Invasive Species of Concern in Maryland

Blooming from early-June well into July, poison hemlock, Conium maculatum, is noticeable all along roadsides and in moist meadows. Native to Europe and the Mediterranean, poison hemlock is widely distributed throughout the U.S., including Maryland. This member of the Apiaceae, or Parsley family, can be confused with other members of the family, like parsley, fennel or wild carrot (the familiar Queen Anne’s Lace), but unlike those edible plants, all parts of poison hemlock can be deadly to humans and livestock. The Greek philosopher and teacher, Socrates, condemned to die for impiety in 399 B.C.E., was killed by drinking the juice of poison hemlock. July is a good time to make sure you can identify this plant, so the Maryland Invasive Species Council has chosen poison hemlock as the July Invader of the Month.

Poison hemlock is an herbaceous biennial or perennial, with branching stems growing up to 7’ tall in open wet areas, along the edges of streams, roadside ditches or waste places. It has a smooth stem, often streaked or spotted with purple, especially on the lower portion. These red or purple blotches are called the “blood of Socrates.” The flowers are very small and white, with five petals, one usually larger than the other four. They bloom in small clusters on stems of unequal length; the clusters are grouped into “umbels” or little umbrellas about 4” wide at the ends of the stalks. The leaves are broadly triangular in outline and as much as 15” long, but are finely divided 3 to 4 times into smaller sections and have a lacy appearance from far away. The roots of poison hemlock are fleshy, creamy white and could be mistaken for carrots or parsnips. In most cases, they are branching or divided and stringy rather than single like parsnips or carrots and white rather than orange. Unlike fennel, which smells like licorice, poison hemlock has a “musty” or “mousy” smell.

Common elderberry is a native plant that has an overlapping bloom time with poison hemlock. Check the pictures below to see some of the characteristic differences between them.

Poison hemlock was spread across the U.S., likely from multiple introductions. The plant has a long history of medicinal use in the Old World, and may have been brought to the U.S. for that purpose. Conium maculatum contain numerous alkaloids, the most potent of which is coniine. Coniine is a neurotoxin, which can cause death by blocking the connections between muscles and nerves. It is described as causing paralysis beginning in the feet and ankles and ascending up the body, and causes death by paralyzing the respiratory muscles, thus depriving the brain and heart of oxygen. Children who use the hollow stems as pipeshooters or whistles can be affected. The alkaloids are volatile, and coniine can be absorbed through the skin, so poisoning could occur from smelling or touching the plant. Someone affected by contact with poison hemlock can be treated by artificial respiration on a ventilator until the toxins have been removed from the system.

Scientists have found that U.S. populations of poison hemlock increase their production of poisonous alkaloids, which the plant manufactures to deter herbivores, when they are reassociated with a European caterpillar, the hemlock-eating moth, Agonupterix alstroemeriana, which eats only that plant. Plants in New York and Washington states, where higher numbers of the caterpillars were found, produced more toxins than plants in Illinois. The plants in Illinois showed higher levels of damage from herbivores.

Although it has a thick taproot, poison hemlock can be dug or grubbed out with shovels or spading forks. Long sleeves and protective clothing are a wise precaution. Given the potential danger of the plant’s chemical constituents, controlling it with an herbicide approved for use near water, such as aquatic forms of glyphosate, is likely safer.

For more information about other Invasive Species of Concern, visit www.mdinvasivesp.org or call the Maryland Department of Agriculture at 410-841-5920.

For more information on the Internet:

PLANTS Profile (USDA NRCS)

CalPhotos

GRIN (USDA ARS, Germplasm Resources Information Network)