Target Poison Hemlock Now To Reduce Populations This Spring

DR. MARCELO ZIMMER AND DR. J.D. GREEN

LEXINGTON, KENTUCKY

Poison hemlock (Conium maculatum) has become widespread throughout most of Kentucky.  Although this plant is often seen along roadways, fence rows, and other non-cropland sites, it has expanded out into grazed pasture lands and hay fields.  It has also become an increasing concern in residential locations when it is observed in areas that are not frequently mowed, such as vacant and abandoned lots. The concern not only stems from its invasive nature, but also from the fact that it is one of the most toxic plants in the world.  Throughout history, the toxicity of poison hemlock has been well known for accidental deaths of humans and other animals.

Description

Poison hemlock is classified as a biennial weed that reproduces only by seed.  It is capable, however, of completing its lifecycle as a winter annual in Kentucky if it germinates during the fall months.  New plants emerge in the fall or late winter, forming a cluster of leaves that are arranged as a rosette on the ground (Figure 1).  The individual leaves are shiny green and triangular in appearance.  Although poison hemlock is most noticeable in late May and June during the flowering stage of growth, the vegetative growth stage is readily observed during the cooler months of the year (Figure 2) with its parsley-like leaves, which are highly dissected or fern-like.

As the plant begins to produce flower stalks in the spring, the leaves are alternately arranged on the main stem.  Each individual leaf is pinnately compound with several pairs of leaflets that appear along opposite sides of the main petiole.  As the plant matures, poison hemlock creates a taproot and grows upwards to about 6 to 8 feet tall.  At maturity, the plant is erect, often with multi-branched stems (Figure 3).  Poison hemlock has smooth hollow stems with purple spots randomly seen along the stem and on leaf petioles (Figure 4).  These features allow us to distinguish poison hemlock from wild carrot (Daucus carota), also known as Queen Anne’s lace, which has hairy stems and lacks purple spots.  Poison hemlock flowers, when mature, are white and form a series of compound umbels (an umbrella-shaped cluster of small flowers) at the end of each terminal stalk. 

Poison hemlock can be associated with areas that have adequate moisture throughout the year, as well as drier environments.  It can grow abundantly in well-protected areas such as fence rows and the edge of tree lines.

Toxicity

Exposure to poison hemlock toxicity occurs primarily through ingestion.  When ingested, even small amounts can result in possible death to all mammals.  The principal toxin in poison hemlock is coniine and a few other toxic alkaloids, which are present in all parts of the plant, including the seeds and roots.  A well-known case of human toxicity was the death of Socrates, a Greek philosopher, who was sentenced to death in 399 BC by ingestion of a poison hemlock potion.  There have been some concerns expressed that toxicity, such as dermal reactions, may occur simply by being in proximity to poison hemlock plants.  However, it is unlikely that most people will experience skin rashes who come in direct contact with poison hemlock as opposed to exposure to other plants such as wild parsnip (Pastinaca sativa) or other potentially toxic plants within the carrot plant family (Apiaceae). Despite a lower risk of skin exposure, personal protective equipment (gloves, safety glasses, etc.) should always be worn if attempting to hand-pull or remove poison hemlock plant parts from a field.

If consumed, all classes of livestock are known to be affected by poison hemlock.  Cattle, horses, and goats are the most susceptible domestic animals, although other animals can be affected as well.  Symptoms of poisoning can occur rapidly, anywhere from 30 minutes to 2 hours, depending on the animal, quantity consumed, and other factors. Initial symptoms can include nervousness, trembling, muscular weakness and loss of coordination, dilation of pupils, coma, and eventually death from respiratory paralysis.  Lethal doses for cattle are in the range of 0.2 to 0.5% of the animal’s body weight.  Poison hemlock is also known to cause fetal deformation when pregnant animals consume the plant.

Fortunately, most animals tend to avoid grazing poison hemlock if other forage is readily available.  However, animals may be more prone to consume green plants in poorly managed or overgrazed pastures during the late winter and early spring when other forage species are more limited.  Also, animals may be more attracted to consume poison hemlock when plants are treated with an herbicide. Toxicity may be somewhat reduced in dried plants, but the potential for toxicity still exists, particularly when sufficient amounts are consumed in dried hay.  Therefore, extreme caution should be considered before feeding animals hay known to contain large quantities of poison hemlock.  

Control

The best overall management practice for reducing poison hemlock populations is to prevent new seed production each year, which can be a challenge since a fully mature plant can produce 35,000 to 40,000 seeds.  Once plants flower, it is generally too late to apply herbicides.  Whereas, mechanical control efforts (if feasible), such as mowing or cutting down individual plants, should be initiated just before peak flower production to reduce the number of new seed being added to the soil seedbank.

As a primary strategy, make note of areas known to contain poison hemlock and begin to look for the emergence of new plants in the fall and during the winter months.  Herbicide treatments are more effective for poison hemlock control when applied during the fall (October/November), or late winter (February/March) when plants are actively growing and in the rosette stage of growth.  Herbicide products containing 2,4-D alone can be effective when applied to young plants that are small and actively growing. As poison hemlock rosettes become more mature, premixtures of 2,4-D + dicamba, 2,4-D + triclopyr, or aminopyralid are needed for best results.  Spot treatments with products containing 2,4-D, triclopyr, or glyphosate can also be used, depending on the location.  Always consult herbicide product labels for approved sites of application and for precautions that should be considered when applying herbicides.   ∆

DR. MARCELO ZIMMER AND DR. J.D. GREEN

UNIVERSITY OF KENTUCKY