Giant Hogweed: why the sap burns, and what to do if you find a stand
Phytophotodermatitis explained in plain English — and the steps to take before anyone gets hurt.
Giant Hogweed (Heracleum mantegazzianum) is the only invasive plant we treat that is genuinely a public health hazard. Its sap contains furanocoumarins that bind to skin cells and react violently to UV light — what looks like a sunburn at first can blister, scar, and flare up again every summer for years.
How the burns actually work
The reaction is called phytophotodermatitis. Sap touches the skin, you go out in daylight, and within 24 to 48 hours the area reddens, blisters, and turns dark brown. The pigment can take months to fade and the affected skin stays photosensitive long after.
What to do if you find a stand
- Do not strim, mow, or cut it — that aerosolises the sap.
- Keep children and pets at least 5 m clear and rope the area off if it's near a path.
- Photograph it from a safe distance and send the photo for confirmation.
- Wash any contact site immediately with cold water and soap, then keep it covered from sunlight for 48 hours.
How we treat it
PPE-led cut, dig and stem-injection during the rosette stage in April–June, before the plant bolts. Most stands need two to three seasons of follow-up to exhaust the seed bank. We post site signage during works and notify any adjoining landowners — encroachment from riverbanks is the single most common pattern we see.
Reviewed by
The survey team
PCA-certificated surveyors, Japanese Knotweed Elimination
PA1 / PA6A certificated surveyors who carry out the site visits, write the management plans and sign off the reports issued to lenders and solicitors.
Reviews the technical, legal and lender statements in this article.
What this page can't tell you
- Guidance is general. Identification, treatment choice and timescales can only be confirmed by a site visit — photographs alone are indicative.
- Lender and insurer requirements vary by institution and change over time. We state what is typically accepted, not what any named lender will accept on your case.
- Guarantee (IBG) eligibility depends on the plan chosen, the survey findings and the underwriter's acceptance. It is not automatic.
- Legal points summarise published law and guidance current at the review date. They are not legal advice.
Last reviewed · Questions on anything stated here: 07582 869979
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