The following descriptions come from the King County, Washington, website. The phone number below for poison control is good nationwide.
“Poison-hemlock can be confused with wild carrot (Daucus carota, or Queen Anne’s Lace), as with many other members of the parsley family that resemble it. It has hairless hollow stalks with purple blotches. It can get quite tall, sometimes up to 8 feet or higher. It produces many umbrella-shaped flower clusters in an open and branching inflorescense.
“Poison-hemlock is acutely toxic to people and animals, with symptoms appearing 20 minutes to three hours after ingestion. All parts of the plant are poisonous and even the dead canes remain toxic for up to three years. The amount of toxin varies and tends to be higher in sunny areas. 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. One individual had a severe reaction after pulling plants on a hot day because the toxins were absorbed into her skin. The typical symptoms for humans include dilation of the pupils, dizziness, and trembling followed by slowing of the heartbeat, paralysis of the central nervous system, muscle paralysis, and death due to respiratory failure. For animals, symptoms include nervous trembling, salivation, lack of coordination, pupil dilation, rapid weak pulse, respiratory paralysis, coma, and sometimes death. For both people and animals, quick treatment can reverse the harm and typically there aren’t noticeable aftereffects. If you suspect poisoning from this plant, call for help immediately because the toxins are fast-acting — for people, call poison-control at 1-800-222-1222 or for animals, call your veterinarian.”