Tuesday, September 8, 2009

Plant Alert: Giant Hogweed by Mary Small

Recently I was directed to an on-line article in which a popular local author recommended the use of Giant Hogweed (Heracleum mantegazzianum) as an exotic ornamental. While an unusual and attractive plant, it is on the Federal noxious weed list - for very good reasons. If moist skin (say from perspiration) contacts plant sap and the affected area is exposed to sunlight, painful, burning blisters result. Some people develop painless red blotches that turn into purplish or brown scars that can persist for several years. If sap should get in eyes, temporary or permanent blindness can result.

Giant hogweed crowds out native species in various ecosystems including streams/rivers and disturbed areas. It prefers moist soil. Here’s a description of the plant taken from this King County, Washington web site: “A member of the parsley family, its most impressive characteristic is its massive size. It reaches a height of 10 to 15 feet when in flower and has hollow stems, 2 to 4 inches in diameter with dark reddish-purple spots and bristles. Coarse white hairs at the base of the leaf stalk are also purplish, and each purple spot surrounds a blister-based hair. The deeply incised compound leaves grow up to 5 feet in width.

Giant hogweed flowers mid-May through July, with numerous white flowers clustered in an umbrella-shaped head that is up to 2.5 feet in diameter across its flat top. The plant produces flattened, 3/8-inch long, oval dry fruits that have a broadly rounded base, and broad marginal ridges.” It has not yet been found in the Colorado countryside, nor is it in an adjacent state. It is similar in appearance to cow parsnip (Heracleum maximum or H. lanatum), which does grow in Colorado. However, cow parsnip does not have purple blotches. In the author’s Colorado garden, it grows to about 6 feet with a 4 foot spread. Management of giant hogweed requires persistence regardless of the digging, mowing or herbicides used. The seeds are viable for 7 years. Do not purchase seed of this plant or transplants. We do not need this exotic pest in Colorado.