Heracleum mantegazzianum • Class A

The Giant Hogweed closely resembles the cow parsnip, except for its huge size, reaching heights up to 15 feet tall. The hollow, sturdy stalks are covered in dark reddish, purple splotches. The stems have coarse white hairs at the base of the leaf stalk. The leaf stalks are also purplish. The huge compound leaves are up to 5 feet in breadth. It has white umbrella-shaped flower clusters, up to 2.5 feet in diameter across the flat top. The seeds are winged and spread through wind and water. Seeds remain viable in the ground for approximately 7 years. 

Giant Hogweed is a public health hazard. The plants’ defense mechanism against predators is its clear, watery sap which contains a toxic chemical compound (or glucoside) known as furocoumarins, which are photo-toxins and carcinogens. These chemicals make the skin highly sensitive to the sun and other sources of ultraviolet light, (a condition called phytophotodermititis). Skin contact with this sap followed by sun exposure produces painful, burning blisters that develop into purplish or blackened scars. Furocoumarins are also cytotoxic. Temporary or even permanent blindness can result if the sap enters the eye. 

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 also grows along streams, where it forms a dense canopy, choking out the native vegetation. It is less effective than native plants at binding soil, so infestations lead to increased soil erosion. 

Giant Hogweed is very invasive. One plant can produce 20,000 seeds, allowing it to spread quickly. 

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. 

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. 

• There are currently no known biological control agents available to control.  

• Spot spraying with an herbicide containing the active ingredient glyphosate (example: Roundup Pro, Glyfos, etc.) 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. 

• 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.  

 Download our Flyer or visit Washington State Noxious Weed Control Board Here. Photo by Leo Michels