The term “hemlock” causes frequent confusion because it refers to two distinct biological entities: a highly toxic herbaceous plant and a non-toxic coniferous tree. These organisms are completely unrelated, belonging to different families of life, yet they share a common name that can have serious implications for public safety. This article defines and differentiates between these two forms of “hemlock” to provide clarity for identification.
The Highly Toxic Hemlock Plant
The name “hemlock” most commonly refers to the poisonous plants of the carrot family, Apiaceae, including Poison Hemlock (Conium maculatum) and Water Hemlock (Cicuta maculata). Poison Hemlock is an invasive biennial plant known for its smooth, hollow stems that often display distinctive purple splotches, growing up to ten feet tall. Its leaves are finely divided, resembling parsley or ferns, and when crushed, the plant emits a strong, unpleasant odor often described as “mousey.”
This plant produces small, white flowers that form umbrella-shaped clusters, or umbels, similar to those of its edible relatives, which leads to dangerous misidentification. All parts of the Poison Hemlock plant, including the roots, leaves, and seeds, contain highly toxic piperidine alkaloids, such as coniine. These compounds are neurotoxins that initially stimulate and then severely depress the central nervous system.
Ingestion of the plant causes progressive muscle weakness and paralysis, eventually leading to respiratory failure and death. Water Hemlock, another dangerous species, contains the neurotoxin cicutoxin, concentrated primarily in the roots. This toxin causes violent convulsions, tremors, and severe cramps, and it is considered one of North America’s most toxic native plants. Both toxic species thrive in moist, disturbed areas like ditches, fields, and stream banks, and their toxicity remains a serious hazard to humans and livestock.
Characteristics of the Hemlock Conifer Tree
In contrast to the toxic plant, the Hemlock conifer tree belongs to the genus Tsuga in the pine family, Pinaceae, and is non-poisonous. The common name is thought to have originated from a perceived similarity between the scent of its crushed foliage and that of the poisonous plant. Species like the Eastern Hemlock (Tsuga canadensis) are large, evergreen trees that can reach heights of over 100 feet and are known for their graceful, drooping leading shoots.
The tree’s “leaves” are short, flat needles, typically about a half-inch long, that have blunt tips and grow in flattened sprays along the branches. A defining feature of these needles is the presence of two distinct white lines, or stomatal bands, on their undersides. The cones are small, generally less than an inch long, and hang downward from the branches.
The bark of older hemlock trees is rough and deeply furrowed, ranging in color from gray to reddish-brown. Historically, it was a significant source of tannin for the leather industry. Hemlock trees play an important ecological role due to their high shade tolerance, often forming dense stands that provide shelter for wildlife. The wood is used commercially for lumber and paper pulp, and the needles can be safely used to make a vitamin C-rich tea.
How to Differentiate the Plant and the Tree
The primary distinction between the poisonous plant and the harmless tree lies in their fundamental structure and growth habit. Poison Hemlock is an herbaceous plant with a biennial life cycle, meaning it is non-woody and dies back after two seasons. The Tsuga tree, by contrast, is a large, woody, perennial conifer.
The stems and foliage offer simple visual cues for identification. The toxic plant has a hollow stem that is smooth and often marked with purple blotches, and its leaves are delicate and fern-like. The tree has a solid, deeply furrowed trunk covered in rough bark, and its foliage consists of short, flat, non-toxic needles arranged on woody twigs.
A final difference is the smell and location. Crushing any part of the Poison Hemlock plant releases a distinct, unpleasant, musty odor, while the Tsuga tree has a pleasant, woodsy, or resinous scent. The plant is typically found growing in open, disturbed, and moist areas like roadsides and ditches. The tree is a forest dweller, forming part of the canopy in cool, moist temperate forests.