California Invasive Plant Council

Conium maculatum

Common names: poison-hemlock

Conium maculatum (poison-hemlock) is a biennial forb (family Apiaceae). Poison-hemlock has spread throughout California in areas below 5,000 feet (1,500 m) elevation, excluding the Great Basin and Desert provinces and is commonly found in dense patches along roadsides and fields. It also thrives in meadows and pastures and is occasionally found in riparian forests and flood plains, but prefers disturbed areas. All parts of poison-hemlock are toxic to humans and animals when ingested; handling plants can cause contact dermatitis in some people. Poison-hemlock can spread quickly after the rainy season in areas that have been cleared or disturbed. Once established, it is highly competitive and prevents establishment of native plants by over-shading.

Cal-IPC Rating: Moderate?

Explanation of Cal-IPC ratings

• High – These species have severe ecological impacts on physical processes, plant and animal communities, and vegetation structure. Their reproductive biology and other attributes are conducive to moderate to high rates of dispersal and establishment. Most are widely distributed ecologically.
• Moderate – These species have substantial and apparent-but generally not severe-ecological impacts on physical processes, plant and animal communities, and vegetation structure. Their reproductive biology and other attributes are conducive to moderate to high rates of dispersal, though establishment is generally dependent upon ecological disturbance. Ecological amplitude and distribution may range from limited to widespread.
• Limited – These species are invasive but their ecological impacts are minor on a statewide level or there was not enough information to justify a higher score. Their reproductive biology and other attributes result in low to moderate rates of invasiveness. Ecological amplitude and distribution are generally limited, but these species may be locally persistent and problematic.
• Alert – An Alert is listed on species with High or Moderate impacts that have limited distribution in California, but may have the potential to spread much further.
• Watch – These species have been assessed as posing a high risk of becoming invasive in the future in California.

CDFA Rating: None?

Explanation of CDFA ratings

• A – A pest of known economic or environmental detriment and is either not known to be established in California or it is present in a limited distribution that allows for the possibility of eradication or successful containment.
• B – A pest of known economic or environmental detriment and, if present in California, it is of limited distribution.
• C – A pest of known economic or environmental detriment and, if present in California, it is usually widespread.
• D – An organism known to be of little or no economic or environmental detriment, to have an extremely low likelihood of weediness, or is known to be a parasite or predator.
• Q – An organism or disorder suspected to be of economic or environmental detriment, but whose status is uncertain because of incomplete identification or inadequate information.
• * – An asterisk next to the rating indicates that a plant is included in the CCR Section 4500 list of California State Noxious Weeds.
• -* – This plant is included in the CCR Section 4500 list of California State Noxious Weeds, but is otherwise not rated.

More details

Assessment(s)

Plant Assessment Form - Information gathered by Cal-IPC on the impacts, rate of spread, and distribution of invasive plants in California. Does not include management information.

Species ID Card

Invasive Species ID Card - To support field identification of early detection species, Cal-IPC has designed a set of Species ID cards that can be downloaded, printed double-sided, and trimmed to size.

Weed Management Notes

• Management Notes - Information on management techniques and effectiveness from the University of California Cooperative Extension’s Weed Research & Information Center.

Cal-IPC Newsletter Articles

• Roundup of Arundo projects reveals commitment, strategic weakness. Kelly, Mike. Vol 07, Issue 2
• Results of the CalEPPC questionnaire at Symposium ’98 in Ontario. DiTomaso, Joe. Vol 06, Issue 4
• Highways as corridors of dispersal. Madison, John. Vol 04, Issue 1

Cal-IPC Symposium Presentations

Presentations are linked where available. Where a presentation is not available, find more information by reading the abstract in the Cal-IPC Symposia Archive.

• Cross-county collaboration on weed management for success on Delta habitat restorations. Davis, Elizabeth; Weise, Ben; Williams, Amy (2022)
• Mapping weeds and rare plants to inform management and conservation in the Zaca and Jesusita fires. Calloway, Stephanie; Schneider, Heather; Knapp, Denise (2018)
• Coyote Creek floodplain reclamation project: Re-establishing native plant habitat. Codianne, Jennifer; Dumont, Leo (2007)
• A test of repeat flaming as a control for poison hemlock (Conium maculatum), Cape ivy (Delairea odorata), and periwinkle (Vinca major). Bossard, Carla; Moore, Ken; Chabre, Cammy; Woolfolk, Andrea; King, Jorden; Johanek, Dana (2005)
• Efficacy and safety of new herbicides on the horizon. DiTomaso, Joseph M. (2005)
• Fire working group. Clines, Joanna; DiTomaso, Joe; Goode, Suzanne (2004)
• Manual removal of cape ivy in Volunteer Canyon Bolinas Lagoon Preserve, Audubon Canyon Ranch, Stinson Beach, California. Blumin, Len; Gluesenkamp, Daniel (2002)
• Potentially allelopathic effects of poison hemlock (Conium maculatum) on native plant revegetation at Wilder Ranch State Park. Hillman, Jannell M. (1997)

Other Conium maculatum Information

• Bugwood - National database from the Center for Invasive Species and Ecosystem Health at the University of Georgia.
• CalPhotos - Images of plants taken mostly in California.
• Calflora - Distribution map and records of this species in California.
• CalWeedMapper - Distribution map of this species in California with ability to determine regional priorities.
• EDDMapS - Distribution of this species in North America.
• Jepson Interchange - Information on this plant's taxonomy, biology, and distribution from UC Berkeley's Jepson Herbarium.
• USDA PLANTS Database - Information on identification and distribution, with links to websites in individual states.

Additional photos