By JEFF BURBRINK
Purdue Extension Educator
Poison hemlock is a weed that seemed to burst onto the scene last year during the drought.
In the past, it could be found in waste areas like along railroad tracks and ditches, but in 2012, poison hemlock seemed to be everywhere, including backyards, gardens, fields, even along the river at Bonneyville Mill Park. This spring it was found growing in the landscaping next to my office.
At first glance poison hemlock (Conium maculatum) may appear like wild carrot (Daucus carrota) or some kind of giant parsley, but it is not a mistake that you should make.
Although poison hemlock is more known for poisonings as a result of ingesting, for example the death of the Greek philosopher Socrates, the plant’s natural oils may absorb through the skin. If you find yourself hand-pulling poison hemlock, it would be a good idea to wear gloves.
Both poison hemlock and wild carrot belong to the parsley family (Apiaceae). Both have the characteristic small, white flowers and leaves that expand at the bases sheathing the stems. You can tell poison hemlock apart from its cousin, wild carrot, by the presence of purple blotches on the stem. The leaves of poison hemlock are also sharper in detail compared to wild carrot.
As it matures, the plant can be as much as 6 to 10 feet tall. Individual leaves can be 2 feet long. The stems are hollow. In fact, children have been poisoned by using the hollow stem as a pea shooter.
Control is best done when the plant is relatively small. Although several herbicides are available for controlling poison hemlock, herbicides should be used only on seedlings or small, low-growing rosettes and not on fully mature plants.
In addition, it is best to hand pull individual plants or small infestations, which are typical of gardens and landscapes. Herbicides such as 2,4-D, triclopyr, and glyphosate, available to both residential users and small noncommercial operations, may be a more effective option with larger infestations.
The broadleaf selective herbicide 2,4-D is most effective when applied soon after plants reach the rosette stage. Both the amine and ester formulations of 2,4-D are effective. Using 2,4-D may make poison hemlock more attractive to livestock but doesn’t change its toxicity, so some caution must be exercised if using 2,4-D in grazed pastureland or in silage production.
Like 2,4-D, triclopyr is also a broadleaf selective herbicide that is most effective on smaller plants. It doesn’t kill most grasses. Apply it during the seedling to rosette stage of growth.
Glyphosate is a nonselective herbicide, meaning it can kill desirable plants that are accidently sprayed. Apply to actively growing hemlock plants before they begin to bolt. Cooler temperatures can reduce the effectiveness of glyphosate.