Two Poisonous Plants You May Encounter This Summer

I have received several calls about plants that have either just shown up, or have increased in density and distribution with the additional precipitation we have received in many parts of Powell County this summer.

Two such plants are poison hemlock and Western waterhemlock. Ingestion of either by humans or livestock can result in illness or death.

Poison hemlock, the most toxic of the two, was used to create the potion Socrates was forced to drink in 329 B.C. Some Native American tribes have used juice made from the hemlock plant to taint the tips of arrows. Roots of hemlock have been mistaken for wild parsnips and eaten by people who then become ill by succumbing to toxicity.

Poison hemlock, also called poison parsley, is a member of the plant family that includes wild carrots, parsley, and parsnip. Like other members in this plant family, poison hemlock flowers form white, umbrella-shaped clusters. Poison hemlock has large glossy green fernlike leaves. This plant was imported by Europeans during the 1800s. Poison hemlock escaped gardens and now infests roadsides, creeks, irrigation ditches, cultivated fields, and pastures. Poison hemlock typically grows in wet soils, but can occasionally tolerate semi-dry soils.

A biennial plant, poison hemlock forms a large rosette during the first year of growth, and the second year produces tall stems with flowers. It grows usually from three to eight feet tall and under favorable conditions could reach heights of 10–12 feet. The large, dried stems have been known to remain toxic for up to three years. Poison hemlock can be differentiated from a carrot, parsley or parsnip plant by the numerous purple spots along the stem. Poison hemlock typically produces an unpleasant smell that closely resembles mouse urine, especially noticeable when the leaves are crushed.

While all parts of poison hemlock (leaves, stem, fruit, and root) are poisonous, leaves are especially poisonous in the spring up until the time the plant flowers. All classes of livestock and wildlife are susceptible to hemlocks, though animals typically avoid these plants as long as there is quality feed available.

Digging both poison hemlock and Western waterhemlock can be effective when caught early and the plant population is small. Take care when digging or mowing hemlock and always wear protective clothing when handling. Treating poison hemlock with herbicides is most effective in late spring or early summer on seedlings or small rosettes. Several combinations of herbicides are effective, including 2,4-D and dicamba or metsulfuron. Glyphosate and triclopyr have also been effective. Repeated applications may be required until the seed bank has been depleted. Burning hemlock after pulling is not recommended as the smoke may contain toxins.

The toxic substance in Western waterhemlock, a plant native to Montana, is a brown or straw-colored liquid called cicutoxin, a poisonous unsaturated alcohol with a strong, carrot-like odor. It is found principally in the tubers or roots but is also present to a lesser extent in the leaves, stems, and immature seeds. Western waterhemlock grows from three to seven feet tall with hollow stems which may have purple striping and are enlarged at the base. The mostly hollow, large tuberous roots have partitions that form distinct chambers, and slender individual roots that grow from the thick, fleshy tubers at the base of the main rootstalk. Western waterhemlock's flowers are also white and grouped in umbrella-shaped clusters, however the leaves of this species are noticeably different than others in the family as they are not fern-like in appearance. These leaves are arranged more like groups of feathers that are divided one to three times into narrow-toothed and lance-shaped leaflets one to four inches long. Prominent veins run from the midrib, or center of each leaflet and branch off to the teeth-like tips. Western waterhemlock is a perennial that reproduces both from seed and overwintering root structures. Tea colored, somewhat kidney shaped seeds arranged in groups of two can be transported by water or soil to other locations from plants growing along irrigation ditches and can remain viable in soil for up to three years.

Application of chemicals for western waterhemlock also most effective in late spring or early summer and may require repeated applications to deplete the seed bank. Several types of herbicide can be effective, including Glyphosate, 2,4-D, and a tank mix of picloram and 2,4-D.

Plants in the carrot family share the characteristic of umbrella shaped flower head and includes hundreds of plants, both edible and toxic. Accurate identification is important for avoiding accidental poisoning as well as effective management. Contact your local county weed coordinator, extension agent, or the Montana Weed Control Association for more information.