How to Manage Pests

Identification: Weed Photo Gallery

Poison hemlock

Scientific Name: Conium maculatum (Carrot Family: Apiaceae)

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Poison hemlock is a tall biennial broadleaf weed native to Europe. With the exception of deserts and the Modoc Plateau, it is found throughout California up to an elevation of 3300 feet (1000 m). Poison-hemlock infests roadsides, ditch banks, and pastures. Sometimes it invades vineyards and orchards planted in low-lying riparian areas. It contains piperidine alkaloids and all plant parts are highly toxic to humans and animals when consumed.

Seedling

Cotyledons (seed leaves) are light green, narrowly lance to egg shaped, three to five times longer than broad, with prominent veins underneath. Stalks below the cotyledons are often purple-tinged. The first true leaves are smooth and deeply cut two to three times, like a parsley leaf, and attach alternate to one another along the stem. Leaf stalks of young true leaves tend to be purple-tinged as well. When leaves are crushed the plant gives off a musty odor that has been described as parsniplike.

Young plant

The young plant exists as a large basal rosette of leaves for the first year.

Mature plant

Poison-hemlock matures in July and August and grows erect up to almost 10 feet (3 m) tall. In overall outline its leaves are triangular, but are divided into leaflets that are lobed or deeply divided again. Leaf stalks usually have purple streaks or spots. Upper leaves are nearly stalkless. Crushed leaves give off a musty odor that can smell similar to parsnip.

Flower

Flowers bloom from April through July. The flower head grows from the end of the flowering stem. It consists of several loose, umbrella-shaped clusters of small, white flowers. Each cluster has a stalk, 1/4 to 3/4 of an inch (0.6–1.9 cm) long.

Fruit

Fruit are grayish brown, egg shaped to nearly round, with longitudinal ribs. With time the fruit separate into two halves; each contains one seed.

Seeds

Seeds germinate in April and May, are grayish brown, and about 1/8 of an inch (0.13 cm) long and less than that in width.

Reproduction

Reproduces by seed.

More information

• Broadleaf ID illustration
• Calflora's distribution map
• See UC IPM's Pest Management Guidelines for agricultural information