How to Identify and Remove Poison Hemlock

Poison hemlock is a highly toxic plant that takes over yards, meadows, roadsides, and ditches. It was introduced into the United States as an ornamental garden plant in the 1800s but quickly became known as problematic, and although it is now naturalized in almost every state, it is classified as an invasive plant. Poison hemlock, a member of the carrot family (Apiaceae), looks like other plants and changes in appearance as it ages. Once you know how to identify poison hemlock, there are multiple methods used to eliminate the plant, though consistency is key. Read on to learn how to remove poison hemlock, as well as prevent it from returning to your property.

Common Name	Poison hemlock
Botanical Name	Conium maculatum
Plant Type	Herbaceous, biennial
Mature Size	2-10 ft. tall
Soil Type	Moist
Bloom Time	Spring, summer
Flower Color	White
Hardiness Zones	4-8 (USDA)
Native Area	Europe, Africa
Toxicity	Toxic to humans and animals

Which Parts of Poison Hemlock Are Toxic?

All parts of the poison hemlock—leaves, flowers, seeds, stems, and roots—are highly toxic. Merely touching it or brushing up against accidentally causes severe, long-lasting injury.

How to Identify Poison Hemlock

The plant changes in appearance during its two-year life cycle. Every stage of the plant's life has clear identification markers. Practice utmost caution when looking at the plant for identification and abstain from getting too close or touching it without proper protective gear.

In the first year, poison hemlock grows in a large rosette of basal leaves up to 2 feet long. The dark green leaves are alternate, mostly triangular, lacy, and deeply divided. The leaves and stems are hairless.

In the spring of its second year, after the plant emerges from its winter dormancy, poison hemlock develops branching and grows a flowering stalk towering between 8 to 10 feet. The stem is sturdy yet hollow. It might appear ridged because of its veins. A telltale characteristic of the plant is the purple blotches or spots on the stem. The leaves on the upper part of the stem are not as large as the basal leaves.

The bloom time of poison hemlock varies greatly from year to year depending on weather conditions. Between May and August, white flower clusters, either flat-topped or slightly convex like an umbrella, appear on the end of the stalks. The clusters are 2 to 3 inches in diameter and each flower consists of five small petals.

After the plant sets seeds, it dies. The stems with the seed capsules, each containing two round and ribbed seeds, remain on the plant. The seed capsule splits and releases the seed when mature.

Plants That Look Similar to Poison Hemlock

There are non-toxic native plants, toxic native plants, and invasive toxic plants that at first glance look similar to poison hemlock. Most of them, however, have distinct features that set them apart.

• Angelica(Angelica atropurpurea) shares purple stems with poison hemlock. The leaves are compound and sharply toothed, not lacy.
• American elderberry(Sambucus canadensis) also has white flowers that are much larger than those of poison hemlock. Elderberry has oblong, sharply serrated leaves and woody stems without any purple spots.
• Common ragweed(Ambrosia artemisiifolia) has very lacy leaves, but the stems are hairy. The inconspicuous green flowers in the late summer are located at the end of the branches.
• Common yarrow(Achillea millefolium) leaves are feathery and fern-like and the plant is much smaller, about 3 feet.
• Tall meadow rue(Thalictrum dasycarpum) can have purple spots on green stems. The leaves are lobed and not lacy.
  
• Cow parsnip(Heracleum lanatum)is as tall as poison hemlock and also toxic. Its flower head is distinctly different, much larger, and up to 1 foot across. The stem is green without any purple spots and the deeply lobed leaves are much larger than poison hemlock.
• Spotted water hemlock(Cicuta maculata) and western water hemlock (Cicuta douglasii) are two related native species that are both highly toxic, just like poison hemlock. While it might take a trained eye to distinguish the flowers–umbels of small white flowers on tall stems that reach up to 6 feet in height—from those of poison hemlock the stems are different. The stems of water hemlock are pale green with purple stripes, which distinguishes them from the spotted or blotchy stems of the poison hemlock. The location also gives you cues, as water hemlock grows primarily in wet locations such as ditches, stream banks, pond margins, and marshes.
• Giant hogweed(Heracleum mantegazzianum) grows much taller than poison hemlock, up to 15 to 20 feet in height, and it is just as invasive and toxic. The stems may have purple spots like poison hemlock butthe leaves look different, they are deeply incised and not lacy.
• Queen Anne’s lace(Daucus carota) is often confused with poison hemlock because of its lacy leaves. The stem of Queen Anne’s lace, however, is hairy and it does not have purple blotches. Also, Queen Anne’s lace has lacy bracts under the white flower heads.The plant is also toxic.
• Wild parsnip(Pastinaca sativa)is a tall plant that you should never touch because it’s so toxic. The similarities with poison hemlock end here. Wild parsnip has celery-like leaves and yellow flowers. The stems do not have purple blotches.

Where Does Poison Hemlock Grow

Poison hemlock grows in full sun to light shade. While it prefers moist soil, it is otherwise not finicky about its location. Also called a “pioneer species,” it quickly populates disturbed sites such as cleared woodlands. It grows along roadsides, along farm fields, in ditches, marshes, and meadows.

Poison hemlock spreads via seeds (up to 30,000) that drop near the plant. Seeds are also dispersed through wildlife and water.

How to Get Rid of Poison Hemlock

Removing poison hemlock depends on the age of the plant and the level of infestation. Wear protective clothing (long sleeves) and avoid letting any skin touch the plant.

• By hand: Remove small first-year seedlings by hand after rain when the soil is moist so the taproot is easier to pull out. Use a trowel or a shovel if the rosette is already big.
• Mowing: Repeatedly mow over a heavy infestation and a large area to weaken the plants. Mowing must be done in the early summer before the plant has set seeds, otherwise, you will disperse the seeds even more. Mowing in the late summer after poison hemlock has gone into seed also poses a greater health hazard.
• Herbicide: If you cannot get the infestation under control manually, use a broad-spectrum herbicide in the first year of the plant’s life cycle. Keep in mind that it will also kill all the other plants so apply it in a highly targeted way and when there is no wind to avoid herbicide drift. Applying an herbicide does not prevent seeds that are already germinating in the soil (poison hemlock seeds remain viable for up to six years) so you will likely have to repeat the herbicide application when new seedlings emerge.
• Disposal: Any pulled plants as well as seed heads should be disposed of in the garbage; do not compost them, as the toxins will remain. Do not burn the plant residue, which releases toxins into the air.
• Replace with native plants: Eradicating a severe poison hemlock infestation takes persistence, several growing seasons, and possibly a combination of manual and chemical methods. After you have fully cleared an area, replant it promptly with desirable, preferably native plants, to prevent poison hemlock from re-establishing itself.

Preventing Poison Hemlock from Spreading

Because poison hemlock spreads by seed, getting rid of the entire plant before it blooms and sets seed is key. If you discover the plant too late in the season when it has already formed seed heads or the infestation is large, the least you should do is remove the entire seed heads promptly before the seeds fall, usually between September and December. Wear protective gear when you do that and dispose of the seed heads in the garbage.

From: thespruce