April Is the Best Time to Control Poison Hemlock
Published: Friday, April 17, 2026
The weed I have had the most phone calls about over the past five years is poison hemlock. It first appeared as an invader on non-tilled waste areas, like the spaces along roads, railroad tracks, ditches and fence lines, gradually working its way into other areas like pasture and hayfields. All parts of the plant are poisonous to both humans and animals, making it more than just a nuisance.
Most of the calls I receive about hemlock will occur in June and July, but April is the best time to do something about it. Once it flowers (when most people realize they have a problem), there is little that can be done.
A native of Europe, poison hemlock is a member of the Apiaceae (parsley) family. It has a biennial growth pattern, being a low-lying rosette the first year, eventually 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.
Poison hemlock has fern-like leaves, pinnately compound and arranged alternately on the stem. The plant reproduces with massive numbers of seeds that are flattened and ribbed. Seeds mature in August/September and are easily spread via mowing/agricultural equipment.
Poison hemlock is easily confused with other members of the parsley family. Wild carrot (also known as Queen Ann's Lace) or wild cow parsnip, both of which have white umbrella-shape flower clusters are very similar. Wild carrot has a hairy stem, while cow parsnip has a ribbed stem. Neither have purple spotting.
Another look-a-like is wild parsnip, which has a flat-topped cluster of yellow flowers, and a coarse, green stem that is deeply grooved. Ohio State Extension has a comparison chart of wild carrot, poison hemlock, wild parsnip, cow parsnip and giant hogweed at https://ohioline.osu.edu/factsheet/anr-102.
Poison hemlock spreads via seed, so effective management must prevent new seed production, prevent spread of existing seed, and exhaust the existing seed supply in the soil seed bank. Prevention: Poison hemlock seed often is inadvertently spread by mowing, road maintenance or agricultural equipment. Most importantly, mow infested areas several times along roadsides, ditch banks and field edges before seed matures.
Poison hemlock seed maturation may vary from year-to-year depending on weather patterns. Mowing should occur from late April through early to mid-July. Avoid working, recreating in or walking or driving through infested areas during seed dispersal periods. Also, clean clothing, shoes, ATVs or vehicles following activity in infested areas.
The most effective control may be a combination of mowing to prevent seed production, followed with herbicide applications to rosettes and resprouts.
Manually pulling the plants can be effective for single plants or very small infestations. Pull or dig up all plants, place in trash bag and dispose of with regular trash. Always wear protective clothing, including gloves and eye protection, to prevent the plant from contacting skin.
Mowing or cutting may be effective control but must be repeated often because the taproot can send up new shoots after a single mowing. Tilling or grubbing can kill hemlock and prevent seed production but is generally not recommended because of soil disturbance. Again, protect your skin and eyes while mowing.
Chemical control is effective for large infestations and for spot spray applications to individuals and clumps. Herbicide application should be performed while the plant is actively growing and before flowering. First-year basal rosettes may be sprayed from midsummer through fall. Second-year plants begin bolting flower stalks in April and begin flowering in mid-May to June. Follow-up treatments will be required, as seeds already present in the soil sprout. Follow label directions and use a surfactant to increase effectiveness.
Glyphosate herbicides containing at least a 41% concentration of glyphosate and follow label directions to mix a 2% spray solution. Thoroughly wet all surfaces of the plant but not to the point of runoff. Use caution: Glyphosate is nonselective and will damage or kill any plant it contacts.
Growth regulator herbicides like 2,4-D or triclopyr also work well providing they are applied while the plants are small. These broadleaf-specific herbicides will not harm grasses. They are most effective on first-year rosettes or very small second-year plants.
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