Phytophotodermatitis: Wild Parsnip, Cow Parsnip, and Giant Hogweed

Wild parsnip, cow parsnip, and giant hogweed can each cause a real skin burn -- but not through contact alone. All three produce furanocoumarins in their sap, compounds that only become harmful once activated by ultraviolet light. This is phytophotodermatitis: a genuine phototoxic chemical reaction, not an allergic one, and it behaves very differently from poison ivy.

This is phototoxicity, not allergy

The active compounds are linear furanocoumarins -- psoralen, xanthotoxin, and related molecules -- found in the clear, watery sap of these plants. On their own, they do little. Once on skin and exposed to UVA light, they insert themselves into the DNA of skin cells and form damaging cross-links when struck by sunlight, triggering a real chemical burn. Because both sap contact and sun exposure are required, someone can touch a plant at night or in shade and see no reaction until hours later, once they have been in the sun -- a delay that often disguises the actual cause.

The reaction is often delayed a day or more

Symptoms typically do not appear immediately. Redness and a burning sensation resembling a bad sunburn commonly develop 24 to 48 hours after both sap contact and sun exposure have occurred, sometimes progressing to blistering. This delay is part of what makes the injury dangerous: by the time it is obvious, the affected skin has already been re-exposed to sunlight multiple times, and the person may not connect it back to a plant encountered a day or two earlier.

Giant hogweed: the most severe, and not native

Giant hogweed (Heracleum mantegazzianum) is native to southwestern Asia and has spread as an invasive species through parts of North America and Europe. It is genuinely enormous -- reaching 8 to 20 feet tall, with leaves up to 5 feet across and white flower umbels spanning up to 2.5 feet. Its stem, 2 to 4 inches thick, is marked by irregular raised purple-red blotches and stiff, coarse bristles. Of the three plants here, giant hogweed causes the most severe burns and carries the clearest reputation for real medical urgency, particularly with eye exposure.

Cow parsnip: a native look-alike, real but generally milder

Cow parsnip (Heracleum maximum) is native across most of North America and is commonly mistaken for giant hogweed because it shares the same genus and a similar general look -- tall, white-flowered, broad-leaved. It is smaller, typically 6 to 10 feet tall, with leaves closer to 2 feet across. Its stem is a helpful identification point: mostly green, sometimes with faint, regularly spaced purple spots, and covered in soft hairs rather than the stiff bristles of giant hogweed. Cow parsnip sap still causes real phytophotodermatitis, but reactions are generally reported as less severe than giant hogweed's.

Wild parsnip: a different genus, yellow flowers, still phototoxic

Wild parsnip (Pastinaca sativa) is a different genus within the same carrot family (Apiaceae), and the easiest of the three to tell apart at a glance: its flowers are yellow, not white. It is smaller and less dramatic-looking than either hogweed species, which can make it easy to underestimate, but its sap contains several of the same furanocoumarins -- xanthotoxin, bergapten, and others -- and reliably causes phytophotodermatitis with sun exposure.

If sap touches skin

Cover the exposed area from sunlight immediately -- clothing or a bandage is enough to block UV exposure while more permanent steps happen. Wash the area thoroughly with soap and cool water as soon as possible, since removing the furanocoumarins before they are absorbed reduces the reaction. Keep the area protected from sun for at least 48 hours afterward, since the compounds remain reactive during that window. If blistering, a large affected area, or facial involvement develops, medical evaluation is appropriate -- the resulting burns can take weeks to months to heal and may leave lasting dark discoloration (post-inflammatory hyperpigmentation) even after the burn itself resolves.