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Portrait of Giant hogweed

Nature · Ferns, grasses & other plants

Giant hogweed

Neophyte, introduced in the 19th century · flowers June–July · 1.5–5 m · riverbanks, waste ground and verges

Heracleum mantegazzianum — Apiaceae

Giant hogweed is a monstrous cow-parsley relative from the Caucasus region, brought to Britain in the 19th century as a garden curiosity and now widespread along rivers and waste ground. The danger is its sap: it contains furanocoumarins that make skin hypersensitive to sunlight, causing severe blistering that can recur for months or years. It is listed on Schedule 9 of the Wildlife and Countryside Act 1981, so it is an offence to plant it or cause it to grow in the wild.

How to spot them

  • ▸Enormous size: 1.5–5 m tall, with stout, hollow, green stems heavily blotched purple-red and covered in stiff white bristles
  • ▸Huge, deeply lobed, jagged leaves up to 1.5 m wide (Woodland Trust; the RHS gives up to 3 m long)
  • ▸Enormous flat-topped white umbels up to 60 cm across, June to July, then large flat, oval seeds about 1 cm long
  • ▸Flowers in its second year or later, then dies: a monocarpic plant that seeds heavily
  • ▸Look-alikes: common hogweed is far smaller, with unblotched, softer stems; cow parsley is lacy and fern-like; wild angelica has smooth purple-tinged stems and rounded umbels

How to know it

Sap containing furanocoumarins sensitises skin to sunlight, causing severe blisters that may recur for months or years

Giant hogweed — phototoxic; wash the sap off at once and keep the skin covered from the sun.

A plant listed on Schedule 9 of the Wildlife and Countryside Act 1981, it being an offence to plant it or cause it to grow in the wild

Giant hogweed.

Purple-blotched, bristly hollow stems up to 5 m, topped by white umbels 60 cm across

Giant hogweed.

Introduced to Britain from the Caucasus region in the 19th century as an ornamental curiosity

Giant hogweed — Heracleum mantegazzianum.

A monstrous relative of cow parsley whose leaves look like jagged rhubarb, up to 1.5 m wide

Giant hogweed.

Blister damage can be triggered by contact with the sap followed by sunlight; the eyes are especially at risk

Giant hogweed — never strim or cut without full protective clothing.

Quick-fire facts

  • •Safety: the sap causes phytophotodermatitis: if it gets on skin that is then exposed to daylight, it can blister badly, with blisters lasting weeks and sensitivity recurring for months or years
  • •First aid (per Woodland Trust, RHS and councils): wash the skin thoroughly with soap and water straight away, keep the area out of sunlight, and seek medical advice if blistering or if sap reaches the eyes
  • •Law: listed on Schedule 9 of the Wildlife and Countryside Act 1981 (England and Wales); having it on your land is not illegal, but planting it or causing it to grow in the wild is an offence, and authorities can order control
  • •Grows 1.5–5 m tall and forms huge rosettes before flowering in its second year (Woodland Trust)
  • •Umbels up to 60 cm across appear June to July; seeds are flat, oval, about 1 cm long
  • •Introduced in the 19th century as a garden ornamental and widely planted by ponds and rivers; the Kew record often cited is 1817
  • •Common hogweed, Heracleum sphondylium, is native, far smaller and its sap can also cause mild skin reactions
  • •Control: pull before flowering or deadhead; material is controlled waste; the RHS advises professional removal as safest

Drill it — 6 cards

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