Conium maculatum

Conium maculatum_poison hemlock_plantb_JM DiTomaso
Photo courtesy Joseph DiTomaso

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.

Rating: Moderate

Cal-IPC Resources

  • 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.
  • CalWeedMapper - Statewide maps, climate models, and reports.
  • Cal-IPC News - Our quarterly newsletter. Each issue is available as a pdf.
  • Cal-IPC Symposium Proceedings - Presentations and papers from our annual Symposium.
  • Don't Plant a Pest! - Select your region to find non-invasive alternatives to ornamental species. Also see our statewide brochure on trees.

Other Resources

Symposium Presentations

  • Codianne, J. and L. Dumont (2007). Coyote Creek floodplain reclamation project: Re-establishing native plant habitat. California Invasive Plant Council Symposium 2007. San Diego, CA, California Invasive Plant Council.
  • Blumin, L. and D. Gluesenkamp (2002). Manual removal of cape ivy in volunteer canyon – Bolinas Lagoon Preserve, Audubon Canyon Ranch, Stinson Beach, California. California Exotic Pest Plant Council Symposium 2002. Sacramento, California, CA.
  • Bossard, C., K. Moore, et al. (2005). A test of repeat flaming as a control for poison hemlock (Conium maculatum), Cape ivy (Delairia odorata), and periwinkle (Vinca major). California Invasive Plant Council Symposium 2005. Chico, CA.
  • Clines, J., J. DiTomaso, et al. (2004). Fire working group. California Invasive Plant Council Symposium 2004. Ventura, CA.
  • DiTomaso, J. M. (2005). Efficacy and safety of new herbicides on the horizon. California Invasive Plant Council Symposium 2005. Chico, California, CA.
  • Hillman, J. M. (1997). Potentially allelopathic effects of poison hemlock (Conium maculatum) on native plant revetation at Wilder Ranch State Park. California Exotic Pest Plant Council Symposium '97. Concord, CA.
  • Hillman, J. M. (1999). Potentially allelopathic effects of poison hemlock (Conium maculatum) on native plant revegetation at Wilder Ranch State Park. California Exotic Pest Plant Council Symposium '99. Sacramento, CA.

Cal-IPC News Articles

  • DiTomaso, J. (1998). Results of the CalEPPC questionnaire at Symposium '98 in Ontario. CalEPPC News. 6: 4.
  • Kelly, M. (1999). Roundup of Arundo projects reveals commitment, strategic weakness. CalEPPC News. 7: 4-9.
  • Madison, J. (1996). Highways as corridors of dispersal. CalEPPC News. 4: 9.
  • Pitcairn, M. (2000). All weeds that have approved biological control agents, accidental introductions and others. CalEPPC News. 8.