Washington State Weeds

Poison Hemlock

poison hemlock

Conium maculatum • Class B

Family Name: Apiaceae family (ay-pee-AY-see-ee)
Common: Carrot/celery family (formerly Umbelliferae)
Genus: Conium (koh-NI-um) 
Meaning: From the ancient Greek koneion, the name given Poison Hemlock, referring to the plant and the poison derived from it
Species: maculatum (mak-yuh-LAY-tum)
Meaning: Spotted
Description:

Poison Hemlock is a very tall biennial plant that can grow up to 12 feet in height. It is a member of the parsnip or wild carrot family and its fernlike, finely divided leaves are similar to those of its parsley/carrot cousins. They grow in leaflets of 3 and have a mouse-like odor. The stem is strong, smooth, and has purples splotches on it. It has a long, white taproot, which may be branched. The plant has white flowers that grow in small, erect clusters. Each flower develops into a green, deeply ridged fruit that contains several seeds. Each plant is capable of producing up to 38,000 seeds a season, which can remain viable in the soil for up to 6 years. 


 Why Is it a Noxious Weed?

All parts of this plant are extremely toxic to animals and humans. The lower portions of the stem and root are particularly deadly. It can quickly invade large areas of pasture or vacant land.


Where Does it Grow? It is often found on poorly drained soils, particularly near streams, ditches, and other surface water. It also appears on roadsides, edges of cultivated fields, and waste areas.

Facts:

Poison hemlock remains toxic for several years after being pulled. Bag up and dispose of all dead plant material to prevent accidental poisoning of wildlife or children. Do not compost this plant. 

Of the eight known alkaloids in poison hemlock, the principle toxins are the Piperidine alkaloids, coniine and coniceine. Coniine is more common in the seed and in mature plants, but coniceine makes up 98% of the total alkaloids in the early vegetative stage. Coniceine is 8 times as toxic as coniine. Dead canes remain toxic for up to 3 years.

CONDITIONS THAT LEAD TO P.H. POISONINGS: Leaves are mistaken for the edible plants, parsley, carrot, Bur Chervil; seeds mistaken for Anise; root for Wild Carrot or parsnips; stems used for straws to blow through. It takes only 4 oz of P.H. vegetation to be fatal to adults, less for children; 16 oz is fatal for a horse or cow. 

Studies indicate that Poison Hemlock has allelopathic characteristics. Research shows that mowing Poison Hemlock during fruit development stage just after flowering or before seed set  intensifies or concentrates the coniine and other forms of the alkaloids which then act as allelochemicals in the soil. 

Control Options:

Hand removal is recommended for small infestations. When pulling this plant, be sure to remove the entire tap root. Wear gloves, long sleeves, and wash thoroughly after working with poison hemlock. Mowing and cutting of this plant is not effective for control, as the plant simply develops new seed stalks in the same season the cutting occurs.  

  • Spot spraying with an herbicide containing the active ingredient glyphosate (Roundup Pro, Glyfos, etc.) or products containing the active ingredient imazapyr (Habitat® and Arsenal®), may be used effectively while the plant is actively growing, prior to seed production. Spray each plant thoroughly on the stems and leaves, enough to be wet but not dripping. Be aware, glyphosate & imazapyr are non-selective and will injure any plants that they come in contact with, including grass. After herbicide application the area must be monitored in succeeding years to check for new plants from the existing seed bank. 

  • When using herbicides, always read and follow all label instructions and obey all label precautions. (Note: pesticide product registration is renewed annually and product names and formulations may vary from year to year.)


More Information:

 Download our Flyer or visit Washington State Noxious Weed Control Board Here. Photo by Leo Michels

 


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