Giant Hogweed vs Cow Parsnip: Native or Hazard?

Do not touch either plant with bare skin. Giant hogweed sap contains chemicals called furanocoumarins. When sap meets sunlight and moisture, it can cause a severe burn within 24 to 48 hours, with painful blisters and scars. Cow parsnip contains the same class of chemicals and can also cause a reaction, usually described as less severe. If sap gets on your skin, wash it off and keep that skin out of the sun.

The decision tree

Work through these in order. Stop as soon as you have a clear answer, and do not part the foliage with your hands.

1. Flower head wider than 1 foot, with 50 or more rays? That points to giant hogweed.
2. Flower head 1 foot or less, with 15 to 30 rays? That points to cow parsnip.
3. Stem 2 to 4 inches across, with coarse white hairs and a thick collar of hairs where the leaf stalk meets the stem? That points to giant hogweed.
4. Stem 1 to 2 inches across, with fine white hairs and no collar? That points to cow parsnip.
5. Leaves deeply cut and up to 5 feet wide? That points to giant hogweed.
6. Leaves less deeply cut and about 2 to 2.5 feet wide? That points to cow parsnip.
7. Still not sure? Treat it as giant hogweed. Do not touch it.

Why height alone fails

The two plants overlap in height. New York’s lookalike table puts cow parsnip at 5 to 8 feet and giant hogweed at 7 to 14 feet. NC State Extension reports cow parsnip up to 15 feet and giant hogweed up to 20 feet. A tall cow parsnip and a young hogweed can both be around 7 or 8 feet, so a single height measurement cannot settle it.

Purple blotches do not settle it either. Both plants can have purple splotches on the stem. The differences that hold up are the size of the flower head, the number of rays, how deeply the leaf is cut, and the kind of hairs on the stem.

Which one is native?

Cow parsnip is native to North America. It is the only Heracleum native to the continent, and it belongs in wet meadows, stream edges and damp open ground. Do not dig it out because it looks alarming.

Giant hogweed is native to the Caucasus Mountains. It was introduced to the United States in the early twentieth century as an ornamental. It is a federal noxious weed, and New York prohibits its possession with the intent to sell, import, purchase, transport, introduce or propagate under 6 NYCRR Part 575.

Is cow parsnip dangerous?

Yes, just less so. NC State Extension lists cow parsnip as having low-severity poison characteristics, with photosensitization, sunburn, dermatitis and eye irritation possible after sap exposure plus sunlight. The active compounds are furanocoumarins, the same family implicated in giant hogweed burns.

That means “it is only cow parsnip” is not permission to handle it bare-handed. Wear long sleeves, long pants, boots and eye protection if you have to work near either plant, and wash up afterward.

If sap gets on someone’s skin and a rash, blistering or burn appears, or you are unsure what to do, call Poison Control at 1-800-222-1222 (free, confidential, 24 hours) or use poison.org. For trouble breathing or a severe reaction, call 911.

What to do if you find giant hogweed

Do not try to dig or cut a mature plant yourself. Photograph the whole plant, then a close-up of the stem, a leaf and the flower head. Send the photos and the location to your state agriculture department or extension office and ask them to confirm it.

For the species profile, see giant hogweed. For the New York rules, see invasive plants in New York. For the other pairs, see look-alikes. For native and invasive plants by category, see plants.