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Washington State Weeds

Giant Hogweed

giant hogweed in pierce county

 Heracleum mantegazzianum  • Class A

Family Name: Apiaceae family  (ay-pee-AY-see-ee)
Common: Carrot/celery family (formerly Umbelliferae)
Genus: Heracleum (Hair-uh-KLEE-um) 
Meaning: Named for Hercules, who was supposed to have used it first for medicine
Species: mantegazzianum (man-tee-gaz-zee-AH-num)
Meaning: Named Paolo Mantegazzi, 19th century Italian etnographist (sociologist/anthropologist)

The Giant Hogweed closely resembles the cow parsnip, except for its huge size, reaching heights of up to 15 feet tall.

The hollow, sturdy stalks are covered in dark reddish, purple splotches. The hollow sturdy stems are covered in coarse white hairs and dark purple/red splotches.  Stiff, bristly hairs also surround the base of the leaf stalk. The leaf stalks are also purplish.

It's deeply cut,  compound leaves are up to 5 feet in breadth.

Its white umbrella-shaped flower clusters are up to 2.5 feet in diameter. The seeds are winged and spread through wind and water. Seeds remain viable in the ground for approximately 7 years. 

 Why Is it a Noxious Weed?

Giant Hogweed is a public health hazard. Its clear watery sap contains toxic chemicals which make the skin highly sensitive to the sun and other sources of ultraviolet light.  Skin contact with this sap followed by sun exposure produces painful, burning blisters. Temporary, or even permanent blindness can result if the sap enters the eye. 

  It is also very invasive and forms dense canopies which choke out native vegetation and increase soil erosion.


Where Does it Grow?

It prefers rich, damp soil and grows along roadsides, ditches, vacant lots, streams and rivers, and vacant farmland. Seventy percent of all Giant Hogweed sites are found in urban areas.  It is less effective than native plants at binding soil, so infestations lead to increased soil erosion. 


Most Giant Hogweed plants are monocarpic perennials, dying after they flower and produce seed. Though it may take from four to fifteen years from the time a seed germinates until the plant produces a flowering stem. 

Control Options:
  • When working around Giant Hogweed, ALWAYS wear protective clothing including goggles to avoid sap exposure. Performing any manual control is risky. The sap that causes the burning is contained in all portions of the plant. 
  • Small plants may be pulled, or dug out and the roots carefully removed. 

  • Following control or removal, landscape barrier cloth, or mulch is recommended to prevent seed germination. 

  • Because Giant Hogweed is a Class A noxious weed it must be eradicated completely whenever it is found, therefore biological control is not a viable option. 

  • Spot spraying with an herbicide containing the active ingredient glyphosate applied during the bolt and bud stage, or when the plant is actively growing is effective in controlling Giant Hogweed. Glyphosate products can be used to treat individual plants or small patches, either by spot foliar application, or by stem injection. Using a spot application, spray each plant thoroughly on the stems and leaves, enough to be wet but not dripping. Spot application means the herbicide is applied only to the Giant Hogweed plants, and not on the surrounding plants or soil. Currently, products containing the active ingredient glyphosate are the only herbicides recommended for the control of Giant Hogweed. 
  • To minimize any harmful impact on bees and other pollinators, timing is important.  Ideally, treat plants before blooming.  If treatment after blooming is necessary, do control work early in the morning, or in the evening when bees are less active.
  • When using herbicides be sure to read and follow all label instructions and obey all label precautions. (Note: pesticide product registration is renewed annually and product names and formulations may vary from year to year.  

More Information:

 Download our Flyer or visit Washington State Noxious Weed Control Board Here. Photo by Rebecca Shoemaker, Pierce County Noxious Weed Control Board. 


More Pictures:  

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