Noxious weeds to watch out for this April

maria holding hogweed
Weed Specialist Maria Winkler holds a young giant hogweed. Always wear gloves when handling this plant.

“There it is!” On the shoreline of West Seattle’s Lincoln Park, Weed Specialist Maria Winkler points uphill past a wall of blackberries, fallen trees, and other brush. A recent landslide has formed a narrow shelf of mud above them. On the ledge, barely visible through shrubbery, a small plant with maple-like leaves is growing.

Young giant hogweed growing from a landslide deposit in Lincoln Park.

This is giant hogweed (Heracleum mantegazzianum), one of more than 75 top-priority noxious weeds the King County Noxious Weed Control Program is tasked with eradicating or controlling. Among them, giant hogweed is one of the highest priorities, a “Class A” noxious weed, and for good reason. The plant’s sap contains a dangerous toxin that, if it touches your skin and is then exposed to sunlight, will produce painful blisters and even scars. Giant hogweed’s stem also has stiff, dense hairs and raised maroon or purple blotches.

A giant hogweed stem, with stiff white hairs and purplish bumps.

Though the plant Maria has found is hardly two feet tall, a mature hogweed (usually about three years old) can reach 15 feet. By that time, its stems will be up to four inches wide and its sharply-incised compound leaves will be five feet across. From mid-May to July, mature plants produce a 2 ½ -foot wide umbrella-shaped cluster of small white flowers. Its many seeds can persist in the soil for up to ten years. In other words, it’s a dangerous plant to have in your backyard!

Mature giant hogweed in bloom.

The Noxious Weed Control Program has battled giant hogweed for years, and it’s had a significant impact. For instance, in 2003 Maria had more than 350 giant hogweed infestations in her region (one of eight in King County.) Now she has just over 100. “It’s definitely one of our success stories,” she says.

But, as the young plant above us makes only too clear, the Noxious Weed Control Program’s job isn’t over yet. Maria puts on her sunglasses, grabs her shovel, and starts to climb.

Weed Specialist Maria Winkler reaches the ledge.

Just down-trail from the giant hogweed, a cluster of native cow parsnips (Heracleum maximum) are growing. Cow parsnip looks like giant hogweed and is often confused with its invasive cousin. To make things more confusing, both plants like fertile, damp soil and tolerate shade. However, cow parsnip usually reaches only six feet tall with smaller flower clusters and leaves. Its leaves are also less deeply incised and have soft, shiny hairs on their undersides.

cow parsnip_1
A young native cow parsnip growing at Lincoln Park.

If you think you’ve found giant hogweed, please contact the Noxious Weed Control Program at 206-477-WEED (206-477-9333) or through this infestation report form. Program staff will be able to confirm the plant identification and give advice to the property owner on the best control method for that site. Always wear gloves and other protective clothing when working with giant hogweed.

Below are a few of the other noxious weeds to watch out for this season:

Garlic mustard (Alliaria petiolata) is a biennial noxious weed that can quickly spread in many environments, including stable forest understories. Its rounded to kidney-shaped leaves resemble those of many other species, but it can be distinguished by its relatively hairless leaves, its garlic smell when crushed, and its s-shaped root. Garlic mustard is both self- and cross-pollinating and produces many seeds, so it’s especially important to remove plants before they go to seed.

Garlic mustard growing at Bellevue’s Coal Creek Natural Area.

Because garlic mustard is so difficult to identify and control, please let us know if you find it!

A young garlic mustard infestation at Coal Creek.

Gorse (Ulex europaeus) is a dense, evergreen shrub with erect stems, spreading branches, and spine-like leaves. It can grow up to 15 feet tall and 30 feet wide, spreading outward so that it leaves a core of dead, dry, highly flammable vegetation. It produces many yellow pea-shaped flowers, usually between late February and April, but may have flowers at other times of year. One plant produces up to 18,000 seeds, which can persist in the soil for up to 40 years.

Gorse blooming at Lincoln Park. Note the thorny leaves and clustered pea-like flowers.

If you find gorse, please contact the Noxious Weed Control Program so that we can advise the property owner on the best control method for their site.

Scotch broom (Cytisus scoparius) is a noxious weed that looks similar to gorse. Both plants thrive in sandy or gravelly soils with plenty of moisture, and prefer sun but tolerate shade. However, Scotch broom has simple, small leaves instead of thorns, and its flowers are more spread out along its stems and bloom later than gorse. Because Scotch broom is so widespread in the county, control is recommended but not required.

scotch broom
Mature Scotch broom, with its characteristic small, simple leaves and more widely distributed flowers.

Perennial pepperweed (Lepidium latifolium) is, as the name suggests, a perennial that grows in a variety of habitats, from riparian areas to roadsides and pastures. The plant usually reaches one to three feet tall, with many stems emerging from a somewhat woody root crown. It has alternate, waxy, lance-shaped leaves and rounded clusters of milk-white flowers. It spreads through seed and spreading roots.

uprooted perennial pepperweed
An uprooted perennial pepperweed, presenting its stout, semi-woody root crown and waxy lanceolate leaves.

Young perennial pepperweed is sometimes confused with the native Douglas aster (Aster subspicatus), but the two species are easily distinguished by their flowers. To confirm your identification of a pepperweed plant, please wait until it blooms in June to September before pulling it.

If you find perennial pepperweed, please let us know. We can confirm the identification and advise the property owner on their best removal options.

Lesser celandine (Ficaria verna) is another tricky noxious weed. It is a perennial that grows in a variety of habitats, from damp forests to roadsides. It flowers briefly in early spring, when it reaches up to one foot tall with stems topped by single yellow flowers. Its leaves vary from oblong to triangle-shaped, growing both in rosettes and on stems. By June, the plant persists only as an underground system of roots and tubers. These tubers are easily spread by mowing and other forms of soil disturbance. It also spreads by bulbils and seed. Like Scotch broom, lesser celandine control is recommended but not required in King County.

Lesser celandine in bloom. Note oblong to triangle-shaped leaves and shorter sepals distinct from petals.


Lesser celandine infestation in West Seattle.

Lesser celandine is easily confused with the native marsh marigold (Caltha palustris). However, marsh marigold does not have tubers or bulbils, and has only sepals which resemble petals, while lesser celandine has distinct petals and sepals.

marsh marigold.jpg
Marsh marigold, a native plant that resembles the noxious weed lesser celandine, but is distinguished by its petal-like sepals, instead of green. Photo courtesy of Aaron Carlson / CC BY.

If you see any of the above noxious weeds, or something you think might be a noxious weed, please don’t hesitate to contact us at 206-477-WEED (206-477-9333) or through this the infestation report form.

For more information, please visit:

Thank you!