Regulations (pertaining to the Great Lakes region)
Conium maculatum is a well-established invasive species in Ohio and has been designated as a prohibited noxious weed (ODNA 2000, USDA NRCS 2012). The possession, transportation, transfer, or introduction of poison hemlock is restricted in all Wisconsin counties except: Crawford, Dane, Grant, Green, Iowa, Lafayette, Richland, Rock, and Sauk (WIDNR 2010). The Robert W. Freckman Herbarium of the University of Wisconsin lists an eradication notice for C. maculatum (Herbarium 2012). However, as of 2007 it was not illegal to sell poison hemlock in Wisconsin (Annen 2007). C. maculatum is listed as a noxius weed under Ontario's weed control act. Regionally, the Great Lakes Indian Fish & Wildlife Commission (GLIFWC) classify this species as capable of causing moderate to severe ecological impacts and/or having limited effective control options available (Falck and Garske 2003).
Note: Check federal, state/provincial, and local regulations for the most up-to-date information.
Control
Physical or chemical removal of C. maculatum individuals is relatively easy, but complete eradication may be difficult due a viable seed bank and reintroductions. Management efforts may need to be continued for several years to be effective (J. McHenry pers. comm. in Pitcher 1985).
Biological
The poison hemlock defoliating moth (Agonopterix alstroemeriana) was accidentally introduced to the U.S., but it is now being investigated as a potential biocontrol agent because of its monophagous (feeding on a single food source) association with C. maculatum (Castells and Berenbaum 2006). Hemlock moth larvae feed on the young stem tissue, flowers, and seeds (Forest Health Staff 2006). High densities of A. alstroemeriana have been effective drivers of plant mortality in C. maculatum stands in the western U.S., where several hundred larvae have been reported from a single plant. However, as a chemical defense, alkaloid production appears to increase with A. alstromeriana herbivory, potentially driving surviving populations to higher levels of toxicity over time (Castells et al. 2005). Furthermore, A. alstroemeriana was found to be targeted by a predatory wasp (Euodynerus foraminatus) in Illinois, suggesting that the effectiveness of biocontrol may be lessened in the Midwest and other locations where E. foraminatus exerts top-down pressure on A. alstroemeriana (Castells and Berenbaum 2008). Although A. alstroemeriana is widespread in the U.S., larvae may still be difficult to obtain for biocontrol purposes (Castells and Berenbaum 2006).
Trichoplusia ni, the cabbage looper, is a generalist lepidopteran that is found throughout the US and occasionally feeds on C. maculatum. Overall growth of T. ni is not stunted, but larvae raised on diets enriched with the piperidines found in C. maculatum develop slower. A prolonged larval stage makes T. ni more vulnerable to predators and could reduce overall biocontrol capabilities (Castells and Berenbaum 2008).
Papulio poluxenes, black swallowtail butterfly, will lay eggs on C. maculatum, but a study conducted in central New York found low larvae survivorship (Feeny et al. 1985).
Conium maculatum is capable of being infected by multiple viruses, including ring spot virus, carrot thin leaf thin virus (CTLV), alfalfa mosaic virus (AMV), and celery mosaic virus (CeMV) (Howell and Mink 1981). However, viral infections appear to stunt growth rather than cause mortality, diminishing their potential for biocontrol (Howell and Mick 1981, Pitcher 2004). Another disadvantage to using these types of biological control agents is the potential for them to escape in neighboring habitats, especially agricultural fields (J. McHenry pers. comm. in Pitcher 1985).
Physical
Care should be taken in handling this toxic plant; it is recommended that gloves are worn (Pitcher 2004). If any body part comes into contact with any part of the plant, be sure to wash it thoroughly (OLA and MAFF 2002). Hand pulling or digging out the taproot are effective method of control for small populations, especially when the soil is moist (WIDNR 2008). Mowing close to the ground is another option of mechanical control if the blade is close to the ground (WIDNR 2008). A dust mask should be worn for protection to avoid inhaling toxins while mowing (King County 2011). In both cases, efforts are most effective if completed before the plants flower and multiple follow-up efforts should be taken to prevent regrowth or new growth (Parsons 1973, Pitcher 2004, WIDNR 2008, Woodard 2008). Poison hemlock remains for several years after death and should be removed where there is a risk of consumption by livestock, wildlife, or children (Pitcher 2004).
Chemical
Effective control of large infestations may require chemical agents (WIDNR 2008, King County 2011). Chemical control of rosettes (before flowering) is a common form of management for C. maculatum because it is such a prolific seed producer (Forest Health Staff 2006, Woodard 2008). Application of herbicides early in spring when sprouts are just emerging may also result in effective control, but if C. maculatum has a large presence in the seed bank, multiple applications may be needed (Forest Health Staff 2006).
Several herbicides—including chlorsulfuron, hexazinone, imazapic, glyphosate, metribuzin, metsulfuron, picloram, triclopyr, terbacil, imazapic plus glyphosate, and metsulfuron plus 2,4-D plus dicamba—were found to be the most effective chemical agents (Forest Health Staff 2006, Jeffery and Robinson 1990, King Country 2011, OLA and MAFF 2002, Pitcher 1989, Woodward 2008). The surrounding plant community should be surveyed prior to selecting a herbicide. The effectiveness of all herbicides declines over time, so multiple applications are recommended from spring to fall (Woodard 2008).
Extra care should be taken when using herbicides near desired wild vegetation or agricultural crops. Foliar herbicides applied with a wick will minimize damage to other nearby plants if C. maculatum is growing amongst favorable vegetation (Jeffery and Robinson 1990, OLA and MAFF 2002). Glyphosate and metsulfuron is not recommended for use near croplands (Monsanto 2007, WIDNR 2008). In a study conducted by Jeffery and Robinson (1990), hexazinon, metribuzin, and terbacil controlled poison hemlock with damaging alfalfa when applied while the alfalfa was still dormant. For more in-depth instructions on the use of 2,4-D, glyphosate, and/or metsulfuron, refer to the Pacific Northwest Weed Management Handbook (Prather et al. 2011).
See also:
Midwest Invasive Plant Network - Invasive Plant Control Database
Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label directions.