Poison Hemlock – A Growing Concern

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 the fact that it is one of the most toxic plants in the world.  Throughout history, the toxicity of poison hemlock is well known for accidental deaths of humans and other animals.

Description

Poison hemlock is classified as a biennial 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 send up 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 hollow stems which are smooth with purple spots randomly seen along the stem and on leaf petioles.  There are no hairs on the plant that helps distinguish it from other plants similar in appearance. The 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 having adequate moisture throughout the year, as well as, drier environments.

Toxicity

The risk of exposure to poison hemlock toxicity is primarily through ingestion.  Just small amounts of ingestion 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 by simply being in proximity of 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 or other potentially toxic plants within the carrot plant family Apiaceae.

If consumed, all classes of livestock are known to be affected by poison hemlock.  Cattle, horses, and goats are considered to be the most susceptible domestic animals although other animals can be affected as well.  Symptoms of poisoning can occur rapidly anywhere within 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 considered to be 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 during the late winter and early spring when other forage species are more limited.  Toxicity may be somewhat reduced in dried plants, but the potential for toxicity still exists, particularly when a sufficient quantity is consumed in dried hay. Therefore, extreme caution should be considered before feeding animals hay known to contain large quantities of poison hemlock.  Also, animals may be attracted to consume poison hemlock when plants are treated with an herbicide.

Control

The principal strategy for poison hemlock control is to prevent seed production, which can be a challenge since a fully mature plant is capable of producing 35,000 to 40,000 new seeds.  Once plants have produced flowers it is generally too late to utilize herbicide control methods.  Whereas, mechanical control efforts (if feasible) such as mowing or cutting down individual plants should be initiated just before peak flower production to avoid or reduce the amount of new seed being produced. 

As an overall strategy, make note of areas known to contain populations of poison hemlock and begin to look for emergence of new plants in the fall and during the winter months.  Throughout the fall (October/November) or early spring (late February/March) is the best time of year for herbicide treatment.  Herbicide products containing 2,4-D can be effective when applied to smaller, actively growing plants that are still in the younger rosette stage of growth. As plant rosettes become more mature, premixtures of products containing 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 product labels for approved sites of application and for precautions that should be considered when applying herbicides. ∆

DR. J.D. GREEN: Weed Science Extension Specialist, University of Kentucky