Sunday, May 13, 2001 - Page updated at 12:00 AM

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The Practical Gardener / Mary Robson

Some weeds more than a nuisance; they can be killers

Special to The Seattle Times

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Q: Are any Northwest weeds poisonous?

A: Most of the time, weeds simply bother gardeners by crowding out more desirable plants in lawns and shrub beds. We seldom think of weeds as being dangerous, but several in the Northwest have chemical components that can harm animals or humans if eaten. These generally are found on the Washington State Noxious Weed List, with control required when they are seen.

Tansy ragwort (Tanacetum vulgare) for instance, produces alkaloids that can kill cattle and horses, and it must be controlled in fields and pastures.

One that's vital to know when considering danger to people is poison hemlock (Conium maculatum). This toxic plant was introduced to North America from Europe and has often been mistaken for a garden ornamental. The plant is attractive but can be lethal to people and animals.

Poison hemlock is a member of the wild carrot family and is common along roadsides, waterways and in cultivated areas. It resembles anise or wild parsley and is classified in the family Umbelliferae, the parsley family. All parts of this plant are poisonous to humans: leaves, stems, roots and seeds (particularly the seeds).

Poison hemlock contains volatile alkaloids that have been used as a poison since ancient times. The famous incident of Socrates' death in Athens in 329 B.C. occurred when he was supposedly given the juice of the poison hemlock plant to drink. Children have been sickened by pulling the hollow stems and using them as straws.

How can you identify it? Look closely at the stem of any plant you may suspect of being poison hemlock. The stem is hollow, smooth (not hairy) and marked with purple streaks and blotches.

The purple mottling makes the stems appear menacing in a fairy-tale sort of way. The finely divided leaves are fern-like. Flowers are lacy and white, appearing in early summer, from late May to August. The weed wil be growing and leafing out in early May in alleys, waste places and gardens. An excellent color photo of this is posted on the King County Noxious Weed Board Web site: in the "Environment" category. Click on "Noxious Weeds." Photos of tansy ragwort, mentioned above, and many other plants are also available there.

A water-dwelling relative of poison hemlock, called western water hemlock (Cicuta douglasii), grows in very wet meadows and along streams. Its flowers resemble the poison hemlock and its hollow roots are particularly poisonous. Strangely, waterfowl eat the seeds of this plant, but it cannot be tolerated by livestock or humans.

Poison hemlock is a biennial; it grows from seeds and produces, during the first year, a rosette of fern-like leaves close to the ground. The second season it bolts to form the tall, erect flowering stems from 4 to 8 feet tall. The white flowers develop into green, ridged-seed capsules that turn brown when the seeds mature.

Poison hemlock must be removed. Dig it out, and do not allow this weed to go to seed. Wear gloves when handling it. Don't put it into the compost - dead stalks can remain poisonous for two or three seasons. Don't incinerate it (don't inhale the smoke). Apply a registered herbicide to the rosette stages if you have a large patch. The herbicide 2,4-D applied to the early stages of growth will kill it.

If you need more help identifying this or other weeds, contact your local Noxious Weed Control Board. They will help with photos and information about getting rid of the weed. Call 206-296-0290 in King County; 253-798-7263 in Pierce County; 360-862-7523 in Snohomish County.

Mary Robson is area horticulture agent for Washington State University/King County Cooperative Extension.


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