U.S. DEPARTMENT OF THE INTERIORBUREAU OF LAND MANAGEMENT
 
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Poison Hemlock (Conium maculatum)

Parsley/Carrot Family (Apiaceae)
 

Conium maculatum (poison hemlock).  Photo courtesy of Jan Samanek, State Phytosanitary Administration, Bugwood.org
Photo: Jan Samanek, State Phytosanitary Administration, Bugwood.org

       Conium maculatum (poison hemlock). Photo courtesy of Steve Dewey, Utah State University, Bugwood.org      
Photo: Steve Dewey, Utah State University, Bugwood.org

Conium maculatum (poison hemlock).  Photo courtesy of Richard Old, XID, Services, Inc., Bugwood.org
Photo: Richard Old, XID Services, Inc., Bugwood.org

Description/Habitat:  Poison hemlock (Conium maculatum) was introduced to the U.S. in the 1800s from Europe as a fernlike garden ornamental. It is generally an erect biennial but can sometimes be an annual or short-lived perennial that can reach up to 3m tall, and usually with purple-spotted or purple-streaked stems. Plants exist as a large basal rosette during the first year. When the foliage is crushed it emits a musty odor, similar to mouse excrement.
 
Warning! Poison hemlock contains piperidine alkaloids, and ALL plants parts are highly toxic to humans and animals when ingested. Poison hemlock can cause coma or death from respiratory paralysis after ingestion. It can cause dermatitis, nausea, and headaches if touched or inhaled after continuous handling, cutting, or mowing. Caution should be used when handling this plant.
 
Mature leaves and green seeds contain the highest concentration of total alkaloids, but new vegetative growth contains the greatest proportion of the most toxic alkaloid.  Hence, many livestock poisonings occur in spring and fall when seeds germinate and new foliage emerges. Habitats include roadsides, pastures, fields, ditches, riparian areas, cultivated fields, waste places, and other disturbed, often moist sites.
 
Leaves:  Generally, the large triangular deeply lobed, pinnately compound leaves are between 15 and 30cm long. The leaflets are generally pinnately lobed or dissected. The lower leaf stalks are sheathing at the base. Upper leaves are sessile.
 
Flowers:  The flowers of poison hemlock are arranged in a compound umbel and are often rounded at the top. The main flower stalk is about 2-8cm long with inconspicuous lanceolate bracts below the secondary stems, which are generally 1.5- 5 cm long, and each main stem has 10-20 of them. Bractletsbelow pedicels are mostly 1.5-2mm long and resemble bracts. The flowers are white, small, and lack sepal lobes. The fruits are grayish brown dried pods. Flowering stems senesce in late summer or fall and can persist with a few fruits attached well into winter or early spring.
 
Seeds:  Poison hemlock reproduces by seed only. Most seeds fall near parent plant, but some may move greater distances with water, soil movement, animals, or human activities. Seed dispersal is prolonged and occurs late summer through winter. After dispersal, most seeds can germinate almost immediately in favorable conditions, but a small portion of seeds will remain dormant. Dormant seeds require a period of high summer and/or low winter temperatures before they can germinate. Seeds are not dispersed to a favorable location can become dormant for up to three years. Germination does not require light and in California it typically commences with the first fall rains through early spring. Seedlings can establish rapidly, especially on disturbed sites with bare soil.
 
Flowering Period:  April through July.
 
Management:  At Fort Ord National Monument,only some areas of poison hemlock are being treated, as there are simply not enough resources to combat all of the poison hemlock present. Manual treatment is the most common form of controlling poison hemlock. This method of control is effective since plants do not regenerate when they are hand-pulled or cut below the crown. Removing plants before seeds mature every year will eventually deplete the seed bank. Repeated mowing can also weaken the plants and reduce the size of an infestation, as well as deplete the seed bank if pursued regularly. Removing poison hemlock is most effective after the plant has matured but before it has flowered, and it is easiest to remove the plant after it has bolted.
 
Flaming with a propane torch is another method used to control poison hemlock on Fort Ord National Monument. Flaming during the seedling and rosette stage has been a very successful treatment method but can require more than one treatment to kill a plant. The best time to flame poison hemlock is during its germination period, which occurs typically within the first fall rains through early spring. Flaming is most often performed during the rainy season. Flaming is only done when both the ground and the vegetation are wet to prevent starting a fire. The overall goal is not to set plants on fire, but rather to damage the cell structure of their foliage. Brief exposure to intense heat causes the cells to expand, which disrupts cell walls. The flamed weeds don’t senesce immediately, but within several hours or days they wilt and then die. Flaming must occur when plants are small and have not developed a large root system in order for it to be effective. Flaming poison hemlock requires multiple visits to sites to ensure effectiveness of this treatment method.  As with all methods of weed control, follow up on any regrowth after any treatment.
 
                       

Sources:

Cal-IPC Weed Workers Handbook: A Guide to Techniques for Removing Bay Area Invasive Plants. 2004. The Watershed Project and California Invasive Plant Council: 80-81

DiTomaso, J. M. and E. A. Healy. 2007. Weeds of California and Other Western States. Volume 1: Aizoaceae to Fabaceae. University of California Department of Agriculture and Natural Resources: 132-136


 
Last updated: 08-20-2012