Poison Hemlock In Rockbridge
Poison hemlock (Conium maculatum) is widespread in Rockbridge County and along the I-81 corridor. It has been addressed in this column before but its prevalence in the county prompts me to revisit this topic.
A native of Europe, poison hemlock was introduced to North America as a garden/ornamental plant. It has a biennial growth pattern, being a lowlying rosette with fern-like leaves the first year and bolting to 3-10 feet the second year. The stems are stout, smooth, with distinctive purple spotting. Flowers are small, white and found in umbrella-shaped clusters in early summer (late May and June). The plant reproduces prolifically via seeds that mature in July/August and are easily spread via mowing/ agriculture equipment. Poison hemlock may be confused with wild carrot (Queen Anne’s lace) or wild cow parsnip, both of which have white umbrella-shape flower clusters. Wild carrot has a hairy stem, while cow parsnip has a ribbed stem. Neither have purple spotting.
The blooming stalks of poison hemlock occur at the end of its life cycle in the second year so the main control practice at this stage is to destroy the stalks and blooms with a tractor-mounted rotary mower (commonly referred to as “brush hogging”) in an attempt to prevent the blooms from fully maturing and shedding seed. Herbicide treatment of the stalks is not especially effective at preventing the blooms from producing seed but an herbicide treatment now may get some of the vegetative (first year) hemlock plants growing underneath the bolting stalks of the second-year plants.
The poison hemlock is often growing in locations a tractor/mower deck cannot access and the only method of cutting it down is a heavy-duty weed eater/or blade. If someone uses hand-held equipment to cut-down poison hemlock stalks, they should be fully covered (long sleeves, gloves, face shield) and should launder the clothing and shower after the cutting work is done. Physical contact with poison hemlock can have varying degrees of effect on people but, in general, a severe rash with burning and itching irritation is likely to result where the plant contacts the skin The stalks will degrade on their own over the winter, but to many, the stalks are unsightly and the extent of degradation through the fall and winter is slow and uneven.
The most effective and safest means of control is to use a broad-leaf herbicide on the vegetative (1st) year plants. The best seasonal windows for treatment are September or early October and late April/early May. 2,4-D with Dicamba, Remedy™, Crossbow™, Grazon Next™ are all effective herbicide choices Poison hemlock in its second year can also be effectively suppressed in May before it shoots out its heavy stalk – but it bolts rapidly so you need to have your chemical and application equipment ready to go. If a person prefers to not use herbicides, the only other option is repeated mowing.
Copies of the Purdue University Extension publication that was a source for this column are available by calling the Rockbridge Extension Office or by sending email to stanleyt@ vt.edu.
