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Giant hogweed: the plant that burns you in sunlight

Giant hogweed sap does not burn on contact. It removes your skin's ability to protect itself from ultraviolet light, so the burn arrives hours later when the sun hits the sap on your skin. The resulting blisters are severe, and the affected skin can remain photosensitive for years.

  • 4High risk
  • 0Medium
  • 0Low
  • 4Zones tracked

Identifying it

It is enormous — commonly two to four metres tall — with a thick green stem blotched with purple and covered in coarse white bristles, deeply divided jagged leaves up to a metre across, and a white umbrella-shaped flower head that can be half a metre wide. It grows along riverbanks, railway lines, ditches and waste ground. Several harmless relatives look similar at a distance, but nothing else combines that height with the purple-blotched bristly stem.

The delayed burn

Phytophotodermatitis develops roughly 15 hours to three days after exposure: redness, then painful fluid-filled blisters, then dark pigmentation that can persist for months or years. Children are often affected because hollow hogweed stems make appealing toys and telescopes.

How to avoid it

  • Learn to recognise it and simply do not touch it — no part of the plant is safe.
  • Never strim, mow or cut it; that aerosolises the sap.
  • Keep children away from the hollow stems, which are the classic source of severe facial burns.
  • Wear full cover — gloves, long sleeves, eye protection — if you must work near it, and wash everything afterwards.
  • Report stands to the local authority or landowner; it is an invasive species in most of its current range.

If it happens

  • Wash the skin immediately with soap and cold water, and keep it completely out of sunlight.
  • Cover the area and keep it shaded for at least 48 hours — this is what limits the burn.
  • Do not burst blisters; see a doctor for anything more than a small area.
  • Flush eyes with water and seek medical care at once if sap gets in them.
  • Protect the affected skin with high-factor sunscreen for months afterwards.

General guidance only — not a substitute for medical advice or local emergency services.

Where we track it

4 zones in the Dangers Here hazard database, by danger level.

High

  • Baltic & Eastern Europe
  • Great Britain
  • Northern Ireland & Irish east coast
  • US Pacific Northwest

Questions

Where is giant hogweed a risk?

We track 4 zones for this hazard. 4 are rated high risk, including Baltic & Eastern Europe, Great Britain, Northern Ireland & Irish east coast, US Pacific Northwest. 0 are rated medium and 0 low.

How do you avoid giant hogweed?

Learn to recognise it and simply do not touch it — no part of the plant is safe. Never strim, mow or cut it; that aerosolises the sap. Keep children away from the hollow stems, which are the classic source of severe facial burns.

What is the first aid for giant hogweed?

Wash the skin immediately with soap and cold water, and keep it completely out of sunlight. Cover the area and keep it shaded for at least 48 hours — this is what limits the burn. Do not burst blisters; see a doctor for anything more than a small area.

Identifying it

It is enormous — commonly two to four metres tall — with a thick green stem blotched with purple and covered in coarse white bristles, deeply divided jagged leaves up to a metre across, and a white umbrella-shaped flower head that can be half a metre wide. It grows along riverbanks, railway lines, ditches and waste ground. Several harmless relatives look similar at a distance, but nothing else combines that height with the purple-blotched bristly stem.

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