Giant hogweed — how to recognise it, what to do after skin contact, how to control it

Giant hogweed (Heracleum mantegazzianum, German Riesen-Bärenklau) and Sosnowsky’s hogweed (Heracleum sosnowskyi, dominant in Poland) are two species of the same genus, the two most dangerous invasive plants of Central Europe. They look almost identical, cause identical burns and share the same legal status — this guide treats them together. Recognising them at a glance and clear action after contact can spare serious eye or skin burns that in extreme cases require hospitalisation and leave permanent scars.

Why they are so dangerous

In the sap of both species there are furanocoumarins — compounds that on the skin ALONE do nothing, but on contact with UV radiation (sunlight) trigger a phototoxic reaction. In practice: touch the plant on a cloudy evening and you feel nothing — and in the morning, after going out into the sun, at the contact site there appears a burning redness, then fluid-filled blisters and finally grade II or even III burns. The reaction may take up to 48 hours after exposure, and discolouration and skin sensitivity may last for months. Contact of the sap with the eye may lead to permanent corneal damage.

This is why the first rule is: shield your skin from sunlight for 48 hours if you had contact with any suspicious plant from this group.

What they look like — four distinguishing features

Both species are huge and, when fully grown, hard to confuse with anything else in Central European flora. Key features:

• Scale. The adult plant reaches 2–4 m in height (with good moisture even 5 m). Native umbellifers — angelica, common hogweed, wild carrot, caraway — stay within 1–2 m.
• Stem. Thick as an adult’s arm (4–10 cm in diameter), hollow inside, with dark red or purple blotches and streaks and rough hairs. Native umbellifers have much thinner stems without pronounced dark blotches.
• Umbel. Large, white, at the top of the stem, 30–80 cm in diameter (record up to 1 m), made up of dozens of rays and hundreds of small, white, five-petalled flowers. Blooms in VI–VIII.
• Leaves. Huge (up to 1.5 m long), deeply cut into several sections, light green and rough, growing directly from the ground on long stalks.

For comparison — helpful contrasts to plants known from the database:

• Carrot has a similar shape of umbel flower, but the whole plant fits in a bucket — a hogweed towers over a person.
• Fennel and dill have fine, thread-like leaves and a distinct aroma — hogweed has large, lobed leaves and smells sharp, unpleasant.

What to do immediately after contact

The order matters:

1. Shield the skin from sunlight — for 48 hours. This is the most important step, more important even than washing. Go inside, put on long sleeves and trousers, avoid sun exposure for 2 days.
2. Wash the contact spot with cool water and soap. Best within 15 minutes, in a shaded room. Gently, without rubbing.
3. Remove contaminated clothing wearing gloves, wash it separately at high temperature.
4. Do not rub the eyes. If the sap got into the eye — immediately flush with plenty of cool water for at least 15 minutes and consult an ophthalmologist the same day (the risk of permanent corneal damage is real).
5. See a doctor for: blisters, contact with face or eyes, extensive burn, in children. Furanocoumarin burns are treated like ordinary thermal burns, but require monitoring — they may develop within 48 hours.

Do not use home „rescue” ointments before consultation; a moist cool-water compress while waiting for help is fine, alcohol and oil — no.

How to legally control it in the garden

The landowner has a legal duty to control both species (they are on the EU list of invasive species of Union concern, Regulation EU 1143/2014, transposed into national law). Similar rules apply in Poland, Germany, Switzerland and, with the Wildlife and Countryside Act, in the UK.

Step 0 — personal protection. Full impermeable overall, tight-fitting goggles, rubber gloves, mask, rubber boots. Work on a CLOUDY day or after dusk — without sun that activates phototoxicity.

Step 1 — single plants. Dig up the WHOLE plant with its taproot at least 20 cm below ground before flowering (best in spring, when the plant is young and you can see its distribution). The root alone has reserves to regrow. Do not cut off at the base with a spade — hogweed regrows from the root and merely waits a year.

Step 2 — mowing is NOT enough. Cutting the shoot only shortens the plant’s life by a season; from the taproot it regrows just as strongly. Mowing makes sense only as a supplement — in autumn or right after digging, to prevent seed ripening on remnants.

Step 3 — flower heads into the bag. If the plant has already flowered, seeds must not be left — they ripen within weeks of flowering and remain viable in the soil for 5–10 years. Cut umbels are collected into sealed plastic bags and disposed of as hazardous waste (not to compost, not to the brown bio bin). Contact the municipality — many councils collect such bags for free.

Step 4 — chemical means. Glyphosate (Roundup and others) works best on young specimens in early spring, applied precisely to the plant with an injector or brush. Mass spraying requires in most municipalities a permit and authorisation to apply plant protection products. The rules of safe garden chemistry are outlined in the guide to plant diseases in the section on control methods — the rule „groups and mechanisms, not brands” and honesty about EU approvals apply here just as well.

Step 5 — when to call the authorities. Stands of more than a few plants, specimens near paths and playgrounds, situations where control with own means is impossible — report to the local environmental authority (in Germany the municipal Umweltamt; in Switzerland the cantonal Fachstelle Neobiota; in Poland RDOŚ or the municipal office; in the UK the local council). The report brings no penalty, only support: many councils run control programmes and fund professional contractors.

Where they came from in Europe

Giant hogweed is native to the south-western Caucasus. It reached Western Europe in the second half of the 19th century as an ornamental plant for Victorian gardens — the imposing scale and exotic look made it a desired garden accent. It escaped and colonised riverbanks in Britain, Germany, Switzerland and the Scandinavian countries.

Sosnowsky’s hogweed is native to the Caucasus. It was introduced to Poland and other Eastern Bloc countries in the 1950s as a promising fodder plant — the large green mass and high yields convinced decision-makers it would be ideal cattle feed. Result: cattle would not eat it (smell, bitterness), and the plant escaped from cultivation and began to colonise abandoned fields, riverbanks and roadsides. Today both species are treated in Europe as one of the most serious problems of nature conservation and public health at the same time.

Summary — three rules

1. Don’t touch if you suspect invasive hogweed. Scale and thick, blotched stem are a warning sign. Any doubt — treat as a dangerous plant.
2. Shield the skin from sunlight for 48 h if you had contact. The reaction is activated only under UV, not immediately.
3. Control with protection (full overall + cloudy day) or report to the local authority. It is a plant one may and should destroy — the legal duty lies with the landowner, but authority support is available.

Seasonal timings of garden work are gathered in the garden calendar; a list of plants worth planting instead of invasive ones — bee- and pollinator-friendly plants.