(Backyard Gardener - Photo Illustration - MetroCreativeConnection)

Hello Mid-Ohio Valley farmers and gardeners! The summer growing season continues. The next 2 weeks would be a great time to plant squash such as zucchini and yellow crookneck in addition to another crop of sweet corn and cucumbers.

This week I want to talk about an invasive plant spreading rapidly across the valley, poison hemlock (Conium maculatum). I have seen this weed along roadsides, abandoned fields and even hay and crop fields.

It was imported into the U.S. as an ornamental in the late 1800s from Europe, West Asia, and North Africa. Rogue plants remained relatively rare until around 30 years ago. Since that time, poison hemlock has been become a very common invasive threat here in West Virginia and Ohio.

Poison hemlock is one of the deadliest plants found in North America. All parts of the plant are poisonous including the leaves, stems, seeds, and roots. However, the toxins must be ingested or enter our body through our eyes, nasal passages, or cuts in our skin to induce poisoning.

Contact with the plant should be avoided due to the ability of plant sap to cause rashes or blisters. The toxins may be absorbed through the skin and lungs, so be sure to wear gloves and a mask when handling these plants.

Poison hemlock contains toxic alkaloids that can cause severe poisoning in cattle and other livestock if consumed. Even small amounts can be deadly. These alkaloids affect the central nervous system, leading to symptoms like muscle tremors, difficulty breathing, and eventually death. The two principal alkaloids are coniine and ɣ-coniceine.

Be cautious! The toxicity remains in dried plants and hay. Feeding hay contaminated with poison hemlock could also result in fatalities. This is why it’s crucial to avoid baling hay that contains poison hemlock, especially if the plant has not been completely removed.

The risk of toxicity persists in hay, even after it has been dried and stored. Extension advises not to feed hay from fields where poison hemlock has been present.

One of the key challenges in managing poison hemlock is identification. It resembles both wild parsnip (Pastinaca sativa) and Queen Anne’s lace (Daucus carota). All these plants belong to the carrot family Apiaceae.

When scouting, pay attention to the distinct differences that can help differentiate poison hemlock look-alikes. First, poison hemlock has purplish spots or streaks on its smooth, hollow stems. This is a crucial feature for proper identification.

The old name for the carrot family was Umbelliferae, which refers to the umbel flowers. The flowers are a key family feature with short flower stalks rising from a common point like the ribs on an umbrella.

Poison hemlock has white flowers while wild parsnip blooms are yellow. The wild parsnip’s flat-topped yellow flowers have a characteristic Umbelliferae arrangement, while poison hemlock’s white flowers are arranged in distinct clusters. Poison hemlock leaves are fern-like and often have a strong, unpleasant odor when crushed.

Concerning Queen Ann’s Lace, both plants share similar white, umbrella-shaped flower clusters. However, Queens Ann’s Lace stems are solid green, covered in fine, fuzzy hairs. While the hemlock stems are smooth, hairless, and marked with distinct purple or reddish blotches. Hemlock grows much taller, reaching up to 8 feet while wild carrot is much shorter.

Poison hemlock is characterized by smooth hollow stems between nodes with distinct purple spots or striations that appear as larger blotches in younger plants. Leaves are pinnately compound, glossy and are arranged alternately on the stem. It has an umbel-shaped bloom about 12 inches in diameter and white in color.

Poison hemlock has a biennial life cycle. The first year is spent in the vegetative stage as a low-growing basal rosette. The rosettes use carbohydrates acquired through photosynthesis to produce a robust root system.

Plants flower or “bolt” during the second-year reproductive stage to produce erect multi-branched stems topped with umbrella-like flowers. Mature poison hemlock plants can tower as much as 6-10 feet tall.  Plants die after producing seeds.

Poison hemlock is a prolific seed producer. Research has shown that seed production ranges from 1,700 to as high as 39,000 seeds per plant with seed germination rates averaging around 85%. Seeds remain viable for 4-6 years. 

This means that management tactics must account for new plants arising annually from the “seed bank” until there are no longer any viable seeds to contribute to infestations. It is critically important to control poison hemlock before it flowers and produces seeds.

Poison hemlock is most susceptible to control with herbicides in the fall, when in the low-growing rosette stage and early spring before stem elongation occurs. Its fern like leaves are easy to identify. It is preferred to spray in the fall, but several products can work well in spring. 

Mechanical control including grubbing or hand-weeding is practical when the weeds are present in small numbers. Wear long gloves and a long-sleeved shirt while handing the plant. Tilling or cultivation is not recommended as the entire root system may not be removed and it creates a risk of inhaling toxic vapors.

Chemical control is practical to manage larger populations of poison hemlock. Herbicides effective against poison hemlock are Crossbow (2,4-D + triclopyr), Remedy Ultra (triclopyr); Cimarron Max and Escort (metsulfuron-methyl), Curtail (clopyralid), dicamba, and glyphosate. Mixing glyphosate and dicamba can improve control compared with either applied alone.

Poison hemlock is an aggressive, invasive weed which may take several years to control. Contact me at the Wood County WVU Extension Office (304)-424-1960 or e-mail me at jj.barrett@mail.wvu.edu with questions.

Good Luck and Happy Gardening!