Who’s Who? Noxious Weeds and their Look-Alikes


Perennial pepperweed can be hard to find when growing among cattails—and look-alikes make the job even harder.

Weed Specialist Mary Fee paces through the wetlands at Dumas Bay Wildlife Sanctuary, scanning the cattails around her. Every few dozen feet she stops to inspect a smaller plant growing among them. She’s looking for perennial pepperweed (Lepidium latifolium), a Class B Noxious Weed in King County. The plant is hard enough to find among the dense wetland foliage, but that’s not Mary’s only problem: like many other noxious weeds, perennial pepperweed has a number of deceptive look-alikes. These plants can make identifying noxious weeds a tough job!

Perennial pepperweed is a 2- to 6-foot-tall herbaceous perennial with many stems that grow from a woody root crown. You’ll often find it growing in salty areas, such as beaches, marshes, and wetlands, but it can adapt to a wide range of areas. Its leaves are waxy, serrated or entire, have distinct whitish mid-veins, and appear alternate on the stems. Basal leaves are lance-shaped and up to 12 inches long with stalks nearly the same length, while stem leaves are smaller with shorter stalks. Pepperweed grows from a rosette and blooms June-September, when dense rounded clusters of small white flowers appear near branch ends. The plant spreads mainly through root fragments, though it also produces seeds in small egg-shaped pods.

At Dumas Bay, we find two plants that might be confused with perennial pepperweed. The first is Puget Sound gumweed (Grindelia integrifolia) which, despite its name, is native to the Pacific Northwest. It’s easy to mix up this plant with pepperweed. Like pepperweed, gumweed is a usually multi-stemmed perennial with a woody root crown and serrated or entire leaves growing alternate on its stems. It also occurs in many of the same habitats: salt marshes, beaches, and other shoreline areas. Luckily, when the two plants are in bloom they’re easy to tell apart: gumweed has yellow composite flowers with bracts covered in a white, sticky “gum” (hence the name). If you find a plant that isn’t blooming, check its leaves: unlike pepperweed, gumweed’s stem leaves lack stalks. At ½ to 2 ½ feet tall, the whole plant is also usually shorter than pepperweed.

You might also confuse perennial pepperweed with bittersweet nightshade (Solanum dulcamara), a widespread invasive plant (and “Weed of Concern”) in King County. While bittersweet nightshade often appears as a vine, you can also find it growing as a standalone shrub. Unlike the two above plants, nightshade’s leaves tend to have lobes near their bases. The roots also grow mostly horizontally, in contrast to the deep root systems of pepperweed and gumweed. And again, this plant’s flowers clearly distinguish it from the above-mentioned look-alikes: bittersweet nightshade has small star-shaped purple flowers with central yellow cones that appear between mid-May and September. It also produces numerous bright red berries that are somewhat toxic to people and animals (though not as dangerous as those of deadly nightshade (Atropa belladonna)).

Later that day, Mary and I visit a tansy ragwort (Senecio jacobaea) infestation just southeast of Saghalie Park in Federal Way. Scattered on a slope we find small plants sprouting through a layer of wood chips.  They look just like tansy ragwort—only something’s not quite right.

Tansy ragwort is a generally biennial plant that spends its first year as a basal rosette with ruffled leaves. In the second year, the plant reaches up to 6 feet tall. Its ruffly-looking leaves have deeply cut, blunt-toothed lobes, and are dark green above and whitish-green below. Mature plants flower between June and October, producing clusters of bright yellow daisy-like flowers—usually with 13 petals—at the ends of its stems. One plant can produce up to 150,000 wind-dispersed seeds, which remain viable in soil for up to 15 years.

Tansy ragwort is an especially dangerous plant because it often grows in pastures and is toxic to people and animals. While livestock tend to avoid live tansy ragwort plants because of their bitter taste, when plants are cut and dried with hay they lose their bitterness—but not their toxicity.

The first plant we find is common groundsel (Senecio vulgaris). Like tansy ragwort, this winter annual starts life as a basal rosette and mature plants have deeply lobed leaves. However, common groundsel only reaches 5-10 inches tall at maturity, and its yellow flower heads lack rays.

We also find woodland groundsel (Senecio sylvaticus), which looks even more similar to tansy ragwort. Like common groundsel, woodland groundsel is an annual, but mature plants reach up to 2½ feet tall. Its yellow flower heads have ray flowers, as do those of tansy ragwort, but woodland groundsel’s ray flowers are tiny—less than 2 mm long—while tansy ragwort’s are 4-10 mm long. When not in flower, you might also be able to distinguish the two groundsels from tansy ragwort by their leaves: tansy ragwort’s ruffled leaves have more of a three-dimensional appearance, while the groundsels’ deeply lobed leaves are flatter.

Tansy ragwort is also easily confused with common tansy (Tanacetum vulgare), a Class C noxious weed in King County. This perennial grows 2-6 feet tall and produces bright yellow flowers at about the same time as tansy ragwort does. But up close, they’re easier to tell apart. Common tansy’s flowers look like buttons and lack ray flowers. Its alternate leaves also appear much more fern-like, in contrast to tansy ragwort’s deeply lobed, ruffled leaves.

A final tansy ragwort look-alike is common St. Johnswort (Hypericum perforatum), another Class C noxious weed. Common St. Johnswort is an herbaceous perennial that usually reaches 1-3 feet tall. Its yellow, star-shaped flowers have five petals, and its oval leaves have distinct black or transparent dots.

Throughout the day, Mary and I also visit a series of goatsrue (Galega officinalis) infestations. Goatsrue is a 2- to 6-foot-tall herbaceous perennial with multiple hollow, upright stems growing from a deep taproot. It has alternate leaves with 13-21 leaflets. From June to October, purple to white pea-like flowers appear in clusters at stem ends.

goatsrue flowering
Goatsrue has alternate leaves with 13-21 leaflets. From June to October, purple to white pea-like flowers appear in clusters at stem ends.
goatsrue leaves
Note the absence of tendrils at leaf end.
Goatsrue has white to purple pea-like flowers.

Goatsrue only occurs in a few places in King County, but it has multiple look-alikes that are more common. For instance, you might confuse goatsrue with one of the vetch species growing in our area. However, goatsrue tends to grow much more upright than the clambering form of vetch, and goatsrue lacks the tendrils at leaf tips shared by vetch species.

Goatsrue also looks very similar to wild licorice (Glycyrrhiza lepidota), a native perennial. Wild licorice’s leaves are somewhat similar to those of goatsrue, as are its cream-colored flowers. However, wild licorice usually reaches only 3 feet tall and has solid stems, in contrast to the hollow stems of goatsrue. Wild licorice also has seed pods with hooked bristles, while goatsrue has smooth seed pods.

Not sure if you’ve found a noxious weed? Don’t worry! Send us a photo by emailing us at noxious.weeds@kingcounty.gov or mailing it to: King County Noxious Weed Control Program, 201 S Jackson St, Suite 600, Seattle WA 98104. And feel free to call us at 206-477-WEED (206-477-9333)!