University of Maryland Extension

Toxic Plant Profile: Hemlock

Sara BhaduriHauck
poison hemlock flowers

There are two types of hemlock: water hemlock and poison hemlock. Although counterintuitive, water hemlock is ten times more poisonous than poison hemlock.

Water hemlock is a native perennial that thrives in wet areas. This is the most poisonous thing an animal can eat. All parts of the plant are poisonous, but the root is the most potent. Eight ounces (just a handful) of the root will kill an adult horse, cow, or human. The toxic compound is cicutoxin which targets the nervous system, causing paralysis to the respiratory system, seizures, and ultimately death. Most often, animals that accidentally ingest water hemlock are killed quickly and found dead. Water hemlock is an erect, branching plant that can grow up to 8 feet high. It has a hollow stem and very distinctive small, white or light green flowers arranged in an umbel configuration.

Poison hemlock is an invasive biennial that is commonly found in disturbed soils. Toxicity varies with plant maturity; it’s most toxic in the spring before it flowers. Like water hemlock, the root is most poisonous although all parts of the plant are toxic. Poison hemlock contains several alkaloid toxins that are structurally similar to nicotine. Like cicutoxin, these alkaloids target the central nervous system and cause paralysis of the respiratory muscles. Although there is no antidote, affected animals may spontaneously recover if they have eaten only a small amount of the plant. Poison hemlock looks quite similar to water hemlock. It is also a tall growing, branching plant with an umbel of white flowers. The leaves, however, are more finely divided and somewhat fernlike. The stems can have purple blotches, and the plant can have an unpleasant musky odor.

Since hemlock is so poisonous, livestock managers should scout pastures and hay fields for these weeds and take steps to prevent animals from accessing them. Glyphosate is usually an effective control, especially when applied before the plant flowers, but it may need to be reapplied for complete control. Don’t compost hemlock plants because the toxin doesn’t break down.

When working with hemlock plants, take precautions to protect yourself. The toxin is most potent when ingested but can also be absorbed through the skin or inhaled. If you’re pulling plants by hand, wear gloves; if you’re mowing them, it’s a good idea to wear long sleeves and a face mask. (Mowing, however, won’t kill the hemlock plant.) When you’re finished, take a shower and wash your clothes separately from your other laundry.

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