Common Name: poison hemlock Scientific Name: Conium maculatum Code: COMA2 Group: Dicot Family: Apiaceae Growth Habit: Forb/Herb Duration: Biennial WARNING: Poison Hemlock is extremely poisonous and should be handled with extreme care and NEVER ingested. It is recommended one wears gloves during handling and carefully monitoring children in areas of infestation.

What does it look like?

Poison hemlock is in the carrot family and its fern-like leaves are delicate and finely detailed, similar to that of wild carrot. When crushed, the leaves have an unpleasant odor. It is easy to identify poison hemlock by the stem that is ribbed, hollow, and marked with purplish streaks and splotches. The small white flowers grow in many umbrella-shaped clusters, each on its own stalk. First-year plants produce ground-level rosettes, while second-year plants grow from two to ten feet tall.

Poison hemlock reproduces only by seed, which can be scattered by wind, mud, animals, human clothing, shoes, machinery, and water. Seeds are fully developed by mid-June and ninety percent of seed dispersal generally occurs from September through December and some in late February. The seeds can germinate in a variety of conditions including a wide range of temperatures and light availability. Seeds can remain viable in soil for up to 3 years, and 85% can germinate as soon as they leave the parent plant.

With a long seed-dispersal period, seed dormancy, and non-specific germination requirements, poison hemlock seedlings can emerge almost every month of the year, except April, May, and July. Late winter and early spring have the highest rates of germination, with the majority of vegetative growth occurring in the winter months (Bossard et al. 2000).

Where does it grow?

Native to Europe, North Africa and Asia, poison hemlock has now spread throughout the United States, Canada, Australia, New Zealand, and South America. It currently exists throughout California in areas below 5,000 feet.

Poison hemlock tends to grow in dense patches along roadsides and fields, some meadows and pastures, and occasionally in riparian forests and floodplains. It prefers disturbed areas with moist soil and some shade, but can grow in dry, open areas as well (Bossard et al. 2000).

Is it in our watershed?

Poison hemlock has been observed growing in Petrolia near the Mattole Valley Community Center. It is important to be aware of this plant and report any sightings to keep it under control.

Why is it a problem?

Poison hemlock is an aggressive invasive that spreads rapidly after the rainy season, especially in areas that have been disturbed or cleared. It prevents growth of native plants by outcompeting and overshading, and also interferes with plant species important for agricultural production and feed for livestock.

Poison hemlock can be lethal to vertebrates such as livestock, wildlife, and humans, by respiratory paralysis after ingestion. Symptoms include nausea, vomiting, convulsions, loss of muscle power, pupil dilation, slowing of heartbeat, and eventual respiratory failure. In livestock, death usually occurs within two to three hours after ingestion. The leaves and seeds are often confused with those of edible seeds and spices, including carrots, and most human poisonings result from eating (Bossard et al. 2000).

How do you get rid of it?

Manual Removal: Hand pulling is quite effective, especially before seed sets and in the spring when soils are moist. Because this plant is a biennial, it is not necessary to remove the entire root system. If seeds and flowers have already present on the weed, pull it and place in plastic bag or similar container to prevent further spread.

Mechanical Removal: Mowing of poison hemlock in the spring has proven effective in killing mature plants, but regrowth can occur along with new seedling growth. A second mowing in late summer is recommended to eradicate remaining plants or re-sprouts. Due to the long seed viability, it may be necessary to conduct a third year of mowing (Bossard, 2000).

Author:
Unity Peterson, Invasive Plant Program Coordinator, Mattole Restoration Council, Petrolia, CA.

Photographs:
Unity Peterson, 2005

References:
Bossard, CC., J.M. Randall and M. Hoshovsky. (eds.) 2000. Invasive Plants of California’s Wildlands. University of California Press, Berkeley, CA. Online version: http://groups.ucanr.org/ceppc/Invasive_Plants_of_California's_Wildlands/