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Alligatorweed
(Alternanthera philoxeroides)
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Photo credit: © Dean Morley, https://flic.kr/p/8j8qge
Poison Hemlock
(Conium maculatum)
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Species at a Glance
Poison hemlock, also called deadly hemlock and poison parsley, is a weedy plant in the parsley family that is acutely toxic to both people and animals. It has a biennial growth pattern with first year growth forming low-lying rosettes of lacy leaves with reddish or spotted stems, followed by extensive growth of 1-3 m (3-10 ft) during the second year.
Identification
Leaves: Bright green, fern-like, shiny leaves are highly dissected and alternately arranged on the stem, dividing three to four times. They can reach up to 30 cm (12 in) long and 10 cm (4 in) wide and have a strong musty smell.
Flowers: Small, white, five-petaled flowers form in umbrella-shaped clusters in June and July.
Fruits/Seeds: Light brown, barrel-shaped seeds are paired, flattened, and 0.3 cm (0.1 in) long with conspicuous longitudinal ribs. They form in green, ridged capsules that eventually turn brown.
Stems/Roots: Stout, extensively branched, and erect stems have distinct ridges. They are hollow, except at the nodes, and the lower portion of the stems have purple spots. The roots have an odor similar to carrots or parsnips.
Similar Species
Poison hemlock may be confused with other members of the carrot family that have white umbrella-shaped flower clusters, including Queen Anne’s lace (Daucus carota) and cow parsnip (Heracleum L.). Queen Anne’s lace has hairs on the leaves and stems that are not found on poison hemlock. Cow parsnip differs from poison hemlock by its ribbed stem and its palmately compound leaves, which radiate at the end of the stalk in a semicircle. Neither look-a-likes have purple spotting on the stems.
Habitat
Poison hemlock grows at low elevations bordering pastures and cropland and gradually invading perennial crops. It can frequently be found in stream and ditch banks, riparian woodlands, and floodplains. It tends to be more competitive in wet soils conditions but can survive in dry sites.
Spread
Reproducing only by seed, poison hemlock relies on spread primarily through water, birds, animals, and human activity such as agricultural equipment and mowing. Up to 40,000 seeds per plant are produced, which often drop next to the parent plant and regenerate, forming dense stands. Seeds also have a long dispersal period (from September to late February) because the plant stalks can persist through winter and seeds can remain viable for up to six years.
Distribution
A native of Europe, poison hemlock was introduced to North America in the 1800's as a garden and ornamental plant. Since then, it has been extremely successful at distributing itself in nearly every state in the United States and is present in all Mid-Atlantic states.
Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.
Environmental Impacts
Poison hemlock grows aggressively in moist pastures and meadows where it has the potential to outcompete more desirable native species. Early spring growth makes it more likely to be eaten by animals when there is limited forage available. All parts of the plant are highly poisonous and can be fatal to humans and livestock. Symptoms of poisoning can occur in as little as 20 minutes and death can result from respiratory paralysis after ingestion.
Video
Note
Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).