War on Weeds—weeds are everybody’s problem

Giant hogweed (Heracleum mantegazzianum) is a biennial or perennial that is native to southwestern Asia. This plant can grow up to 15 ft tall and its sap causes photodermatitis that initiates severe blistering. It can grow in riparian areas and disturbed moist sites. 

Its most distinguishing feature is its size. Stems can be up to 15 ft tall and 4 inches in diameter. It has hollow, ridged stems with white hairs at nodes and at the base of petioles. The stems are green with purple blotches. Leaves are lobed, compound, and deeply notched. Leaves can be up to 5 ft wide. Giant Hogweed produces large umbrella- shaped clusters of small white flowers. Seeds can be viable in the soil for up to 10 years and are easily transported in water and by animals.  

Do not confuse giant hogweed with cow parsnip, which is a native that lacks purple blotches on stems. 

Integrated Pest Management (IPM) Options: 

Prevention—Be careful what
you plant.

Mechanical—Giant Hogweed can be dug, mowed, and or cut. Be sure to use extreme caution, the sap can cause severe blistering. Remove flowers to ensure seed is
not produced.  

Cultural—Maintain healthy competitive desirable plants to compete with weed seedlings. 

Chemical—Triclopyr and Glyphosate are herbicide active ingredients that can provide good control of this weed. Herbicide applications are most effective in the fall after the first frost, but it might take multiple applications throughout the year for several years to eradicate this weed. Always read and follow herbicide label directions.

Justin Hatch, University of Idaho Extension Agriculture Educator in Caribou and Bear Lake Counties. 208-547-3205 [email protected]