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Issue No. 8, Article 8/May 19, 2006

The Bane of Socrates--Becoming More Familiar to Illinois Farmers?

The spectrum of weed species encountered in agronomic production fields can change or shift in response to changes in crop production practices. Changes in weed spectrum can occur through modification of tillage practices, selection of herbicide-resistant biotypes, an increased preponderance of species that emerge later during the growing season, or introduction of species not previously encountered or considered troublesome. Weed species that complete their life cycle over a two-year period may find a home in agronomic production systems where tillage is reduced or eliminated.

A contemporary example of a such a biennial weed species is poison hemlock (Conium maculatum), commonly found in pastures and along railroad rights-of-way, roadsides, and other nondisturbed areas, but becoming increasingly common in reduced- and no-till cornfields and soybean fields. Weed control practitioners have reported great consternation in attempting to control this plant species, especially prior to corn or soybean planting. This article describes the biology of poison hemlock and potential options for its control.

An Exotic Plant Species

Like many other weedy plant species, poison hemlock is not indigenous to the United States. It was introduced, often as an ornamental garden plant, from its native lands of Europe, northern Africa, and western Asia. It escaped from cultivation as an ornamental and now is known to occur in nearly every state of the contiguous United States. Since it is an exotic plant species, very few natural control agents exist in the United States. Herbarium records indicate poison hemlock collections have been made from at least 64 of the 102 counties in Illinois, ranging from Jo Daviess County in the north to Pulaski County in the south, and from Adams County in the west to Vermilion County in the east.

The Least Desirable Characteristic

Perhaps the most well-known attribute of poison hemlock is its toxicity to mammals. Poisonings of humans, cattle, horses, pigs, sheep, goats, and poultry have been reported. The plant contains several different but closely related alkaloids, the most active of which is coniine, a neurotoxin of the central nervous system. All parts of the plant are recognized as being very poisonous, including the roots and seeds. When the plant is dried, the levels of toxic components are reduced but not eliminated. Livestock rarely graze on poison hemlock by choice, perhaps due in part to the plant's rather unpleasant odor, but they may ingest the plant if it is inadvertently harvested along with alfalfa or other forage. Extracts from poison hemlock were often used to execute criminals and political prisoners in ancient Greece, and hemlock is reputedly the poison used to kill the philosopher Socrates.


Poison hemlock is a member of the Apiaceae plant family, often referred to as the carrot or parsley family, which includes between 2,500 and 3,000 species. The scientific name of poison hemlock, Conium maculatum, provides some clues about certain characteristics of the species. Conium is derived from a Greek word that means to spin or whirl, a reference to the deleterious effects the plant's poison can have on the human body. The name maculatum is Latin for spotted or bearing spots, in reference to the characteristic purple spots common on the stems of the plant.

Purple spots or blotches are commonly found on the stems of poison hemlock plants.

Poison hemlock is a herbaceous plant widely considered to have a biennial life cycle, although some authors have suggested it can exist as an annual (winter) or short-lived perennial (monocarpic) under certain conditions. Seedling plants have been reported to emerge over a long period during any given year, including (depending on geographic location) spring, summer, fall, and winter emergence. This prolonged emergence may be at least partially attributed to the long period of seed dispersal from the parent plant. Mature plant stalks can frequently persist throughout the fall and winter months, and researchers have determined that seed dispersal from these stalks can occur even during the winter. Seeds dispersed from the parent plant are mostly nondormant, allowing germination and seedling emergence essentially any time conditions are favorable.

