What does it look like?
Giant hogweed is a perennial herb that can grow up to 6m tall in ideal conditions, and has large, serrated leaves up to 50cm long. It has stout dark reddish-purple stems, and spotted leaf stalks with sturdy bristles. The stems are 5-10cm in diameter, hollow, grooved, and covered in fine bristles and red-purple spots. When the plant is two to three years old, it produces large, umbrella-like clusters of greenish-white flowers. Giant hogweed usually grows on the banks of rivers, creeks or in rich, moist soils.
Why is it a problem?
Giant hogweed is poisonous to humans. The sap is phototoxic, and prevents the skin from protecting itself from sunlight, causing skin irritation and swelling. It forms dense colonies that compete with and exclude native vegetation that grows along the banks of rivers or streams. One plant can produce up to 50,000 viable seeds which are spread by water. When it dies down in winter, it leaves infested banks bare of vegetation and susceptible to erosion or invasion by weeds.
Control methods
Physical control
Always wear gloves and eye protection, and cover arms and legs when working near this plant. Contaminated clothing and tools are potentially hazardous also. Wash any skin that comes in contact with the plant immediately. Hand pull whole plants (during spring and summer) before they set seed.
Herbicide control
- Cut down (during summer): Cut the stem below ground level during flowering but before seed sets. Spray regrowth in February or March if necessary with glyphosate (150ml/15L). Or,
- Overall spray (spring) using glyphosate (150ml/15L). Spray again in February or March if necessary.
- Seeds may remain dormant in the soil for at least 5 years, so prevent germination by oversowing with other species.
- CAUTION: When using any herbicide or pesticide, PLEASE READ THE LABEL THOROUGHLY to ensure that all instructions and safety requirements are followed.
Related links
Management Programme
National Pest Plant Accord (NPPA)
The NPPA is designed to prevent the sale, distribution and propagation of a set list of pest plants (the Accord list) within New Zealand. If allowed to spread further, these pest plants could seriously damage the New Zealand economy and environment.
More informationUnwanted Organism
An unwanted organism is any organism that's capable of causing harm to natural or physical resources (like forests and waterways) or human health. A number of introduced pests in New Zealand are classed as unwanted.
Taxonomies
FamilyApiaceae
TypePlants
GroupGroundcover
HabitatLand
Management ProgrammeNational Pest Plant Accord (NPPA)Unwanted Organism
