Top Priority Noxious Weeds on Vashon Island this Month

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With the Vashon Strawberry Festival coming up and the Unofficial Mayor Contest in full swing, Vashon Island might seem like an idyllic world of its own, separated from the rest of King County. In some ways, it is; but when it comes to noxious weeds, the island’s mainland connection is clear.

The residents of Vashon struggle with some of the same regulated noxious weeds as their counterparts in other areas of the county. In May, we highlighted the island’s shiny geranium (Geranium lucidum) troubles after Weed Specialists Maria Winkler and Minwook Park discovered a number of alarmingly large patches there. This time of year, three other weeds are in full swing: giant hogweed (Heracleum mantegazzianum), tansy ragwort (Senecio jacobaea), and perennial pepperweed (Lepidium latifolium).


Shiny geranium grows throughout Vashon Island. It has two peak blooms per year: in spring and fall.

On July 3rd, I traveled with Maria to Vashon to find these high-priority weeds. Our first stop: a ravine on the east part of the island, where she had discovered an infestation of giant hogweed (Heracleum mantegazzianum). Giant hogweed is a menacing plant. It grows up to 15 feet tall, with 3-5-foot-wide incised leaves, reddish purple bumps and stiff white hairs on its stems, and 2-foot-wide umbrella-shaped clusters of white flowers. Most important, in sunlight the sap can cause severe skin blisters and even scars.

For more information on giant hogweed identification and control, visit our website.

Giant hogweed grows to 15 feet tall and produces 2-foot-wide clusters of small white flowers.
Giant hogweed has 3-5-foot-wide, deeply incised leaves.
Giant hogweed’s stems have reddish-purple bumps and stiff white hairs. Watch out for the sap, which can cause blisters and even scars!
Giant hogweed is a perennial, usually flowering in the third year or later. Mature plants on Vashon are currently at the tail end of their bloom and going to seed.
Weed Specialist Maria Winkler surrounded by giant hogweed plants (note the large incised leaves in the foreground).

Without a doubt the most widespread regulated noxious weed on Vashon (and many other parts of King County) is tansy ragwort (Senecio jacobaea). This plant is a biennial that often grows in rural areas, such as in pastures and on roadsides. Like giant hogweed, tansy ragwort is an especially big concern because it’s toxic. Animals that eat the plant can sicken and even die. Fortunately, its bitter taste repels many of them. However, when it’s cut and dried (e.g. in hay), it loses its bitter taste while keeping its toxicity. That’s when the weed is an especially big worry.

For more information on tansy ragwort identification and control, visit our website.

Tansy ragwort is toxic to animals. Eating it can make them sick or even kill them.
In its first year, tansy ragwort grows as a basal rosette of deeply ruffled leaves.
In its second year, tansy ragwort grows 2-4 feet tall and produces clusters of daisy-like flowers with 13 yellow ray petals and an orange-yellow center.
Tansy ragwort blooming on Vashon Island, June 3, 2018.
common tansy
One of tansy ragwort’s look-alikes is common tansy (Tanacetum vulgare). From a distance, it’s easy to confuse this non-regulated noxious weed with tansy ragwort.
Up close, though, common tansy has distinct fern-like leaves.
Common tansy also has button-like flowers with no ray petals.

Tansy ragwort is just starting to bloom on Vashon (a bit behind those in other parts of the county). Maria and I found the weed growing in pastures and on roadsides, though on some properties it had disappeared since her last visit. Kudos to all the Vashon landowners controlling their tansy ragwort!

Last up, perennial pepperweed (Lepidium latifolium) is a usually 1-3-foot-tall (sometimes 6), multi-stemmed plant that grows in wet areas, farms, pastures, roadsides, and other habitats. It thrives especially in salty sites. True to form, on Vashon it grows mostly on beaches and in wetlands along the coast.

Weed Specialist Maria Winkler presents a perennial pepperweed plant on Vashon Island.

This time of year, many mature perennial pepperweed plants are producing dense rounded clusters of small white flowers at stem ends. These flowers help to distinguish it from one common look-alike, Puget Sound gumweed (Grindelia integrifolia). You can also identify perennial pepperweed by its hairless, waxy, bright green or gray-green leaves with white mid-veins and slightly serrated edges; and by its semi-woody root crown above deep roots.

For more information on perennial pepperweed identification and control, visit our website.

Perennial pepperweed has hairless, waxy, bright green or gray-green leaves with white mid-veins and slightly serrated edges.
Perennial pepperweed produces rounded clusters of small white flowers at stem ends.


Native species Puget Sound gumweed (Grindelia integrifolia) looks similar to perennial pepperweed, but it only reaches 1/2 to 2 1/2 feet tall and its stem leaves lack stalks.
grindelia integrifolia flowers_CC
Native species Puget Sound gumweed is definitely easiest to distinguish from pepperweed when in bloom, with composite yellow flowers that have a sticky white “gum.” Photo by Bureau of Land Management / CC BY 2.0.

Maria and I found perennial pepperweed growing in a variety of habitats, including beaches, wetlands, and a roadside waste pile. If you find pepperweed blooming on your property, pull it now before it’s too late!

Perennial pepperweed grows out of a junk pile on Vashon Island, July 3, 2018.

Enjoy the summer, wherever you are in King County, but keep an eye out for those noxious weeds, too!

For questions or more information, feel free to contact us at (206) 477-9333 or