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King County urges landowners to watch for deadly noxious weeds this spring


Springtime is when dangerous and potentially deadly noxious weeds begin to sprout and bloom across the Pacific Northwest, and the King County Noxious Weed Program encourages everyone to be aware of these dangerous invaders.


While some telltale signs of spring are welcome in King County, such as longer daylight hours, other RSZ-giant-hogweedharbingers of the new season aren’t as warmly received – such as the return of dangerous and potentially deadly noxious weeds.

Poison-hemlock and giant hogweed are two of the more dangerous noxious weeds that start to grow during springtime in King County. The King County Noxious Weed Program encourages everyone to be aware of these toxic weed species, and urges caution when working around them this spring and summer.

Noxious weeds are non-native plants that impact natural resources, agriculture, and human health. The County’s noxious weed program is available to help residents learn to identify and control these and other harmful noxious weeds.

The list of species that property owners and public agencies are required or advised to control in King County is found on the County’s noxious weed website, Help is also available by calling the King County Noxious Weed Control Program at 206-477-9333.

Poison-hemlock (Conium maculatum) is acutely toxic to people and animals. In western Washington, it is common on roadsides and in open fields and natural areas. All parts of the plant are poisonous and even the dead canes remain toxic for up to three years. Eating the plant is the main danger, but it is also toxic to the skin and respiratory system. When digging or mowing large amounts of poison-hemlock, it is best to wear gloves and a mask or take frequent breaks to avoid becoming ill.

Young poison-hemlock can be confused with parsley and carrot greens, wild carrot and with many other members of the parsley family that resemble it. Poison-hemlock can be identified by its hairless, hollow stems with purple or reddish blotches and its strong, musty odor. First-year plants are low and clump-forming and second-year flowering plants are 5 to 10 feet tall. The flowers are white and arranged in numerous, small, umbrella-shaped clusters. Plants sprout in fall, grow over the winter and early spring, and flower May to June.

King County noxious weed experts recommend landowners remove this plant from their property. If the plant is near where people are growing or collecting food, people should notify the property manager or agency in charge as soon as possible.  This is especially important in p-patch or community gardens where new gardeners or newcomers to the area may be unfamiliar with this plant.

Giant hogweed (Heracleum mantegazzianum) is a Class A Noxious Weed in Washington, and public and private landowners are required to control this plant when it occurs on their land. Because of the risk of injury when handling this plant and the difficulty of distinguishing it from the native plant cow parsnip, noxious weed experts recommend landowners contact the King County Noxious Weed Control Program for a positive identification and advice on control methods before removing.

A public health hazard, hogweed's clear, watery sap has toxins that cause photo-dermatitis. Skin contact followed by exposure to sunlight produces painful, burning blisters that may develop into purplish or blackened scars. People should always avoid skin contact with this plant and wear proper clothing, shoes, and eye protection when attempting any control measures.

True to its name, giant hogweed reaches a height of 10 to 15 feet when in flower and has hollow stems that are 2 to 4 inches in diameter, with dark reddish-purple raised spots and stiff bristle-like hairs. Coarse white hairs are also at the base of the leaf stalk. The sharply toothed compound leaves grow up to 5 feet in width.

Giant hogweed blooms from mid-May through July, with numerous white flowers densely packed in a large umbrella-shaped head that is up to 2-½ feet in diameter across its flat top. Similar in appearance to native cow parsnip that is often found on trails in the mountains, only much larger, the purplish blotches on giant hogweed are more raised and bumpy, while hairs on the leaves’ undersurface are shorter.