Giant Hogweed

Common name:  Giant Hogweed
(also known as Wild rhubarb, cartwheel flower, wild parsnip)
Scientific name:  Heracleum mantegazzianum
Management programme:  Organism of Interest (OOI)

Why is it a pest?

Giant hogweed is poisonous to humans and care should be taken when removing this plant. Exposure to dust or sap can cause skin irritation, blistering, swelling, and long-term scaring. If a person gets the sap in their eyes, it can cause temporary or permanent blindness.

This perennial herb (plant with a life cycle of three or more years) can form dense colonies that suppress native vegetation. When it dies down during winter, it can leave infested banks bare of vegetation and susceptible to erosion or reinvasion by other pest species.

Giant hogweed has been recorded in coastal Otago from Oamaru to the Catlins and in Central Otago:

 

Reported Giant Hogweed

 

What does it look like?

  • Size: Giant Hogweed can grow up to 6 m tall.
  • Leaves: Large, serrated leaves up to 100cm long, with deeply divided leaflets (a leaf-like part of a compound leaf), forming a rosette (circular arrangement) at the base. 
  • Flowers: Large umbrella-like clusters of greenish-white flowers in summer, followed by dry, flattened, oval light brown fruit (1cm long).
  • Distinctive feature: Grooved, hollow stems are spotted reddish purple with sturdy bristles containing toxic sap. 
  • Root system: Forked or branched taproot (main root) growing vertically downward.
  • Habitat: Giant hogweed typically grows in rich, moist soils and around water ways.

When can I spot it best?

Between December and February is the best time to spot giant hogweed as it is flowering and fruiting.

What are the rules?

There are no specific rules associated with Giant Hogweed as it is an Organism of Interest in the Otago Regional Council’s Regional Pest Management Plan (2019-2029).

Giant Hogweed is declared an Unwanted Organism in the National Pest Plant Accord . This means it cannot be sold or be in a place where plants are being sold. It cannot be propagated, bred, multiplied, communicated, released, caused to be released, or otherwise spread.

How can I control it?

TAKE GREAT CARE!

Wear protective gear to cover your arms, hands and eyes. If you get the dust or sap on your skin, wash the area immediately.

Physical control:

  • Pull the whole plant out before it seeds in spring/summer. 

Chemical control:

  • Cut stems below ground level in summer and spray any re-growth with glyphosate as required.
  • Spray the whole plant with glyphosate in spring, and again in summer if required.

Be aware, seeds can remain dormant for up to 5 years. To effectively control this pest, continue to monitor the site and consider planting native species, as these can prevent giant hogweed from growing back again. To find native planting advice visit Otago Native Planting Guide.

Management programme