Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Giant Hogweed © Hans G. Schabel

The giant hogweed invades roadsides, empty lots and woodland edges.
© Hans G. Schabel

June 2014

Slaying the Godzilla weed

Giant Hogweed is noxious.

Story and photos by Hans G. Schabel

July 18, 2011 was not the kind of day when anyone would choose to engage in physical outdoor activity. A rapidly moving thunderstorm had rumbled through Portage County in the morning, soaking everything and adding to the already tropically charged atmosphere. I was worried that the “Godzilla Squad,” a force of 20 weed fighters who had volunteered to help contain the giant hogweed (Heracleum mantegazzianum) invasion on our property, would fizzle in the face of temperatures in the 90s and a heat index much higher.

A week later would have been too late. By then the plants could have shed multi–millions of ripe propagules. Fortunately, the Godzilla Squad did show up for this first of battles in a three–year war against a noxious invader from the mountains of central Asia.

What we now know to be a dangerously invasive exotic was originally introduced as an ornamental for bee pasture or as forage for pigs and cattle into many parts of temperate Europe and North America, including at least six states and provinces in the Midwest.

Its long–distance spread generally relies on soil transport or the help of plant enthusiasts, who collect its seeds for garden use, or its showy dry umbels for flower arrangements. Short–distance spread is facilitated by wind and animals, and even in the treads of muddy shoes and vehicles. Because of its localized occurrence in Wisconsin, mostly in counties neighboring Michigan, the giant hogweed is not listed among the 44 serious “Invasive Plants of Wisconsin.” It nevertheless deserves to be taken seriously. Aside from very aggressively displacing native flora, its sap is phototoxic and can cause skin inflammation and even blindness.

At first, the giant hogweed on our land seemed more like an attraction than a liability. With leaves up to six feet across and massive stalks, sometimes exceeding 20 feet, crowned by showy umbrellas of white flowers, there are no comparable native herbaceous plants in the northern hemisphere. The plant’s flowers attract numerous insects, among them butterflies. The foliage appears to be candy for deer and the caterpillars of the black swallowtail. As a result, it was a welcome newcomer to fill a few niches on our farm, which otherwise tended to be unattractively weedy.

But by the summer of 2011 everything had changed. The honeymoon was over. An abundance of snow was followed by a wet, cool spring, just like in the Caucasus Mountains, the home of the giant hogweed. In addition, a recent timber harvest allowed much more light to reach the forest floor. As a result, every single seed of the giant hogweed that had accumulated and remained dormant for years seemed to spring to life, especially in rich, moist, cool depressions, along edges and trails. The Trojan horse had spilled its hidden army. The showy siren had metamorphosed into a weed from hell! It was time to declare war.

Rallied by Anne Graham, chair of the invasive species committee of the local chapter of the Audubon Society, members of several nature conservation organizations and some neighbors graciously pledged their support. In the process we learned some lessons.

That first summer, in our attempt to contain the giant hogweed, we relied almost exclusively on cutting the still green flower heads and carefully stuffing them into heavy–duty plastic bags. These were then “roasted” in the sun for days, before being disposed of in a waste bin. While this effort greatly reduced the amount of soon–to–be–released fresh seed, it was not sufficient to contain the bulk of the weed population. There were plenty of individuals in the vegetative stage left, poised to become flower bearers the following year.

More seriously, I noticed that all the flower stalks that we had cut produced secondary “stealth crops” of small, hard–to–detect flowers later in the summer and into fall. I tried to chase these down one by one, but this extremely tedious and labor–intensive procedure proved to be like cutting Medusa’s hair: the plants simply kept breeding tertiary and even quaternary sets of flowers, of which many undoubtedly escaped undetected.

To counter the giant hogweed’s tenacity, another volunteer force ratcheted up the battle the second summer. This time we showed up about one month earlier than the previous year. The umbels were still in mid–bloom, and carried only a few seeds approaching maturity. After cutting the inflorescences, we simply dumped them in the trail for subsequent shredding by a lawnmower. The decapitated, mature plants were then dug up in their entirety and hung upside down for drying away from neighboring shrubs. Younger plants were cut by lawn mower or scythe throughout the remainder of the growing season wherever there was easy access with these tools, while isolated plants in more difficult locations were drenched in Roundup®.

Follow–ups assured that the chemical did its work. I had hoped to avoid herbicides, but felt justified in using this extreme measure to deal with a tough adversary. This procedure of cut, dig, spray and mow succeeded in substantially reducing the population of plants that would have produced flowers the following year. In several spots, eradication was achieved.

Finally, in early June of the third summer, only a few plants in the pre–flowering stage were detected during routine monitoring of several remaining trouble spots. The problem was small enough for me to take care of with the one–time help of a friend. We dug up the maturing specimens, while spraying the remaining immature plants with Roundup® or mowing them to exhaustion.

While the battle appears won, it’s imperative to keep watch on the situation for several years, as it is not known exactly how long seeds of giant hogweed can survive underground. In the meantime, deer and swallowtail caterpillars will have to do without the previous abundance of Godzilla candy, and, where a plant with “presence” is called for, non–invasive rhubarb, also from Asia, will have to stand in.

Hans G. Schabel Ph.D. writes from Custer, Wis.