Poison Hemlock, Wild Parsnip and Giant Hogweed in Michigan: How to Tell Them Apart, and How a Stand Gets Cleared Safely
The Gibson Environmental Services crew
More than 10 years clearing land, cutting trails, and running forestry equipment in Southeast Michigan.
Three tall plants with umbrella-shaped flower heads grow along Southeast Michigan roadsides, fence rows, ditch banks and pasture edges, and all three are worth knowing by sight: poison hemlock, with smooth hollow stems blotched purple and white flowers in June, is poisonous if any part of it is eaten; wild parsnip, with yellow flowers and a grooved stem, has sap that burns skin in sunlight; and giant hogweed, rare here but a plant of real size, does the same with worse burns. Each one has a harmless look-alike, and most of the reports of hogweed in this state turn out to be cow parsnip, a native.
This guide gives the identification side by side, where each grows in our counties, what the sap actually does, the mowing window that stops a stand from seeding, and how we clear a stand of any of them as part of brush hogging or pasture reclamation work: from a closed cab, timed before the seed is made, with the cut material left to dry where it falls. It pairs with the 12-month clearing calendar.
The Three, and Their Look-Alikes, Side by Side
| Plant | Height | Stem | Leaves | Flowers | The tell |
|---|---|---|---|---|---|
| Poison hemlock | 6 to 10 ft in year two | Smooth, hollow, hairless, with purple spots, blotches or streaks | Lacy, fern-like, parsley-like, finely divided | White, flat-topped umbels, June to August | The purple-spotted smooth stem and a musty smell when crushed |
| Wild parsnip | 4 to 5 ft | Grooved, hairless, green to yellow-green | Compound, like a celery or parsnip top, with a mitten-shaped end leaflet | Yellow-green flat umbels 4 to 8 in across, June to July | The only one of these with yellow flowers |
| Giant hogweed | 8 to 15 ft | Hollow, 2 to 4 in thick, with purple blotches and coarse white bristles | Deeply lobed and jagged, up to 5 ft across | White umbels up to 2.5 ft across, late June to July | The size, and the bristly purple-blotched stem |
| Cow parsnip (native look-alike) | 5 to 9 ft | 1 to 2 in thick, mostly green, soft fuzzy hairs | Lobed like a maple leaf, less jagged, to about 2 ft | White umbels about 1 ft across, a few weeks before hogweed | Fuzzy green stem, smaller leaves, flowers earlier; what most hogweed reports are |
| Queen Anne's lace (wild carrot) | 1 to 3 ft | Solid, hairy, green | Carrot-like | White lacy umbels, often with one dark floret in the center, July to September | Hairy stem, carrot smell from the root, a third the height of hemlock |
| Golden alexanders (native) | 1 to 3 ft | Smooth, green | Compound, toothed | Yellow umbels in May to June | Flowers a month before parsnip and stays small |
| Angelica (native) | 4 to 8 ft | Smooth, often purple all over, not spotted | Compound | Ball-shaped greenish-white clusters | The round flower heads |
Two of these are biennials: hemlock and parsnip spend their first year as a low rosette of leaves and bolt into the tall flowering plant in year two, which is why a ditch that was knee-high green last year is head-high white this year. Hogweed can sit as a rosette for several years before it flowers, then flowers once and dies. The rosettes are the stage the follow-up work is aimed at.
Where They Grow in Southeast Michigan
Poison hemlock is established across the state and has been reported in Oakland County; it likes moist ground along roads, fence rows, pasture edges, creek banks and the low corner of a field. Wild parsnip has been in Michigan for a very long time and lines the road shoulders and railroad edges; a roadside survey found it along a twenty-mile stretch near Pinckney and Hamburg in Livingston and Washtenaw counties. Giant hogweed is the rare one: the state plant diagnosticians confirm only a handful of real finds in a year out of hundreds of reports, and the confirmed sites are mostly elsewhere in the state. If a plant on your land is taller than you, with a bristly purple-blotched stem thicker than your wrist and leaves wider than a door, photograph it and leave it standing until it is identified; if it is a little shorter, with a soft fuzzy green stem, it is cow parsnip, and the sap of cow parsnip can blister too.
What the Sap Does
- Wild parsnip and giant hogweed: the sap makes skin react to sunlight. Redness and blisters appear within about two days of sap plus sun, and the dark marks can last for months; hogweed burns are deeper, and sap in the eyes has caused lasting damage. Overcast days reduce the reaction; sweat and humidity make it worse.
- Poison hemlock: every part is poisonous if eaten, to people and to livestock, and the toxins stay in the plant after it is mowed or baled. Skin contact bothers some people and not others. The real exposure is mowing or trimming it from an open seat, which throws plant material at the operator, and breathing the dust.
- If sap gets on skin: wash with soap and cool water right away, keep the spot out of the sun for two days, and see a doctor for blisters or anything near the eyes.
- Working near any of them: long sleeves and pants, waterproof gloves, eye protection, and a closed cab on the machine. Smoke from a brush pile that holds them carries the oils, so they stay out of the pile.
