Phototoxic Plants: Wild Parsnip and Giant Hogweed

Why this matters

A crew member comes in two days after trimming a roadside strip with blistered streaks down both shins that look like a chemical burn, and nobody can say what caused it, because nothing hurt at the time. It was plant sap and sunshine. Wild parsnip and giant hogweed are spreading along roadsides, ditches and property edges across the northern states, and the injury they cause is a real burn that can scar the skin dark for months. It is also almost entirely prevented by sleeves, long pants and gloves on the days the crew is cutting the wrong plant.

How the burn works

These plants carry chemicals called furanocoumarins in their sap. On skin, the chemicals do nothing on their own. When ultraviolet light from the sun hits that skin, they react with it and damage the cells. The result is phytophotodermatitis: a burn caused by plant sap plus light.

Four facts set it apart from poison ivy:

• It is not an allergy. Poison ivy affects people who are sensitized to it. Phototoxic sap burns everyone whose skin gets sap and sun together. There is no "immune" tech.
• Both ingredients are needed. Sap in the shade, or sap washed off before the sun reaches it, causes little or nothing. Sap on bare skin in full summer sun causes the worst burns, and moisture from sweat makes it worse.
• It shows up late. Redness typically appears about 24 hours after exposure and blisters within 48 hours, so the tech rarely connects it to the job. The pattern gives it away: streaks, drips and splash marks matching where sap ran or sprayed, not a sunburn that covers all exposed skin evenly.
• It leaves a mark. After the blisters heal, the skin often stays dark brown in the same streaks for months, sometimes longer. A tech who is burned once often gets the marks rechecked by a doctor who wonders what happened.

Ultraviolet light passes through cloud cover, so an overcast day reduces the risk but does not remove it.

The two plants that matter most

Wild parsnip. An invasive member of the carrot family, now common along roadsides, ditches, railroad edges, abandoned fields and the unmown margins of properties across much of the Midwest and Northeast and into the Great Plains and Pacific Northwest. In its first year it is a low rosette of leaves; in its second year it sends up a stem 2 to 5 ft tall with flat-topped clusters of small yellow flowers in early to mid summer. The stem is hollow and grooved, and the leaves are divided into toothed leaflets along a central stalk. Flowering season, roughly June and July, is when its sap is most potent and when crews are trimming roadside edges. It is the plant most likely to burn a lawn or landscape crew.

Giant hogweed. A much larger, much rarer invasive from the Caucasus, established in parts of New York, Pennsylvania, New England, the Great Lakes states, and Washington and Oregon. It grows 7 to 14 ft tall, sometimes taller, with white flower clusters up to about 2 ft across, a thick stem marked with purple blotches and coarse white hairs, and deeply cut leaves that can be several feet wide. Its sap causes the same burn, often more severely, and sap in the eyes can cause serious eye injury. It is on the federal noxious weed list (7 CFR Part 360), which restricts moving or propagating it, and several states, New York among them, run eradication programs that want sightings reported.

The look-alikes

The carrot family is full of plants with umbrella-shaped flower clusters, and a crew does not need to be botanists, just able to tell which ones change the PPE:

The kitchen version. Lime and other citrus juice carry the same chemistry. A tech who squeezes limes into lunch on a tailgate and then works an afternoon in the sun can get the same streaked burns on the hands and forearms. It is a useful way to explain the mechanism to a crew, and it happens.

Covering skin when you trim, mow or dig

The exposure that burns people is mechanical: a string trimmer, mower, brush cutter or weed whip that breaks the stems and throws sap onto bare legs and arms. Hand pulling with bare hands, and brushing through tall plants in shorts, come next.

• Cover arms, legs and hands whenever cutting through roadside or field-edge vegetation in summer: long pants, long sleeves, and waterproof gloves (nitrile, rubber or coated), because sap soaks through thin cotton and leather. Tuck pants into boots or band them, since the ankle gap between sock and pant is a common burn site.
• Protect the eyes. Safety glasses at minimum when trimming, and a face shield over them when running a trimmer into tall growth that may include giant hogweed or cow parsnip.
• Change the cutting plan for a known patch. Mow wild parsnip before it flowers if the customer wants it controlled, or skip a flowering patch rather than shred it. Leave giant hogweed alone entirely: the trade's job is to stay away from it and report it, not to cut it, and moving cuttings or seed heads off site is what the federal noxious weed rule restricts.
• Clean the tools. Wipe trimmer shafts, handles and guards with soap and water, wearing gloves; sap on a handle can burn the next bare hand that holds it in the sun.

