Poison hemlock is an extremely poisonous plant belonging to the parsley family. Sometimes called poison parsley, this biennial weed has shiny green triangular leaves and white flowers that bloom in July and August. All parts of poison hemlock are dangerous, particularly the lower stem and root. Applying an herbicide can prevent poison hemlock from destroying crops and invading livestock grazing areas.
Herbicides can be used before poison hemlock becomes a serious problem. According to Montana State University, tebuthiuron and sulfonylurea herbicides, such as the brands Spike 20P and Glean FC, are the herbicides to be used for pre-emergence control. These herbicides are recommended for use when the crop is dormant and sprayed to control any emergence of hemlock sprouts from seeds.
Phenoxy herbacides, such as Roundup and Rodeo, are recommended after poison hemlock sprouts have surfaced. According to Washington State University, these herbicides "are the most effective if sprayed in the early spring after emergence." Phenoxy herbicides should be used with water and applied directly to the sprout.
Glyphosate is the herbicide to use when poison hemlock has flowered in the rosette phase. Glyphosates provide effective control of poison hemlock but can be effected by cool temperatures. Montana State University states that glyphosate can be used for spot spraying in areas where there are no other desirable plants.
Poison hemlock is a weed that should be contained with long-term management. Weeds are infectious and spread quickly, and spot spraying the borders of areas with poison hemlock can prevent infestation.
It is important to apply herbicides until the seed bank is depleted; several applications of the chemicals may be required to control poison hemlock. The length of time between applications depends on the herbicide. According to Montana State University, "once poison hemlock is depleted, it is important to implement proper grazing, fertilization, and irrigation management to promote the growth of desired species and to reduce the risk of re-infestation."