What’s That Ferny Weed?

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Poison-hemlock spring foliage

A couple of weeds with “ferny” foliage are coming up in our neighborhood now and it’s important to know the difference. Poison-hemlock, Conium maculatum, as the name suggests is poisonous. That’s the plant pictured above. ALL parts of this plant are toxic to humans and animals, particularly if eaten. Don’t touch this plant and then get your hands near your face. Wear gloves and other protective gear to dig, pull, or otherwise remove it. The King County Noxious Weeds website has further information about poison-hemlock.

Positively identifying this plant isn’t as easy as one might wish. The best diagnostic when the plants are small is to look for purple spots on the stem and the leaf petioles. The spots were just barely visible on the plants I photographed up the road from our house, but I know from observing the site over the years that there’s a big patch of this nasty stuff growing there. The purple spots on the stem persist throughout the growing season so that’s your best clue later in the season, too.

In addition to looking for the purple spots, note that the leaves are twice pinnately divided and have small teeth on the edges. You can see that better in the photo below of a young plant before the leaves are fully open.

Another clue to identifying poison-hemlock in early spring is to look for last year’s stalks. They’re about a half inch or so in diameter, hollow, and nearly white. They’ll probably be laying flat on the ground with one end coming from under this year’s new foliage. Note that the old stems can remain poisonous until they’re decayed away.

Poison-hemlock spring foliage

Poison-hemlock will grow to be 6-8 feet tall by summer, when it will be capped with umbels of tiny white flowers. Find pictures of poison-hemlock in flower, along with more information about the plant, on my Pacific Northwest Wildflowers website. Later in the season you might confuse poison-hemlock with wild carrot, Daucus carrota. Wild carrot is smaller (rarely taller than 2-3 feet) and has hairy leaves and when in bloom has a flat-topped umbel characteristic of the parsley family. Both poison-hemlock and wild carrot are in the parsley family, Apiaceae.

Poison-hemlock is a Class B weed in Washington. That means it’s required to be removed from public land and rights-of-way and recommended that private landowners eradicate it as well.

Common Tansy spring foliage

Another common weed emerging this time of year is common tansy, Tanacetum vulgare. Its leaves are also pinnately divided, but only once. Tansy foliage has a pleasant, somewhat sweet small when crushed. It was introduced from Eurasia as a garden plant and has escaped into the wild, particularly roadsides and other disturbed sites.

Common Tansy spring foliage

Common tansy grows to about 3 feet tall and has button-like clusters of tiny yellow flowers. It’s a rhizomatous perennial (meaning it spreads by underground roots) in the aster family, Asteraceae. Tansy is a Class C noxious weed in Washington, which means it’s widespread and not required to be controlled, although individual counties can designate it for control.

Tansy stems often persist through the winter. They’re rather stiff, about 1/4 inch diameter, and brown — both narrower and darker than last year’s stems of poison-hemlock.

Find more pictures and information about common tansy on my wildflowers website.

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