Class B noxious weeds are flowering and getting ready to seed. They’re incredibly difficult to get rid of, and control is required only in certain areas on the Green and Cedar Rivers and their tributaries because the plants are a big threat to riparian zones. Here’s how to identify the knotweeds:

  • Tall, bamboo-like perennials 4-12 feet tall that spread by rhizomes to form dense thickets
  • Stems round, cane-like, reddish-brown, thick and hollow
  • Stem nodes swollen, surrounded by thin papery sheaths
  • Leaves alternate, heart- to spade-shaped, bright green with smooth edges
  • Flowers small, creamy white to greenish white, appearing in showy, plume-like, branched clusters in leaf joints near stem ends in July and August
  • Fruit 3-sided, black, shiny
  • Rhizomes up to 23 feet long and 7 feet deep
  • Plants die back

For more information on invasive knotweed identification and control, visit these pages:

Knotweed Weed Alert

Knotweed Best Management Practices

Invasive Knotweed Brochure

Himalayan Knotweed Fact Sheet

Knotweed Biology and Control Slide Show

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Mature Canada thistle (Cirsium arvense) patch.

Last, keep an eye on two nonregulated weeds, both thistles that will be blooming and seeding this month. Canada thistle (Cirsium arvense) is a widespread perennial that reproduces both by seed and its extensive root system. Here’s how to identify Canada thistle:

  • Stems up to 5 feet tall, often hairless
  • Leaves long, lance-shaped, spiny, alternate on stems
  • Small purple flower heads in clusters
  • Bracts beneath flower heads lack spines
  • Roots white, running horizontally just below soil surface
  • Roots produce new plants, as do root fragments

For more information on Canada thistle identification and control, visit this page:

Canada Thistle Weed Identification

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Bull thistle (Cirsium vulgare). Photo courtesy of Harry Rose / CC BY 2.0.

Bull thistle (Cirsium vulgare) is a widespread biennial that reproduces by seed. Even though it looks nastier than Canada thistle, it’s actually an easier weed to handle. Here’s how to identify bull thistle:

  • Branching, erect, spiny stems up to 6 feet tall
  • Rosettes first year, flowering plants second year
  • Leaves with white hairs, deeply lobed, with sharp spines
  • Many 2-inch-wide, rose to purple flower heads with spines around base, clustered at branch ends

For more information on bull thistle identification and control, visit this page:

Bull Thistle Weed Alert

Bull Thistle Best Management Practices

As always, if you have any questions about these or other noxious weeds, feel free to contact us at 206-477-WEED (206-477-9333) or noxious.weeds@kingcounty.gov. We look forward to hearing from you!


  1. Hello everyone at Noxious Weeds. I want to tell you that these blogs are amazing! The information and pictures are great and I feel I can properly identify more weeds now that I read these blogs. As a horse and land owner in unincorporated King County, this information is critical for me. Thank you so much for the work you do!

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