Giant hogweed

How to recognise it

Habit

Giant plant of 2–4 m. It first grows for a few years as a large leaf rosette, then flowers once and dies.

Leaf

Enormous, often 1–2 m long, deeply cut into pointed, sharply toothed lobes; green and less rough than in common hogweed.

Flower

White umbels up to 50 cm across with 50–150 rays, in June–August: a large central umbel surrounded by smaller side umbels.

Fruit/Seed

Flat, oval, winged seeds of about 1 cm, in huge numbers. They float and spread along watercourses.

Stem

Hollow and ridged, 5–10 cm thick at the base, with striking purple-red blotches and stiff hairs.

Confusion with

Common hogweed stays smaller (0.5–2 m), has umbels of 10–20 cm, rough leaves with broad, blunt lobes and no large purple blotches on the stem. Wild angelica has smooth, often purple-tinged stems and finer, twice-pinnate leaves.

Flowering: June–August · Pruning: April–July

What you need to know

• Recognising it matters: with sunlight its sap causes severe burns, especially in children who play with the hollow stems.
• It is on the EU list of invasive alien species: never plant or spread it.
• A single plant produces thousands of seeds, which stay able to germinate in the soil for several years.
• Dense stands crowd out native plants and leave bare banks in winter that erode easily.
• Acting early pays off: a few plants can be cleared in an afternoon, a large stand takes years of work.

Giant hogweed: where does it grow best?

• In the wild: banks of rivers and streams, damp rough ground, verges, railway embankments, parks and waste ground.
• Grows in sun and partial shade, on moist to damp, fertile soil.
• It has no place in any garden: if it appears, remove it as soon as possible.
• Be especially watchful after groundworks, along water and where soil has been brought in: seeds germinate easily there.

Plant these instead

• Wild angelica: Native alternative: a 1–2 m umbellifer for damp places, with white to pinkish umbels full of insects.
• Hemp-agrimony: Native alternative: large pink flower heads in July–August by the water, a butterfly magnet.
• Purple loosestrife: Native alternative: purple spikes at the water’s edge, loved by bees.
• Meadowsweet: Native alternative: creamy, fragrant plumes on wet ground.

How to remove it

1. Never plant or sow giant hogweed and never pass on seed or plants: it is on the EU list of invasive alien species.
2. Protect yourself fully before you start: waterproof gloves, long sleeves, long trousers, boots and safety goggles or a face shield. Work in cloudy weather.
3. Dig out small and medium-sized plants in April–May: with a sharp spade, cut through the taproot 10–15 cm below ground and pull out the top part.
4. For flowering plants you cannot dig out: cut off the umbels as soon as the first green seeds form, before they ripen and turn brown, and check again after 2–3 weeks.
5. Put umbels and seeds in a sealed bag with general household waste, never on the compost heap or with green waste. Let roots and leaves dry out on a hard surface or dispose of them with household waste too.
6. Check the spot every spring for the following years for new seedlings and report large stands to your local council.

Control

Digging out

The best method for single plants: cut through the taproot 10–15 cm below ground in April–May. The plant will not regrow.

Cutting umbels

On flowering plants, cut off the umbels when the first seeds are green. Cutting too early produces new umbels; too late and the seeds are already falling.

Mowing

Repeated mowing, every 2–3 weeks from April to August, weakens large stands but does not kill the plants quickly. Protect yourself well against splashing sap.

Covering

After mowing, cover small stands with black, light-proof sheeting for a full growing season.

Follow-up

Come back every year: the seeds in the soil keep producing seedlings for years.

Tip: Tackle giant hogweed in March–April, while the rosettes are still small: the root is easy to cut through and you hardly come into contact with the sap.

Pruning and mowing

Pruning months: April–July.

When: Dig out in April–May; cut off umbels in June–July, when the first seeds are green.

How:

• Always wear full protective clothing and goggles or a face shield.
• Cut through the taproot 10–15 cm below ground: this kills the plant.
• On flowering plants, cut off all umbels, including the smaller side umbels, and remove them in a sealed bag.
• Come back after 2–3 weeks and remove any umbels that reappear.
• Do not use a strimmer that sprays sap around without full protection.

Caution: Sap on the skin plus sunlight causes severe burns that often appear only after a day and stay visible for a long time. After contact: wash the skin straight away with soap and water, keep it out of the sun for at least two days and see a doctor if blisters form.

Pruning: when and how · Pruning calendar and basic rules

How it spreads

Seed

It spreads only by seed, but with thousands of seeds per plant. Most fall within a few metres, but water, mowing machines and moved soil carry them much further.

Seed bank

Seeds stay able to germinate in the soil for several years. Never move soil from a place where giant hogweed grew.

Along water

Seeds float and wash up on banks further downstream during high water. So remove plants along streams and ditches too.

Giant hogweed: how does it get through the winter?

Hardy to −30 °C.

• The leaves die back in autumn; the thick taproot survives winter.
• Plants that have flowered die completely after setting seed, but their dry stems can stay standing for a long time.
• In March the new leaf rosettes appear: the best time to spot them and act.
• Fully hardy: even hard frost does not kill the roots.

Diseases and pests

• Burns: Sap and sunlight cause blisters and burns, sometimes with scars or dark marks lasting months. Always work fully protected.
• Seed bank: Seedlings keep appearing for years after removal. Check every spring and dig them out while young.
• Regrowth after mowing: Mown plants regrow and sometimes flower close to the ground. Cutting through the root is better.
• Spread along water: Seeds arrive from upstream. Work together with your neighbours and report stands to your local council.

Through the year

• March: New leaf rosettes appear. Find them and mark them.
• April: Dig out young plants: cut through the taproot 10–15 cm below ground.
• May: Finish digging out before the flowering stems grow tall.
• June: Flowering begins. Cut off umbels when the first seeds are green.
• July: Look for new umbels and cut them off. Bags of umbels go with household waste.
• August: Remove the last umbels before the seeds turn brown.
• September: Check the spot for late seedlings.
• October: The leaves die back. Note the stands for next year.

Did you know?

• Its scientific name honours the Italian scientist Paolo Mantegazza.
• The plant flowers only once in its life: after a few years as a rosette it produces an enormous umbel, sets seed and dies.
• In the 19th century it was planted in Europe as an admired ornamental in parks and large gardens; from there it escaped along rivers.