WSU Extension

Hortsense

Weeds : Poison hemlock : Conium maculatum
(revision date: 2/12/2019)

Family: Apiaceae (Umbelliferae)
Cycle: Biennial
Plant Type: Broadleaf

Use Integrated Pest Management (IPM) for successful weed management.

Biology
Seedling poison hemlock plants are rosettes of finely divided, fern-like leaves which closely resemble carrot leaves. Mature plants grow six to eight or more feet tall from a large white taproot that is solid throughout. The alternate, fern-like leaves have a distinct, unpleasant odor. The stout, hairless stems are erect and branching, and are marked with purple spots. Small white flowers occur in large, open, umbrella-shaped clusters at the tips of branches. Each flower cluster is supported by its own stalk. The rough, gray-brown seeds are distinctly ribbed. SPECIAL INFORMATION: All parts of this plant are poisonous, particularly the large white taproot and the seeds. This plant can be mistaken for parsley (Petroselinum crispum) or wild carrot (Daucus carota). In WASHINGTON and OREGON, poison hemlock is designated as a Class 'B' noxious weed. Management may be required by law in your county. In addition, it is on the Oregon noxious weed quarantine list, which prohibits sale, purchase, and transport of plants, seeds, and plant parts. Consult your local Noxious Weed Control Board for more information.
Habitat
Poison hemlock commonly grows on the borders of fields, roadsides, and waste areas. It can tolerate poorly drained soils and may also be seen on ditch banks and along streams. It is not usually a problem on maintained lawn and turfgrass.

Management Options

Non-Chemical Management
  • Cultivation (rototilling or hoeing) will effectively eliminate plants.
  • Reduce weed infestation by handpulling weeds.
  • Mowing to prevent seed production is a very effective means of management. In lawns, mowing regularly at the proper height for the grass species may help minimize weed growth and invasion.
  • Careful digging is useful to manage weed populations. However, digging can carry undesirable weed seed to the surface and foster further germination.
Select non-chemical management options as your first choice!

Chemical Management

Apply according to label directions. Glyphosate products should be applied as spot treatments only! NOTE: Some ingredients listed here are only available in combination. Read the label carefully on combination products to make sure the product is suitable for your specific situation.

Landscape areas
  • glyphosate
Turf areas
  • 2,4-D + MCPP + dicamba
Bare ground areas
  • glyphosate
Images
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Caption: Poison hemlock leaves
Photo by: T. W. Miller
Caption: Poison hemlock umbel
Photo by: T. W. Miller
Caption: Poison hemlock fruits
Photo by: T.W. Miller
Caption: Poison hemlock
Photo by: D.D. Tapio
Caption: Poison hemlock flowers
Photo by: T. W. Miller
Caption: Poison hemlock seeds
Photo by: T. W. Miller
Caption: Poison hemlock stem
Photo by: T. W. Miller
Caption: Poison hemlock
Photo by: T. W. Miller
Caption: Poison hemlock seedling
Photo by: T. W. Miller