If sap gets on skin or in the eyes

1. Get out of the sun and wash the skin with soap and cold water as soon as you can. Washing quickly is the single biggest factor in how bad the burn gets.
2. Keep the area covered from sunlight for at least 48 hours after exposure, under clothing, even after washing, because the skin stays reactive for some time.
3. Sap in the eyes means flushing with clean water for at least 15 minutes, then going to medical care. Wear sunglasses afterward.
4. Treat a burn as a burn. Cool compresses, leave blisters intact, and see a clinician for blistering over a large area, any burn on the face, signs of infection, or any eye exposure.

Telling the burn from what it gets mistaken for

The office usually hears about it as "something on the job burned him," and on a lawn or landscape crew there are three other suspects. Getting the cause right matters, because each one changes a different habit:

• Sunburn covers all exposed skin evenly and stops at the clothing line. A sap burn is streaked, splashed or dripped, and can sit right next to unburned skin that got the same sun.
• Poison ivy itches more than it hurts, shows up as clusters and lines of small blisters, and appears on people in proportion to their sensitivity. A sap burn hurts more than it itches, blisters in larger patches, and leaves the dark staining poison ivy does not.
• A herbicide or fertilizer splash appears soon after contact rather than a day later, matches where the product was handled, and is already on the product's label and safety data sheet. If the crew sprayed that day, check the label's first aid section and treat it as a chemical exposure.
• A hot-surface burn from a muffler or engine is one sharp-edged spot, felt immediately.

Write down in the job record what was trimmed, where, for how long, and what the person was wearing. If the case becomes a workers' compensation claim, that description, made the day the burn appeared, is what the clinician and the insurer will want.

Worked example: one roadside strip, two crew members

The job. A lawn crew in Wisconsin trims a 300 ft strip along a county road edge at a commercial property in early July. Full sun, about 85 F. Crew member D runs the string trimmer in shorts and a T-shirt. Crew member E edges along the sidewalk in long work pants and a long-sleeve shirt, no gloves. The strip is thick with yellow-flowered wild parsnip, which nobody identifies.

Day of: D is splattered with green plant debris from the knees down for about 40 minutes of trimming. He feels nothing. He wipes his legs with a rag at the truck and works the rest of the afternoon outdoors in shorts.

Day 1: Red, slightly tender streaks on both shins. He assumes sunburn.

Day 2: The streaks blister, some long and narrow like drips, one patch on the left ankle where sap ran into the sock line. E has two small blisters on the back of one hand, where he pulled a stalk out of the trimmer guard to clear it.

The diagnosis. A clinic identifies phytophotodermatitis from the pattern and the job description. Neither person is allergic; both were burned, in exact proportion to how much skin and sap met the sun. D's legs took 40 minutes of trimmer spray and an afternoon of sunlight. E's covered legs and arms took nothing; his one bare hand took the one stalk he touched.

Months later, D's shins still carry brown streaks.

What the shop changed: long pants and long sleeves required for any roadside, ditch or field-edge trimming from May through August; nitrile-coated gloves on the trimmer and for clearing the guard; a picture of wild parsnip and giant hogweed in each truck's binder; and a rule that a flowering wild parsnip stand gets flagged to the crew lead before anyone runs a trimmer into it.

What would change the answer. On a cloudy day, the same exposure would likely have burned D less, but not zero. If the plant had been giant hogweed, the crew should never have cut it: the right move was to stop, flag the location, tell the customer, and let the state program or a contractor with the proper protection handle it. If D had washed with soap and water right away and kept his legs covered for two days, the burn would likely have been minor or absent, with the same sap on his skin.

References

• NIOSH, Workplace Safety and Health Topics: Poisonous Plants
• New York State Department of Environmental Conservation, giant hogweed identification, health hazards and sighting reports
• USDA Animal and Plant Health Inspection Service, Federal Noxious Weed List (7 CFR Part 360)
• Your state extension service or invasive species program, for wild parsnip identification and local distribution
• See related: Poison Ivy, Oak and Sumac on the Job; Sun Exposure and Skin Cancer for Outdoor Crews