Is there a giant in your backyard or along your stream edge? Giant hogweed is a huge, dramatic plant that was introduced as a garden ornamental from Europe, botanically named Heracleum mantegazzianum. The immense size caused it to be named after the mythical Hercules, and getting it out of landscapes may be like the labors of Hercules. This plant colonizes stream and river edges, and other riparian areas, and establishes itself readily. The rich, moist soil of cultivated gardens suits the plant perfectly. The plant may look attractive, but don't get near it, because the sap is toxic to skin.
Giant hogweed, which may reach 15 feet in height, is an introduced weed that has now been added to the Washington state noxious weed list, as a Class A weed, meaning that eradication is a high priority. The Noxious Weed Board lists non-native, invasive and destructive plants that have been introduced to the state. They do not list natives, but 'exotics' that can poison humans or livestock, or clog waterways, or reduce agricultural crop yields, or degrade public lands.
The Noxious Weed Board brochure comments that "more than 2/3 of our noxious weeds started out as horticultural plantings that escaped." An introduced plant can become a pest because it's often growing without the natural insect, disease, or climate conditions that tended to curb in it in its native habitat.
Giant hogweed is a public health hazard. The toxic sap, clear and watery, contains a substance that causes painful blisters. The blisters emerge when skin coated with the sap is exposed to sun. The combination of sap and sun produces painful burning blisters that may develop into purplish, blackened scars. It's illegal to move the plant within or between states, or to sell the seeds. Don't collect the seed heads to dry them Keep children away from this plant and use extreme caution when handling it. Some people do not react to this sap, but don't take chances.
Again, identification is vital. This plant resembles the common cow parsnip, Heracleum lanatum. Cow parsnip, with a similar leaf and flower, is a native plant to the maritime Pacific Northwest. The distinguishing difference is in scale: cow parsnip seldom exceeds 3 to 4 feet tall, with a maximum of about 8 feet. Giant hogweed soars to 10 or 15 feet. The hollow stems of giant hogweed can be 2 to 4 inches in diameter, with dark reddish or purple blotched stems.
To get more information and color pictures of this weed, contact your local WSU Extension office and order bulletin PNW 429, Giant Hogweed. In King County, the bulletin office number is 206-296-3900; in Pierce County, 253-798-7170; in Snohomish County, 360-338-2400.
Many Washington counties have Noxious Weed Control Boards. In King County, call 206-296-0290. In Pierce County, call 253-798-7263.
Flower heads of giant hogweed can reach 2 ½ feet in diameter, while the smaller cow parsnip seldom exceeds one foot in diameter. Scale is definitely a defining characteristic for giant hogweed. It's tall, broad, and imposing.Each purple spot on the stem surrounds a hair, and there are large, coarse white hairs at the base of the leaf stalk. Leaves, vaguely resembling rhubarb leaves but deeply cut (incised) can be 5 feet across. The huge flower looks like a giant Queen Anne's lace, but the leaves are quite different. Queen Anne's lace has ferny-looking leaves; giant hogweed has big, platter-shaped solid leaves. The giant hogweed flowers from mid-June to mid-July and then produces large flattened elliptic dry seeds.
Shoots die down in the fall. Tall stems mark locations of this weed during winter. How can it be controlled? Keep children away from this plant. Wear protective clothing when handling it if you dig plants; consider wearing eye protection. Chopping out the root is feasible, but may have to be done several times as the plant re-grows. Wash off and exposed skin thoroughly with soap and water.
Keep the plant mowed down during the summer to prevent seeds from maturing. Don't allow pieces of it to land on bare skin. Even after the parent plant is completely removed, the seeds left behind can come up, 7 or 8 years later. Removing the green growth will help to exhaust the root and will weaken the plant, so digging it out and chopping it is feasible.
WSU specialists report that if chemical control is needed, glyphosate (sold as Round-up and other brand names) is reported to control it. Glyphosate must be applied to an actively-growing weed, and not when the weed is in flower. Tim Miller, Extension Weed Scientist for WSU in Mount Vernon, says that the ideal time to spray it with glyphosate is fairly early in the season when leaves are less than two feet tall.
Since many of these plants are now larger, one method would be to carefully cut the plant down and bag it for trash (don't compost it or let the cut stems and leaves lie around unbagged). The plant has a huge root system and will regrow, but the smaller new leaves can then be treated again. Tim Miller says "use great care not to back into the plants when spraying." Repeat applications will be necessary for full control. Do not let this plant go to seed.
Cody Toal at the Noxious Weed Control Board reports that a study on giant hogweed control using glyphosate is proceeding in King County during 1998 and more information will be available when the study concludes. Controlling this plant, and preventing injuries caused by exposure to it, is vital for all maritime Northwest gardeners.
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