August 15, 2009
There is invasive alien out there that is one of the most dangerous plants a horticulturist will come across. Giant hogweed (Heracleum mantegazzianum), also known in some areas as cow parsnip, produces clear sap that causes nasty burns.

Giant hogweed was transplanted from China to England for use in ornamental gardens. It’s covered Britain, spread across Europe and is now moving into southern Ontario.

Landscapers should be aware of danger that giant hogweed presents. Gardeners admire its regal appearance, as the plant can grow up to five metres tall with an umbrella of white flowers nearly a metre across. The hollow stems can be up to 10 cm in diameter, with white hairs and red-purple colouring. The leaves can spread nearly two metres across.

Victims of the plant report that within a day or so of coming in contact with the giant’s sap, dime-sized, brown blisters appeared, as they worked out in the sun. The blisters look like a second-degree burn. Some report pain, but not overwhelming. Sap can also produce painless red blotches that later develop into purplish or brownish scars that may persist for years. If exposed to the sap, avoid sunlight, don’t touch your eyes and wash thoroughly with soap and water. Get medical help if blisters form. Contact with eyes can cause temporary or permanent blindness.

Giant hogweed develops pods that contain upwards of 10,000 seeds. The plant likes wet soil along waterways, where it drops pods into the water to spread seeds downstream. Seeds can lie dormant up to seven years, so expect years of work to eradicate the plants from an area.

Large pockets of the alien can be found along the Grand River in the Kitchener area and in the Milton area along 16 Mile Creek. Woolwich and Waterloo Region councils have added giant hogweed to the local list of noxious weeds. That means weed inspectors can order land owners to destroy it on sight. If they don’t, eradication crews can be ordered in and the cost added to the landowners’ taxes.

For more information on the giant hogweed, go to

Caption: Giant hogweed