Poison Hemlock Identification: Purple-Blotched Stems

The feature that identifies poison hemlock (Conium maculatum) is the stem: smooth, hairless, hollow between the nodes and marked with purple or red spots and streaks. Penn State Extension calls the purple spotting very distinctive, and King County's noxious weed program uses exactly that smooth, spotted stem as the test against the plants it gets confused with, which have hairy stems.

Everything else is supporting evidence. The plant is biennial: in year one it is a low rosette of dark green leaves that most people walk past, and in year two it throws up a branching stalk that University of Maryland Extension records at six to ten feet, carrying fern-like finely divided leaves and flat clusters of small white flowers from June into August. Crushed foliage smells strongly musty.

Do not taste any part of this plant to test it, and keep children and livestock away. All parts are poisonous, and Penn State notes the roots and seeds contain the highest concentration of alkaloids.

Several roadside plants carry flat white flower clusters, and that is where mistakes happen. King County lists Queen Anne's lace and water hemlock among the plants poison hemlock is confused with, and gives the distinguishing feature as the completely smooth stem with purple or red spots — the lookalikes have hairy stems.

So, at the stem:

• Hairless with purple blotches — treat it as poison hemlock
• Hairy, green, unspotted — not poison hemlock, but that alone does not make a plant safe

The carrot family contains several plants that cause burns or poisoning, so "not hemlock" is not the same as "harmless." Penn State notes poison hemlock sap is both toxic and a skin irritant, and University of Maryland Extension records that contact causes dermatitis in susceptible people.

Never eat a wild plant on the strength of a stem check, a photo or an app result. Deciding a carrot-family plant is edible needs an expert with the actual plant in hand, root included.

King County's guidance for working near poison hemlock is to wear gloves, long sleeves and long pants, and to wash any skin that contacts the plant with soap and warm water as soon as possible.

Two further cautions from the same source: do not burn the plants, and cover your mouth and nose with a mask if you mow or string-trim it, because mowing is how plant material ends up breathed in or in your mouth.

If you want it gone, contact your county extension office or county noxious weed program before you start. They hold the control recommendations for your state, and in many states poison hemlock is a listed noxious weed with rules attached to it. Seed production is the reason to act early rather than late — King County records up to 40,000 seeds per plant, viable up to six years, which is why a single overlooked plant turns into a patch.

Swallowed any amount: this is an emergency. King County states that eating even a small amount of any part of this plant can kill people, livestock and wildlife. Call 911 if the person is unconscious, having a seizure or struggling to breathe. Otherwise call Poison Control at 1-800-222-1222 — free, confidential, answered around the clock — or use the webPOISONCONTROL online tool.

Skin contact: Poison Control's first-aid guidance is to remove contaminated clothing and rinse the skin with room-temperature running water for at least 15 minutes, then call for advice. King County adds washing with soap and warm water as soon as possible.

Breathed in dust or smoke: move to fresh air immediately, ventilate the area, then call Poison Control.

Livestock or pets: call your veterinarian now and say what the animal ate and roughly how much. Do not wait for symptoms to appear.

If you want a second opinion — from the app, an extension office, or a local plant group — take these shots from a distance, without handling the plant:

• The stem at mid-height, close enough to show whether it is hairy and whether there are purple blotches
• The whole plant with something for scale
• One leaf laid flat in frame, still attached to the plant
• The flower cluster from above
• The base of the plant where it leaves the ground

The stem photo is the one that decides it, and it is the one people most often skip. With the app, photograph it and the app identifies it. For a toxic plant, treat that answer as the start of a decision rather than the end of one, and confirm anything that matters with your county extension office before you cut, mow or pull.