What is poison hemlock?

Poison hemlock identification is a matter of safety, not just yard care. This tall biennial in the carrot family came from Europe and is now common along Pennsylvania roadsides and stream banks. The PA Department of Agriculture lists it as a Class B noxious weed, and every part of it is toxic if eaten.

In its first year it grows as a low, lacy rosette that’s easy to mistake for parsley or wild carrot. In its second year it shoots up to 3 to 8 feet, flowers, sets seed and dies. It greens up very early in spring, often before most other plants, which gives it a head start along ditches and field edges.

Poisonings are rare because the plant tastes and smells bad, but they do happen, usually when someone mistakes the leaves, roots or seeds for wild parsley, carrot or parsnip. Livestock are most at risk in overgrazed pastures where there’s little else to eat.

Poison hemlock identification: what to look for

Check the stem first. That’s the clearest field mark, and you can see it without handling the plant.

• Stem: smooth and hairless, hollow, often with a whitish waxy bloom, and marked with purple blotches or streaks, especially near the base.
• Leaves: glossy, bright green and finely cut into many small segments, much like flat-leaf parsley or a fern. They give off a musty, mousy smell when bruised, but don’t crush them bare-handed to test.
• Flowers: lots of small white flowers in loose umbrella-shaped clusters, many clusters per plant, in late spring and early summer.
• Seed: small ridged seeds that turn grayish-brown as they ripen.
• Root: a white taproot shaped like a small parsnip.

Where it grows in Pennsylvania

Poison hemlock favors moist, disturbed ground. In central PA it lines roadside ditches, stream and river banks, railroad tracks, fence rows, field edges, trails and woods edges. It’s common in the river valleys, and it often moves into the unmowed back corners of rural yards and pastures.

Seed drops close to the parent plant and travels in moving water, on mowers and in fill, which is why you’ll often see long bands of it following a ditch or creek.

It also tends to show up after ground disturbance. A new culvert, a regraded bank or a cleared fence row can sprout a thick crop of hemlock the next spring if seed was already nearby.

Poison hemlock look-alikes

The most important comparison is with Queen Anne’s lace, or wild carrot. Wild carrot has a hairy green stem with no purple spots, usually stays under 3 feet, smells like carrot, and its flat flower cluster often has a single dark floret in the middle. Poison hemlock is taller, smooth, spotted and smells musty.

Wild chervil has hairy lower stems and blooms earlier. Wild parsnip has yellow flowers and burns skin in sunlight. Native water hemlock grows in wet spots and is even more dangerous. If you can’t be sure, treat the plant as poisonous.

How to get rid of poison hemlock

Gear up before you start: chemical-resistant gloves, long sleeves, long pants and eye protection. Then pick the method that fits the size of the stand.

• Dig rosettes: small numbers of first-year plants can be dug out with a shovel in fall or early spring, getting as much of the taproot as possible.
• Cut flowering plants: once plants have bolted, cut them at the base before seed ripens. Bag the plants and put them in the trash. Don’t compost them, don’t burn them and don’t leave them where animals can reach them, since dried plants stay toxic.
• Skip the string trimmer: mowing or trimming flowering plants throws sap and debris at you. Repeated mowing of first-year rosettes in a maintained area is fine and keeps them from flowering.
• Build competition: reseed bare ditch banks and field edges with grass so new seedlings have less room.

Herbicides: work best on rosettes in fall or early spring, before the plants send up flower stalks. Extension programs commonly list 2,4-D, triclopyr, dicamba and glyphosate for poison hemlock, with effectiveness dropping once it flowers. Many stands grow along ditches and streams, so the product must be labeled for use near water. Read and follow the label every time, since the label is the law. Applying herbicide for hire in PA requires a PA Department of Agriculture pesticide applicator certification, so roadside, streambank and pasture-size stands should go to a licensed vegetation-management company.

Keeping it from coming back

Seed in the soil will keep producing new rosettes for several years. Walk the area each spring and fall and remove rosettes while they’re small, using gloves every time.

Keep edges mowed or planted, and don’t let any plant set seed. Clean mowers and boots after working in a stand so you don’t carry seed to new spots on the property.

If kids or pets use the area, check it before they do each spring. The early-green rosettes are easy to spot when little else is growing, which makes March and April the best time to catch them.

Questions we get about poison hemlock

Similar weeds

Queen Anne's Lace (Wild Carrot)Daucus carotaWild ParsnipPastinaca sativaWild ChervilAnthriscus sylvestris