During its first year of growth, poison hemlock forms a rosette (a dense cluster of leaves growing close to the ground) of finely divided leaves that may reach 1 to 1.5 feet in diameter. When bruised, the leaves exude a pungent odor, described by some to smell like mice. In mid- to late April of its second year, the plant bolts, or produces a flowering stalk that branches and gives rise to umbels (flat or slightly convex clusters of flowers) that are characteristic of this plant family. Flowering begins around mid- to late May and lasts until late June. Seeds are produced in a structure known as a schizocarp that splits apart when mature. Seeds are typically mature by late summer, but seed dispersal from the parent plant may not begin until mid-September. Propagation of this species is by seed only, with reports estimating seed production at approximately 38,000 seeds per plant. After producing seed, the plant usually dies. Seeds are not equipped for long-distance dispersal and are thus dropped close to the parent plant. This limited dispersal often results in dense stands of poison hemlock plants that can outcompete native vegetation. Poison hemlock seed longevity in the soil is relatively short, with a reported range of 2 to 3 years.

Young poison hemlock plant, prior to bolting.

Seeds of poison hemlock.


Leaves are arranged alternately on the stem, triangular in outline, four to five times pinnately compound (fernlike), without hairs and borne on petioles. Individual leaflets are only about 1/4 to 3/8 inch long, with veins that extend to the tips of the finely toothed leaf margins. Basal leaves have a long petiole, while upper leaves have much shorter petioles.

Poison hemlock leaves are compound and arranged alternately on the stem.

Stems (produced during the second year) are erect, much branched, smooth (without hairs), and hollow except at the nodes. Stem color is light green, and lower portions of the stem are spotted or streaked with red or purple blotches. Mature stems are glaucous (having a powdery or waxy coating that gives a frosted appearance and tends to rub off).

Stems of poison hemlock lack hairs.

The inflorescence of poison hemlock is a compound umbel (a flat-topped or convex structure of multiple flowers borne separately on individual pedicels) consisting of multiple white flowers, each flower bearing five petals. Each umbel, composed of 8 to 16 individual umbellets, may be 2 to 5 inches in diameter.

Some confusion may exist when trying to differentiate between poison hemlock and wild carrot, another species in the Apiaceae plant family. Both are biennial species that produce an umbel inflorescence; poison hemlock, however, is much more toxic than wild carrot, so it's important to be able to differentiate between these species. Differences include plant height (poison hemlock may reach 6 to 10 feet at maturity; wild carrot rarely exceeds 4 to 5 feet at maturity); stem coloration (poison hemlock stems have red to purple blotches; wild carrot stems have no such coloration) and hairs (poison hemlock stems have no hairs; wild carrot stems have bristly hairs); flowering time (poison hemlock flowers mid- to late spring; wild carrot flowers later in summer); the pungent odor of poison hemlock foliage; and the hairy leaves of wild carrot.

Stems of wild carrot have bristly hairs.


Integrating cultural, mechanical, and chemical options into a poison hemlock control program will often yield better results than any tactic used alone. Preventing or reducing seed production from nonmature plants and seed movement from mature plants is important to helping curtail the spread of poison hemlock across agronomic production fields. Poison hemlock infestations in such fields usually begin as individual plants or small patches along field margins or edges; when left unchecked these patches can soon encompass more and more area as seeds are moved by human and animal activities. Tactics to reduce or eliminate seed production from small patches of poison hemlock include roguing or mechanical cultivation to physically remove plants, applying herbicides (such as glyphosate) before plants begin to bolt (which may include fall applications to rosette-stage plants), mowing after plants bolt to delay or reduce flower and seed production (although more than one mowing operation may be necessary to adequately suppress seed production), and, if feasible, restricting movement of humans and livestock through mature stands of poison hemlock to reduce seed movement.

Poison hemlock plants invading a soybean field.

Around 1973, a herbivorous insect from the native range of poison hemlock was first identified in the United States in New York state. Agonopterix alstroemeriana, sometimes referred to as the defoliating hemlock moth, is reported to be an insect species that feeds exclusively on one food (monophagous), in this case poison hemlock. During the 1980s populations of this insect were subsequently identified in several western states (California, Oregon, Utah, Washington, Idaho, and Colorado), and entomologists confirmed the existence of established populations in central Illinois in 1993, including several sites throughout Champaign County. While this insect may not completely eliminate poison hemlock populations, its presence will undoubtedly help slow the spread of this invasive plant species.

