Weed of the Month: Poison Hemlock

Published: February 29, 2012

One of the first weeds that you can see "greening-up" right now along roadsides, pastures, and a lot of other areas is poison hemlock (Conium maculatum L.). Poison hemlock is an erect biennial or perennial plant that typically ranges from 4 to 6 feet in height and is primarily a weed of roadsides, pastures, fencerows, and non-crop areas, but in recent years has been found as a weed in no-till corn and soybean fields as well.

Figure 1. Poison hemlock infestations like this one have been common in pastures and roadsides, but in recent years this plant has also appeared as a weed of no-till cropping systems.

Figure 2. Poison hemlock in the rosette stage of growth.

Poison hemlock develops into a basal rosette during the first year of growth (Figures 1 and 2) and then produces an erect vegetative stem and flowers during the second year of growth (Figure 3). Stems are smooth, covered with purple spots, and are hollow except at the nodal regions. Poison hemlock flowers are white and arranged as compound umbels that usually range from 7 to 15-inches in width. Poison hemlock leaves are arranged alternately along the stem and are often glossy in appearance. Upon closer examination, you will see that each leaf is divided into pairs of opposite leaflets that each measure 4- to 10-mm in length (Figure 4). Poison hemlock can produce as many as 38,000 seed per plant (Whittet 1968), with seed dispersal generally occurring when the plants are mowed.

Figure 3. Mature poison hemlock plants in the second year of growth.

Figure 4. Poison hemlock leaves are divided into pairs of oppositely arranged leaflets.

Poison hemlock produces many chemicals that can be toxic to both humans and animals. The most toxic compounds produced by poison hemlock are coniine and y-coniceine, which are alkaloids that primarily affect the reproductive and central nervous systems (Sheley and Pertroff 1999). Coniine is the alkaloid present in the greatest abundance in mature plants and seeds. However, in the early vegetative stages of growth, approximately 98% of the total alkaloids present are y-coniceine (Panter et al. 1988a). This is significant in that y-coniceine is about 8 times more toxic than coniine; therefore consumption of young poison hemlock plants can lead to significant problems (Panter and Keeler 1988). Livestock that are affected the most by poison hemlock consumption are cattle, horses, and goats; sheep and swine are less sensitive due to their liver's ability to metabolize the plant's toxic compounds more efficiently (Cheeke and Shull 1985). Fresh poison hemlock tissue has been reported to be lethal at amounts ranging from 2 to 6 grams of plant material per pound of body weight in cattle, sheep, and pigs (Keeler and Balls 1978; Panter et al. 1988a). Symptoms of poison hemlock consumption and poisoning include dilation of the pupils, reduced heart rate, coma, trembling, nervousness, and respiratory paralysis, which may eventually lead to death (Panter et al. 1988b). The rates at which livestock are affected varies; cattle and sheep are usually affected within a couple hours, while horses may show symptoms within 30 to 40 minutes (Vetter 2004; Cheeke and Shull 1985).

Non-chemical options for the control of poison hemlock include hand-weeding, mowing, and tillage. Growth regulator herbicides and products that contain 2, 4-D, dicamba, or 2, 4-D plus triclopyr are all effective in controlling poison hemlock in a grass pasture or hayland setting. Glyphosate (sold as Roundup or a variety of other trade names) also has some activity on poison hemlock. In no-till corn or soybean fields, glyphosate plus 2, 4-D or glyphosate plus dicamba should be considered for the burndown of this weed prior to planting. Weed scientists at Purdue University have found that dicamba plus glyphosate may offer more consistency and higher levels of poison hemlock control than applications of glyphosate plus 2, 4-D (Nice et al. 2005). The key to the chemical control of poison hemlock plants in any setting is to apply the herbicide when plants are young and in the rosette stage of growth, rather than when bolting and stem elongation has occurred. Once the central stems are present that will eventually support flower and seed production, control of these plants with any herbicide is likely to diminish. In most areas of Missouri, poison hemlock is in the rosette stage right now and has not yet begun to bolt, although this will likely occur soon if we continue to experience above-average temperatures. So, as long as temperatures remain around 50? or so for at least a couple of hours following herbicide application, then now is the time to treat.

Literature Cited

Cheeke, P.R., and L.R. Shull. 1985. Natural Toxicants in Feeds and Poisonous Plants. AVI Publ. Co., Westport, CT.

Keeler, F.D. and L.D. Balls. 1978. Teratogenic Effects in Cattle of Conium maculatum and Conium Alkaloids and Agalogues. Clinic. Toxicol. 12(1):49-64.

Nice, G., B. Johnson, T. Bauman and T. Jordan. 2005. Poison Hemlock – The Toxic Parsnip. Purdue Extension. http://www.btny.purdue.edu/WeedScience/2003/Articles/ PHemlock03.pdf. Accessed February 23, 2012.

Panter, K. E. and R. F. Keeler. 1988. The hemlocks: poison-hemlock (Conium maculatum) and waterhemlock (Cicuta spp.). p. 207-225. In: L. F. James, M. H. Ralphs, and D. B. Nielsen (eds.), The Ecology and Economic Impact of Poisonous Plants on Livestock Production. Westview Press, Boulder, CO, 207-25.

Panter, K.E., T.D. Bunch and R.F. Keeler. 1988a. Maternal and fetal toxicity of poison hemlock (Conium maculatum) in sheep. Am. J. Vet. Res. 49:281-283.

Panter, K.E., R.F. Keeler, and D.C. Baker. 1988b Toxicoses in livestock from the hemlocks (Conium and Cicuta spp.). J. Anim. Sci. 66:2407-2413

Sheley, R.L., and Petroff, J.K. 1999. Biology and Mangement of Noxious Rangeland Weeds. Oregon State University Press. Corvallis, OR.

Vetter, J. 2004. Poison hemlock (Conium maculatum L.) Food and Chem. Toxicology 42:1373-1382. doi: 10.1016/j.fct.2004.04.009.

Whittet, J. 1968. Weeds. 2nd ed. Sidney, Australia. Melbourne: Inkata Press. 487 pp.