Giant hogweed

Do not touch giant hogweed to identify it. It is a huge carrot-family plant whose sap, combined with sunlight, can cause severe blisters. If you think you found it, photograph it from a safe distance and report it to your state agency or extension office.

What the sap does

New York State DEC explains the mechanism clearly: all parts of the plant contain toxic sap with chemicals called furanocoumarins. When those chemicals get on skin and are exposed to sunlight, they cause phytophotodermatitis, a serious skin inflammation. Heat and moisture, including sweat or dew, can make the reaction worse.

The reaction can start as soon as 15 minutes after sap gets on skin. Painful blisters can form within 48 hours and then become dark and pigmented. New York State DEC says scars can last up to six years, though they usually last a few months, and the skin can stay sensitive to sunlight long after the burn heals.

This is not an ordinary rash. The sap is the hazard, and light contact with the foliage, stem, flower or seed can release it.

What to do if you touched it

1. Get out of the sun.
2. Wash the area thoroughly with soap and water as soon as possible.
3. Keep the exposed area covered and protected from sunlight for 48 hours.
4. See a physician if a reaction starts or if you have questions.
5. If sap gets in your eyes, rinse with water, wear sunglasses or cover your eyes, and seek medical care.
6. Protect the skin from the sun until it is fully healed, which can take years.

If you are working near a confirmed stand, New York State DEC recommends long waterproof gloves, long sleeves, pants, boots and eye protection. Keep soap, water and eyewash nearby. Wash tools and clothes, and shower as soon as possible after exposure.

The size test, and its limits

Giant hogweed is defined by its size, but the numbers in official sources differ. Virginia Cooperative Extension says plants typically grow 6 to 8 feet tall, with a maximum of 18 feet. New York State DEC describes plants 7 to 14 feet or more. Wisconsin DNR gives 8 to 20 feet when in flower.

For a homeowner, the practical rule is simpler: a carrot-family plant over your head with a stem 2 or more inches across and purple blotches deserves a second look from a distance. The stems are hollow, ridged and 2 to 4 inches across, with coarse white hairs and a thick ring of hairs at the base of each leaf stalk. The leaves are deeply cut, not just lobed. The white flower heads can be 2.5 feet wide with 50 or more rays.

Purple blotches alone do not settle it. Virginia Cooperative Extension warns that the blotching is common but not consistent, and should not be used in isolation. Size, leaf dissection, stem hairs and flower-ray count together are stronger evidence.

Cow parsnip is the usual false alarm

Cow parsnip is native and much smaller: 5 to 8 feet tall, with leaves 2 to 2.5 feet wide that are less deeply cut and more block-shaped. Its stems are 1 to 2 inches across with fine white hairs, and its flower clusters are no wider than 1 foot with 15 to 30 rays. It blooms earlier, from late May into June in New York.

Cow parsnip is not automatically safe to handle. New York State DEC says it may also cause burns. Treat any large carrot-family plant with white umbrella flowers as a look-only plant until you have a confident identification. Our cow parsnip vs giant hogweed comparison walks through the decision. For the yellow-flowered plant that can also burn skin, see giant hogweed vs wild parsnip.

Report it, do not dig it

Giant hogweed is a federally listed noxious weed under 7 CFR 360.200. New York State DEC says it is established in New England, the Mid-Atlantic region and the Northwest. It is also a Class A noxious weed in Pennsylvania and is prohibited in Wisconsin. New York law prohibits possessing it with the intent to sell, import, purchase, transport, introduce or propagate it.

• New York: call the DEC Invasive Plant Information Line at 845-256-3111, submit a report through iMapInvasives, or contact your local PRISM. Photos should show the whole plant, the stem, the leaves and the flowers or seed, with something for scale.
• Pennsylvania: giant hogweed is on the state noxious weed list. Contact the Pennsylvania Department of Agriculture for guidance before removing it.
• Virginia: contact Virginia Cooperative Extension or the Virginia Department of Agriculture and Consumer Services at 804-786-3515 or NoxiousWeed.VDACS@vdacs.virginia.gov. A permit is required before transporting plant material.

Do not collect a sample unless the agency asks you to. Do not mow or weed-whack a stand yourself. Virginia Cooperative Extension says control should be left to people with training, experience and proper equipment, and New York State DEC publishes a separate control guide for that work.

What to do this week

1. Stay back and take photos with a zoom lens or from a safe distance.
2. Warn children, visitors and anyone who works on the property.
3. Report the location to the agency listed for your state.
4. Do not cut, mow or pull it.
5. If sap contacted skin, wash with soap and water and keep that area out of the sun for 48 hours.

For state rules and reporting contacts, see the New York and Pennsylvania pages. More profiles are listed under invasive plants, and image credits are on the image credits page.