Poison Hemlock (not native)

Conium maculatum

tall plants with white flowers
Central Basin, south side | April 2017

  Poison hemlock is so toxic that in Scotland it is known as “deid men’s oatmeal” 56                                                                               

Poison hemlock (Conium maculatum) is a stately plant with large white flower clusters held high above a clump of green, fern-like leaves. All parts are highly toxic, and with no known antidote, prevention is the only safeguard.

 Ironically, poison hemlock was introduced into the United States as a garden plant, marketed as “winter fern” because of its foliage. It has since spread throughout the United States and much of the northern hemisphere.

Other Common Names:

carrot fern, poison parsley, spotted hemlock, devil's bread

Description 4,11,59,260,340

Poison hemlock is a tall, sturdy, conspicuous biennial herb (occasionally annual) from a fleshy taproot. The entire plant is poisonous!

The first year it produces a basal tuft of large, compound green leaves that resemble fern fronds. Basal leaves are usually two-times pinnately divided; leaflets in turn are pinnately dissected. Basal leaves are reported to 36 inches (90 cm) long.  The second year, a stout, upright, branching stem grows to 6 or 10 feet or more high. The lower portions of many stems, are marked with streaks, splotches and spots of reddish-purple. Cauline leaves resemble basal leaves. They alternate along the stem with petioles clasping the stem, becoming smaller higher on the plant. The leaves have a distinctive odor, variously described as musty, rank or mouse-like.

Flowers occur opposite a leaf, in large, compound slightly domed clusters, or umbels that generally extend beyond the foliage. Each compound umbel may be 3 1/2 inches (9 cm) across and consists of several smaller umbellets of flowers also originating at a common point. The individual flowers are bisexual (rarely pistillate) white, less than 1/8 inch (.2-.35 cm) across.  There are no sepals and the five petals are ovate narrowing to a small terminal point that curls inward, giving some petals a heart-shaped appearance. Five stamens initially curve inward, expanding outward when mature. The single pistil has a two-chambered inferior ovary with a two-parted domed surface topped by a small, flattened structure, the stylopodium, that supports two small styles, each with minute stigma. Flowering occurs April to Sept.7

The fruit of poison hemlock is broadly oval, 1/8 inch long (.2-.32 cm) or less and clearly divided into two flattened halves with a persistent stylopodium. The surface is smooth, with conspicuous pale ribs. When young, the fruit is green. At maturity, it dries brown and the halves split apart from bottom; each half contains one seed.

Basal clump of leaves of first year plant | East Basin, southeast end | March 2020

green stem with purple spots

Purple markings on lower stem | Central Basin, south side | April 2019

oval, green fruit

a compound umbel of developing fruit with 14 umbellettes | Central Basin, south side | June 2019

Distribution 7,46,89,260  

Poison hemlock is native to Eurasia but has now spread throughout the United States and into Canada, Australia, New Zealand, South America and the British Isles.

Poison hemlock is found throughout California, below 5000 feet (1500 m) and excluding the desert regions. It prefers disturbed areas where there is some moisture and a bit of shade.

In spite of eradication efforts, poison hemlock is found in several spots along the trail on the south side of the Reserve, especially on the shady bank east of the Rios trailhead and the trail through the eucalyptus grove at La Orilla. They have been found on Stonebridge Mesa. One plant has come up in the area burned in 2019.

Classification 11,59,143

Poison hemlock is a dicot angiosperm in the carrot family (or parsley family, Apiaceae). Members of this family are mostly herbs and are characterized by having flowers in compound umbels in which the peduncles of primary umbels radiate from one point; the primary umbels consist of many smaller umbellets in which the flower pedicels are also joined at one point. Previously, this family was called Umbelliferae, reflecting this flower structure. Members of the carrot family often have a thick taproot, leaves that are pinnately dissected, and petioles that wrap part way around the stem. The two-seeded dry fruit is also characteristic.

The carrot family includes many important foods such as celery, carrots, and parsnip, and many flavorful herbs such as parsley, cumin, coriander, dill, and caraway. The same family includes two extremely toxic genera: Conium, poison hemlock, and Cicuta, a small genus called water hemlock or cowbane which is native to temperate North America and Europe but not in the Reserve. Poison hemlock may be confused with two non-native and non-toxic members of the carrot family that are found in the Reserve:48 fennel (Foeniculum vulgaris) and wild celery (Apium graveolens).





Jepson eFlora Taxon Page
tall plant with white flowers

Central Basin, south side | April 2018

white flower cluster

Central Basin, south side | April 2017

striped, oval brown fruit

An umbellet of fruit | Central Basin, south side | June 2019


The size, shape and texture of poison hemlock fruit and seed appear poorly adapted for long-distance dispersal. They are round, smooth and heavy, precluding transport by wind. Their toxicity to vertebrates eliminates active transport by birds and mammals. Seeds seem best adapted to formation of large, dense colonies near the parent plant, and this has been reported as a characteristic of its distribution.4

There is a theory that colonization of new areas may be facilitated by modern transportation corridors such as wagon tracks and highways.512 These not only provide a path of disturbance through native vegetation, but divert water off the road surface to the shoulders, often creating moist areas in adjacent depressions. Thus, human pathways may provide a mechanism for slow but steady movement of poison hemlock into new areas.

tall plants with white flowers

Central Basin, south side | April 2019

white flower cluster

A compound umbel | Central Basin, south side | April 2017

striped, oval brown fruit

Central Basin, south side | June 2019

Human Uses 41,56

The early Greeks used poison hemlock for judicial executions. The most famous victim was the Greek philosopher Socrates who is considered one of the fathers of modern western philosophy and who counted Plato among his students. In 399 BC, possibly the result of political infighting, Socrates was convicted of corrupting the minds of the youth of Athens and of not believing in the gods of the state. He was sentenced to death by drinking poison hemlock.

The red-purple blotches on the stems of poison hemlock are sometimes called Socrates’ blood.

green, fern-like leaves

Doubly compound leaves | Central Basin, south side | Feb. 2020

white flower cluster

Photo courtesy of Denise Stillinger | April 2007

developing of oval, green fruit

Central Basin, south side | June 2019

Interesting Facts

Poison hemlock was introduced into the United States as a garden plant sometime in the 1880’s. It was marketed as “winter fern”.260

tall plants with white flowers

East Basin, south side | April 2016

green stem with purple spots

Central Basin, south side | April 2019

green, fern-like leaves

Central Basin, south side | Feb. 2020

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