poison hemlock (Conium maculatum)
SCIENTIFIC SYNONYMS: There are no scientific synonyms for
CONFIRMATION STATUS: Pending confirmation.
TAXONOMY: The currently accepted scientific name for poison hem-
lock is Conium maculatum L.
Because there are many members of the Carrot family with small white
flowers, it can be difficult to correctly identify. Poison hemlock has the
following key characteristics: 1) it is usually tall-growing, 1) the foliage
is at least double pinnate and fern-like in appearance, 2) the foliage has a
bitter rank odor, especially when it is bruised, 3) the hairless stems have
purple spots, 4) there are undivided bracts at the base of the compound
umbel and undivided bractlets at the base of the umbellets, and 5) the
small leaflets are pinnately cleft or dentate.
NATIVE STATUS: Introduced, United States and Canada.
GENERAL BOTANICAL CHARACTERISTICS:
Habit: This biennial plant is 3-7' tall. During the 1st year, it consists of a
rosette of basal leaves spanning about 1–1½' across. During the 2nd year,
it produces a flowering stalk that branches occasionally. The stems are
round, glabrous, light green, and purple-spotted; they often appear to be
ribbed because of longitudinal veins.
Leaves: The alternate leaves are up to 18" long and 12" across, becoming
smaller as they ascend the stems. They are double or triple pinnately com-
pound and triangular-ovate in outline and hairless. The individual leaflets
are about 1/3" long, pinnately cleft or dentate, and hairless. These leaves
have a fern-like appearance and a somewhat filmy upper surface. The
basal and lower leaves have long petioles, while the upper leaves have
shorter petioles. The base of each petiole is partially covered by a sheath.
Flowers: The upper stems terminate in compound umbels of small white
flowers. These compound umbels span about 2-5" across and consist of
about 8-16 umbellets. Each umbellet consists of about 12-25 flowers. A
flower has 5 white obcordate petals, 5 spreading white stamens, and a
white nectar pad in the center. It spans only 1/8" across when fully open.
At the base of the compound umbel are several floral bracts that are ovate-
lanceolate with elongated tips; there are also several bractlets at the base
of each umbel. These bractlets are ovate-lanceolate like the bracts, but
smaller in size.
Fruit/Seeds: Each flower is replaced by a schizocarp that is broadly ovate
and somewhat flattened; it has several longitudinal ribs that are wavy.
Roots: The root system consists of a white taproot.
REGENERATION PROCESS: Poison hemlock has a varied reproduc-
tive capabilities, depending upon geographical region. It reproduces only
from seed, both as a biennial and winter annual, and occasionally as a
short-lived perennial. Seeds germinate in autumn and plants develop rapid-
ly throughout the winter and spring. Some produce flowering stems in the
first spring and die in the summer. Others remain in the vegetative stage
without producing flowering stems until the second spring, thus becoming
a biennial. Plants are more likely to be biennial in very moist situations.
After producing seeds, the plants die in the summer. The spread of hem-
lock is by seeds which can adhere to farm machinery, vehicles, agricultural
produce, mud and clothing as well as being carried by water and to a limit-
ed extent wind.
HABITAT TYPES: Habitats include edges of degraded wetlands and
prairies, low-lying areas along small rivers, banks of drainage ditches,
thickets, woodland borders, fence rows, low-lying areas along railroads
and roads, pastures, and abandoned fields. This plant is usually found in
disturbed areas, but it occasionally invades native habitats. It also invades
native plant communities in riparian woodlands and open flood plains of
rivers and streams.
SITE CHARACTERISTICS: Poison hemlock prefers full sun to light
shade, moist conditions, and a fertile loamy soil.
SEASONAL DEVELOPMENT: The blooming period occurs from late
spring to mid-summer (June through August) and lasts about 1-2 months.
GENERAL DISTRIBUTION: Poison hemlock has been reported
throughout most of the continental United States (except Mississippi
and Florida) and most of the Canadian provinces (except Newfoundland
and Manitoba). It is probable this invasive species from Europe has or
will soon be found in all states and provinces north of the Rio Grande
SKY MEADOWS DISTRIBUTION: To be determined.
IMPORTANCE AND USES: The nectar of the flowers attracts a wide
variety of flies, beetles, sawflies, and wasps, including Ichneumonid and
other parasitoid wasps. The caterpillars of the butterfly Papilio polyxenes
asterias (black swallowtail) feed on the foliage, notwithstanding the ex-
treme toxicity of the foliage. This toxicity is the result of the alkaloid
coniine and other chemicals, which can be found in all parts of poison
hemlock, including the seeds and roots. Mammalian herbivores won't
touch the foliage because of its rank odor and extreme toxicity. Just a small
portion of the ingested plant can be fatal to humans.
Poison hemlock can be a tenacious weed particularly in moist habitats and
along streams. Poison hemlock may act as a pioneer species quickly coloniz-
ing disturbed sites and displacing natives during early successional seres.
The presence of poison hemlock degrades habitat quality and could indicate
a management problem on an ecological preserve.
Poison hemlock is most famous for being used as a poison. It was probably
used to poison Socrates. Poisoning of humans has occurred after the inges-
tion of seeds, leaves and roots. The seeds, however, are the most toxic part
of the plant. Extracts of hemlock have been used as arrow poisons by North
American Indians; however, poison hemlock has also been used medicinally
for many years in treating tumors, ulcers and gout.
Crooked Run Valley