Historically, poison hemlock has been more common in pastures and noncrop areas, so it's not altogether surprising that more research on control has been done in these areas than in cornfields and soybean fields. Products containing tebuthiuron (Spike), chlorsulfuron (Telar), hexazinone (Velpar), metribuzin (Sencor), and terbacil (Sinbar) have been reported to provide good to excellent preemergence control of poison hemlock in pasture and noncrop settings. Spot treatments containing glyphosate or 2,4-D have been more effective than dicamba or bromoxynil.

What about options for poison hemlock control in corn or soybean? Few data are available from which to draft definitive management recommendations, but many anecdotal suggestions and observations have been offered.

Options Prior to Planting

If poison hemlock plants are present before planting, mechanical control options (i.e., tillage) may provide control as effective as or more effective than many herbicides. Poison hemlock appears to be most susceptible to burndown herbicides as a rosette-stage plant, and susceptibility seems to decline rapidly after this biennial species has begun to bolt. Unfortunately, the time before bolting frequently corresponds to periods of low air temperature, which tends to reduce the activity of many translocated herbicides used in burndown applications.

Glyphosate, at rates of 0.75 lb acid equivalent (ae) or less, is generally more effective when applied prior to significant spring growth. Including 2,4-D ester at 0.5 lb ae with glyphosate may improve control. If applications are delayed until after significant spring growth has occurred, consider increasing the glyphosate rate to 1.5 lb ae. A few reports have suggested that adding Basis, Canopy, Canopy EX, or Express as a burndown component has improved control of poison hemlock. Poison hemlock less than 6 inches in height and up to 12 inches in diameter is listed on the Basis label under the section on spring burndown applications. Dicamba performance on poison hemlock has been erratic, and often less effective as a burndown treatment than glyphosate or glyphosate plus 2,4-D. Lumax and Lexar have demonstrated some reasonably good activity on poison hemlock when applied as a component of an early preplant or burndown program. One anecdotal report of success involved applying glyphosate to the poison hemlock plants, followed seven days later by mowing the stand before planting soybean.

Farmers and other weed control practitioners who have experienced difficulty controlling poison hemlock in the spring prior to crop planting may want to investigate fall applications to possibly improve overall success. A fall timing would be better suited for herbicide applications to rosette-stage plants, and higher herbicide rates may provide better control with fewer planting restrictions compared with lower application rates in the spring.

Options Following Crop Emergence

Glyphosate can be applied postemergence in glyphosate-resistant crops. Consider application rates higher than the "usual" postemergence rate of 0.75 lb ae (maximum single in-crop application rate is 1.5 lb ae in soybean ) or perhaps spot treatments with handheld equipment. Dicamba, 2,4-D, Basis and other rimsulfuron-containing products, or Callisto and other mesotrione-containing products may provide some control or suppression in corn.


Poison hemlock is a biennial weed species that is well adapted to invade reduced- or no-till corn and soybean production fields. The toxicity and weedy characteristics of this species warrant appropriate attention and care from weed control practitioners and the general public. Preventing seed production and movement from plants along field margins may slow the advancement of poison hemlock into production fields.


Baskin, J.M., and C.C. Baskin. 1990. Seed germination ecology of poison hemlock, Conium maculatum. Canadian Journal of Botany 68: 2018-2024.

Castells, E., M.A. Berhow, S.F. Vaughn, and M.R. Berenbaum. 2005. Geographic variation in alkaloid production in Conium maculatum populations experiencing differential herbivory by Agonopterix alstroemeriana. Journal of Chemical Ecology 31: 1693-1709.

Holm, L., J. Doll, E. Holm, J. Panchho, and J. Herberger. 1997. World Weeds, Natural Histories and Distribution. New York: Wiley.

Jeffery, L.S., and L.R. Robison. 1990. Poison-hemlock (Conium maculatum) control in alfalfa (Medicago sativa). Weed Technology 4: 585-587.

Mitich, L.W. 1998. Poison-hemlock (Conium maculatum L.). Weed Technology 12: 194-197.

--Aaron Hager

Aaron Hager

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