The Mowing Window That Works
The extension weed scientists who tested it on wild parsnip found the window is narrow and worth hitting: mow after the flower heads have emerged and before the seed has matured. Too early, and the plant sends up a second flower stalk from the base; too late, and the pass spreads ripe seed. In Southeast Michigan that is late June into July for parsnip and the same for hemlock, which flowers a little later. The pass has to repeat, because the rosettes of next year's plants are already on the ground under this year's stalks: a second pass in late summer catches the late bolters, and a low pass over the rosettes in fall or early spring sets up the following year. The seed in the soil lasts about four years for parsnip, so a stand is a three-year project, each year easier than the last.
Giant hogweed is cut the same way, before the seed, with the one difference that the people doing it are fully covered and the machine is a closed cab; the rosettes get the follow-up for several years. Our guide to how often and when to brush hog a field puts these windows on the field calendar next to the thistles and knapweed.
How We Clear a Stand
From a closed cab
The brush hog and the forestry mulcher both ride on a tracked skid steer with a closed cab, which keeps the plant material and the dust off the operator. That is the difference between clearing a stand of hemlock and weed-whipping one.
Timed before the seed
The pass goes in after the umbels emerge and before the seed matures, late June into July, so the stand is cut at the one moment that costs it the most. A fall or early-spring pass over the rosettes follows.
Left to dry where it falls
The cut material lies in the sun and dries; nothing is piled, hauled or burned, and the chopped stalks are gone by the second summer. Hemlock cut in a pasture is kept away from the animals until it has dried and broken down, which is the one case where a strip is left and mowed again.
Repeat passes on the calendar
A second pass in late summer the first year, then one pass a year in the window for two more years. Each year the stand is thinner, and the native grasses and goldenrod that come back through the chopped stalks take the ground.
Pasture and hay
Hemlock in a pasture or a hay field is a livestock problem first: the toxin stays in mowed and baled plants. We cut it before it seeds, keep the animals off the strip until it has broken down, and plan the reseeding; see pasture reclamation.
Common Questions
How do I tell poison hemlock from Queen Anne's lace?
Height and stem. Hemlock stands six to ten feet with a smooth, hairless, hollow stem blotched purple and a musty smell when crushed. Queen Anne's lace stands one to three feet with a solid hairy stem, often one dark floret in the center of the flower, and a carrot smell from the root.
Is poison hemlock in Michigan?
Yes, across the state, and it has been reported in Oakland County. It favors moist ground along roads, fence rows, pasture edges and creek banks, where it flowers white in June and July in its second year.
Can you mow poison hemlock?
Yes, and timed right it is the main tool: a pass after the flower heads emerge and before the seed matures, from a closed cab so the operator stays clear of the plant material, with a second pass in late summer and a low pass over the rosettes in fall or spring. The toxin stays in the cut plants, so livestock are kept off the strip until it has broken down.
What does a wild parsnip burn look like?
Redness and blisters that show up about two days after sap and sunlight meet, in streaks or splashes where the sap landed, followed by dark marks that can last months. Wash with soap and water right away and keep the skin out of the sun for two days.
Is it giant hogweed or cow parsnip?
Usually cow parsnip. Hogweed is eight to fifteen feet with a bristly stem blotched purple and thicker than a wrist, leaves up to five feet across and flower heads over two feet wide. Cow parsnip is five to nine feet with a soft, fuzzy green stem, smaller leaves, flower heads about a foot across, and flowers a few weeks earlier.
When should I mow wild parsnip?
After the yellow flower heads have emerged and before the seed matures, which in Southeast Michigan is late June into July. Earlier and the plant sends up a second stalk; later and the pass spreads seed. Repeat for about three years, because the seed in the soil lasts that long.
What do I do if I touched the sap?
Wash the skin with soap and cool water right away, keep it out of the sun for at least two days, and see a doctor if blisters form or anything reached the eyes. The reaction needs sunlight, so getting the sap off and the skin covered is what matters.
Do you clear poison hemlock, wild parsnip and hogweed?
Yes, as part of brush hogging, pasture reclamation and roadside or fence-row clearing: from a closed cab, timed before the seed, with the cut material left to dry where it falls and repeat passes on the calendar for the next two seasons.
Sources and Further Reading
The identification features, the sap effects, the mowing window and the Michigan distribution notes come from these extension and state pages. How we clear a stand on our own jobs is stated in the text.
Michigan State University Extension: Poison hemlock identification and control
Iowa State University Extension and Outreach: Mowing requires management to be effective (wild parsnip)
Michigan State University Extension: Common plants are being mistaken for giant hogweed
Minnesota Department of Natural Resources: Giant hogweed
Michigan Invasive Species Program: Wild parsnip
University of Minnesota Extension: Poison hemlock
Where We Do This Work
The work in this guide is done across eight Southeast Michigan counties from our base in Royal Oak. Every county page lists the townships we work in most, with a page for each.
A Ditch Bank, Fence Row or Pasture Edge Gone to Hemlock or Parsnip?
We clear it from a closed cab, timed before the seed, and come back for the rosettes. The quote is free.