The term “hemlock” refers to two completely different types of plants: a large evergreen tree and a highly toxic herbaceous weed. These distinct species have vastly different characteristics and interact with deer in separate ways. The difference is based on the plant’s toxicity and structure, which dictates whether a deer treats it as a food source or avoids it completely.
Clarifying the Two Hemlocks: Tree vs. Toxic Herb
The Hemlock Tree belongs to the genus Tsuga and is a large, shade-tolerant conifer found primarily in North America and Asia. As a member of the pine family (Pinaceae), it is a woody, evergreen plant that can grow to be hundreds of years old. Its needles and bark are generally considered non-toxic to humans and animals.
Poison Hemlock, scientifically known as Conium maculatum, is an entirely unrelated plant belonging to the carrot family (Apiaceae). This biennial herb features a hollow, smooth stem often marked with distinct purple splotches. Its danger stems from highly toxic piperidine alkaloids, such as coniine, which affect the nervous system of mammals.
Deer Browsing Habits on Hemlock Trees (Tsuga)
Deer readily consume the foliage of Hemlock trees, particularly the Eastern Hemlock (Tsuga canadensis), despite it not being a preferred food source. This browsing is most noticeable during the winter months when preferred forages are scarce. The evergreen needles provide a reliable, if less nutritious, source of sustenance during periods of low food availability.
Hemlock is often categorized as a “second choice” food, but heavy browsing can severely impact young saplings. Deer typically browse by nipping off the terminal buds and stripping the lower branches, which stunts the tree’s vertical growth. Repeated defoliation can keep young hemlocks trapped below six feet, where they are easily reached by deer and may eventually die.
Studies using deer exclosures have shown that unprotected juvenile hemlocks survive at a rate four times lower than those protected from deer browsing. This sustained pressure from high deer populations is a primary factor constraining the regeneration of hemlock forests. Landowners often use physical barriers, such as fencing, to protect individual trees until they grow taller than the deer’s reach.
Deer Avoidance of Poison Hemlock (Conium maculatum)
Deer, along with most other grazing mammals, generally avoid consuming Poison Hemlock due to its extreme toxicity and naturally repellent characteristics. The plant contains potent neurotoxic alkaloids, primarily coniine and gamma-coniceine, which can cause neuromuscular blockage and respiratory failure in mammals. These toxins are present in all parts of the plant, including the roots, leaves, and seeds, making it dangerous at any growth stage.
Wild animals, including deer, possess an instinctual or learned avoidance of plants that are intensely bitter or have a strong, unpleasant odor. Poison Hemlock has a pungent, mousy smell that acts as a natural deterrent, signaling its unpalatability to grazing animals. This natural defense mechanism is usually effective enough to prevent free-ranging deer from ingesting toxic amounts.
The rare instances of deer or livestock poisoning generally occur under specific, non-natural conditions, such as extreme starvation when no other forage is available. Accidental consumption can also happen when the plant contaminates dried hay or silage. However, for a free-ranging deer with access to diverse forage, the avoidance of this noxious weed is a reliable survival behavior.