Poison hemlock (Conium maculatum) is a plant of importance since it can be poisonous to livestock. Livestock may be attracted to it early spring, even though the plant is not considered very palatable, because it may be one of the few green, growing plants in the landscape. Try to keep livestock from having grazing access to the plant. The poisonous alkaloids are present in all plant parts, although leaves, flowers and seeds tend to have the highest concentrations. Cattle are more sensitive to the effects of poison hemlock alkaloids relative to other livestock. Consuming just five pounds of foliage can be potentially lethal for cows.
A number of poison hemlock plant samples were submitted to the UNL Diagnostic Clinic for identification in the last four years. This species seemed to have flourished in both rural and urban landscapes in recent years. Poison hemlock is in the parsley family and has potential human health risks. The lacy appearance of hemlock leaves can be confused with edible parsley when plants are young. The ingestion of small amounts of poison hemlock can make humans sick as well. One way to distinguish poison hemlock is the heavy smell it produces when leaves or stems are broken or disturbed.
Poison hemlock is a taprooted biennial broadleaf plant. A distinguishing characteristic is the purple to red spots or irregular blotches on the hollow stems. The leaves are finely divided, hairless, and may have a glossy green color. Leaves are alternate on the stem, but this may be difficult to determine in its first year of growth, since it may have a very basal rosette appearance. It produces a stalk in its second year of growth.
Flowers are small, white, and arranged in relatively large compound umbels. When mature, plants can reach 10 feet in height. It is often described as thriving in moist soils and is commonly found in pastures, ditchbanks, and roadsides.
According to the USDA Ag Research Service, "Because of its attractive flowers, poison hemlock was brought to the United States from Europe as a garden plant. It is moving onto rangelands. Poison-hemlock is found at roadsides, on edges of cultivated fields, along creekbeds and irrigation ditches, and in waste areas."
A mixture of 2,4-D + dicamba or Grazon P&D can effectively control poison hemlock when applied in the fall or early spring before they begin to bud. Spot treatments may be effective if three to four warm, sunny days, with nighttime temperatures above freezing follow application. Keep livestock out of treated areas, as the poisonous alkaloids can still be present in dead leaf tissue. Dead plant tissue also tends to be more palatable to livestock than green or growing plants. Repeated herbicide applications may be necessary over a number of years in heavily infested areas for adequate control. Follow all label directions when using herbicides. Poison hemlock can also be controlled with repeated mowing.