Giant Hogweed (Heracleum mantegazzianum)

Giant Hogweed

(Heracleum mantegazzianum)

 Description

  • Very tall (up to 5.5m when in flower), short-lived perennial in the Parsley family. Resembles native Cow Parsnip, but much larger.
    Giant Hogweed Flower
  • Stalks are thick and hollow, and covered with reddish-purple flecks and stiff hairs filled with sap
  • Leaves are very large and dark green, with deeply incised lobes and coarse teeth on all margins. Hairs on underside are stiff and stubby.
  • Forms large white umbrella-shaped flower clusters in June to August.  Flower clusters can be 1 metre in diameter.
  • Reproduces by seed only.
  • Prefers damp, rich soil, and can grow vigorously in various light conditions including full shade.  Most commonly found in ditches, wetlands, riparian areas, wooded ravines, roadsides, and other disturbed sites.
    Giant Hogweed Leaves

Consequences of Invasion

  • Health Risk: Sap (found in leaves, stalks, and stem hairs) contains toxins that can cause severe skin inflammation and painful burns if affected skin is exposed to sunlight. Scars can persist for many years.  Worksafe BC has issued a special Toxic Plant Warning for this species.
  • Ecological Impact: Out-shades and out-competes native species, and reduces biodiversity in its preferred habitat. Shallow root system reduces soil stability compared to healthy native plant complex, and increases erosion hazard on steep terrain and stream banks.
    Giant Hogweed Burn

How was it introduced?  How does it spread?

  • Introduced from Asia by humans as a horticultural curiosity.
  • Each plant can produce up to 100,000 winged seeds that can float for up to 3 days, and can remain viable for up to 15 years.
  • Seeds can be dispersed in wind, flowing water, and by inadvertent animal/human transport when seeds adhere to fur, clothing or equipment.

Integrated Pest Management Options

Prevention

  • Report-a-Weed: report sightings of Giant Hogweed by clicking this link, or downloading one of the smartphone apps, or by calling 1-888-WEEDSBC.
  • Be Plantwise- Choose non-invasive or native plants for your garden, and do not transplant unknown plants into new sites.
  • Practice PlayCleanGo: arrive and leave natural areas clean.  Remove all dirt, plants, seeds and bugs from your equipment, pets, footwear and clothing.  Stay on existing trails, and avoid weed-infested areas.
  • Dispose of exotic (non-native) garden waste responsibly, by bagging and transporting to a designated disposal site (e.g. landfill).

Mechanical Control

**Safety alert**

Anyone handling Giant Hogweed parts should wear waterproof gloves and outerwear, as well as safety glasses to protect skin from toxic sap.  Refer to WorksafeBC’s Toxic Plant Warning for more details.

Crew removing Giant Hogweed in the proper safety gear.
  • Mature plants: manually remove the first 8-12 cm of the central root.
  • Immature plants: mow every two weeks to exhaust the seed bank in the soil; this could take 3-5 years.
  • Disposal: bag and dispose at the landfill, or dry and burn all plant parts.  DO NOT COMPOST!Warning: take special care when controlling Giant Hogweed near waterbodies to prevent the movement of plant parts downstream.

Biocontrol

There are no effective biological control agents available for Giant Hogweed.

Chemical Control

Before considering herbicide use, site characteristics and objectives must be evaluated.  Herbicide labels and pesticide regulations must be reviewed and followed.  Herbicides must never be used within 10m of any waterbody.

  • Effective herbicides include glyphosate and triclopyr.
  • Foliar applications are most effective in spring during active growth, followed by a subsequent summer application for late sprouts.
  • Stem injection methods are effective after heavy sap flow in the spring.
  • Application of pesticides on Crown land must be carried out following an approved Pest Management Plan and under the supervision of a certified pesticide applicator (Integrated Pest Management Act).

Invasive Giant Hogweed vs Native Cow’s Parsnip: What is the difference?

We have been receiving many reports of Giant Hogweed in the area. Thanks very much folks for your reports. To date, all reports have been identified as cow parsnip (Heracleum maximum), a common native plant species. You can watch the video below and read the CKISS blog post on this topic to assist you in identifying the difference between theses plants.

 More Information Available: