A pretty, deadly weed

There's probably an important metaphor here, as well as a garden truism: Lots of weeds are remarkably good looking.

But proceed with caution: Many attractive weeds common in Western Washington need careful control. A particular villain is poison hemlock, botanically known as Conium maculatum. It's poisonous and occasionally fatal to humans and animals.

Though called hemlock, it's not related to the native western hemlock tree, Tsuga heterophylla. In fact, "hemlock" derives from an Anglo-Saxon word "hemleac" which means "shore plant." That describes the growth habit and preference for damp soils for this weed (although it also romps in dry, disturbed locations).

One theory is that European immigrants brought the plant over in the early 1800s as a garden ornamental, but this seems unlikely since the toxicity of the plant has been known for millennia. It's rumored to have killed Socrates in 329 BC. And some of its European common names alert the gardener — "poison parsley," "wode whistle," "spotted cowbane," and "poison snakeweed."

Accidental poisonings are often caused by mistaking poison hemlock for parsley, cilantro, fennel or anise. Humans seldom kill themselves with this one; the Centers for Disease Control report about 100,000 plant poisonings per year, most of them treatable. Plants are listed as the fourth most common cause of accidental poisoning.

The statistical grabber, though, is that 80 percent of the reported plant poisonings happen to children. And, a curious child could use the hollow stem of the poison hemlock as a straw or whistle with dangerous results. Alkaloids in the plant affect the central nervous system, and all parts of the plant contain the poison.

It's easy to learn to identify this common weed. Poison hemlock stands tall, 6-10 feet in maturity. Its height distinguishes it from parsley or cilantro. Stems are hollow and smooth (NOT hairy), marked at the lower stem end by mottled purple blotches and streaks. The purple streaking definitely identifies it. Flowers and leaves resemble Queen Anne's lace, with white flowers and finely divided fernlike leaves.

Poison hemlock is a biennial. It grows from seeds and produces, during the first year, a rosette of fernlike leaves close to the ground. The second season it bolts to form the tall, erect flowering stems. The white flowers develop into green, ridged seed capsules which turn brown when the seeds mature.

The leaves and flowers smell distinctive, rather "mousey" though it could also be called musty.

Do not allow this weed to go to seed. Wear gloves when handling it. Skin contact isn't as dangerous as ingesting it. However, you do want to limit your exposure to the plant and its oils. (A landscaper cutting weeds in a parking lot in downtown Seattle became ill after encountering hemlock and having the chopped bits land all over his face and skin.)

Don't put it into the compost — dead stalks can remain poisonous for two or three seasons.

Apply a registered herbicide to the rosette stages if you have a large patch.

Mary Robson is area horticulture agent for Washington State University/King County Cooperative Extension. She shares gardening tips every Wednesday. Her e-mail is gardeningtips@seattletimes.com