Signs of Fall 4: Giant Hogweed!

Photo by D. Sillman

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Hiking on the woodland trails or across the old fields of Harrison Hills Park (or almost anywhere, for that matter) you see a familiar array of plants. It is quite startling to realize that so many these plants are, in fact, alien species carried to North America sometimes intentionally and sometimes unintentionally by people.

Garlic mustard, Japanese knotweed, barberry (both European and Japanese), honeysuckle (Amur and Japanese and Tartarian), multiflora rose, poison hemlock, mile a minute vine, privet (four different kinds!), tree of heaven, and common mugwort make up a substantial proportion of the plants across the park (and our county and our state!). All of these species are non-native and all have been classified as, and are very recognizable as, invasive species.

But there are also many species we value on the alien, invasive list including butterfly bush, dame’s rocket, orange daylily, coltsfoot, and chicory.  The Native Plant Center even lists forsythia as an exotic invasive species. How could you have watched swarms of monarch and swallowtail butterflies gathering nectar from the park’s butterfly bushes and considered them alien invasive plant? And the sight of coltsfoot and then forsythia blooming in the early spring, and then dame’s rocket in June, and the orange daylilies and the blue-flowered chicory in July seem like true signs of the passing seasons rather than visual evidence of an alien invasion.

We are, according to Charles Mann, living in the Homogeocene! People are moving not only themselves but also wild and domesticated plants around the world so rapidly that the vegetative ecosystems all over the Earth are becoming dominated by the same hardy, generalist species. Travel to Central Europe or to South America or Africa or Asia and you find similar arrays of invasive species alongside all of the pathways through the untended and also the cultivated regions.

Deborah made a set of webpages in which she described the flowering plants of Harrison Hills Park. Of the 161 plants that she has observed in the park, 52 (32%) are alien species. The park is a good place to get a glimpse of the Homogeocene!

Leaf of giant hogweed. Photo by D. Harper, Wikimedia Commons

Every once and a while, though, some new alien invasive plant comes to popular attention often because of some massively exaggerated feature of its anatomy or its ecology. The invasive of this past summer is giant hogweed (Heracleum mantegazzianum).

I first read about giant hogweed on an information posting on weather.com. I was checking on the upcoming day’s weather and I saw a headline at the bottom on the page, “Horror Plant Spreads in US!” The story went on to describe giant hogweed as a member of the carrot family that can grow 14 feet tall and produce a sap that can interact with sunlight and cause third degree burns on unfortunate people who have come in contact with it. The plant shades out native plant species (anything under 14 feet tall, anyway!) and has been found in Virginia, Maine, New York, New Jersey, Massachusetts, Connecticut, Pennsylvania, North Carolina and parts of the Pacific Northwest.

After reading that article, I was glad that I had never run into giant hogweed. The next day, though, I got an email from one of our bluebird volunteers at Harrison Hills Park relaying a request from an acquaintance who thought that they had seen giant hogweed in two locations of the park! Deborah and I were on our way to Ann Arbor for a few days but promised that we would check out the park locations when we got back.

First, though, we went to Internet for more information!

One point of some reassurance was that although Pennsylvania is on the hogweed-positive list, hogweed has only actually been confirmed, according to the very frequently updated “Early Detection and Distribution Mapping System of Invasive Plants,” in two of its counties: Erie and Mercer. Both these counties are quite far away from northern Allegheny County and Harrison Hills Park. Giant hogweed was, though, quite extensively reported from New York State and northern New Jersey, so it seemed to closing in on the borders of Pennsylvania.

Flowers of giant hogweed. Photo by M. Pixel

The second important piece of information was that although a 14 foot tall giant hogweed plant should be remarkably distinctive, there were a number of plant species that were frequently confused with it. Three plants mentioned in the New York State Department of Environmental Conservation “Giant Hogweed Identification” website are cow parsnip, wild parsnip and poison hemlock, and all three of these potential “lookalikes” are found at Harrison Hills Park.  None of these three giant hogweed imposters, though, really manifest the essential features of the plant. Giant hogweed stands between 7 and 14 feet tall. It has huge (2.5 feet across), white flowers with 50 to 150 rays organized into a broad umbrella shape. The leaves are also huge (five feet wide) and deeply incised and lobed. It’s stems are green with spatters of purple and lots of coarse white hairs. Cow parsnip is the closest giant hogweed mimic but its leaves and its flowers are half as large and its flowers are flat rather than domed up like an umbrella. Also its stem are green with no purple splotches.

So, the day after we returned from Ann Arbor, we went up to Harrison Hills to look for giant hogweed. On our walk about the suspected areas we found extensive stands of poison hemlock and wingstem (a tall, native “weed”) but no giant hogweed (and also no cow parsnip which we were assuming to be the stimulus for the hogweed report). Turning a corner around a large pile of plant debris, though, we came face to face with a plant that looked so huge and so out-of-place and so alien that it had to be the hogweed wanna-be.

Photo by D. Sillman

The plant was growing in a cluster of five or six stems. The stems were very woody and not the green, purple-blotchy, hairy stems one would expect of giant hogweed. The leaves were very large (about 4 feet long and four feet wide) but were not deeply incised or lobed (as one would expect from giant hogweed). This was something unusual, but it was not giant hogweed. It turns out that this plant was another potentially nasty, exotic invasive species called the princess tree or, sometimes, Royal Paulownia (Paulownia tomentoas).

The princess tree is a native of central and western China and was brought to this country (and many countries in Europe) as an ornamental or through the accidental distribution of its seeds (which were used in the days before Styrofoam peanuts as packing materials). It is extremely fast growing, prolific in its production of seeds, and able to tolerate a wide range of environmental conditions. It would be a tree valued for its appearance and vigor except for its tendency to rapidly spread and engulf any area into which it is introduced.

We passed along our observations but don’t expect that the princess trees will be cut down. We will keep an eye on them in coming years!

So, we went on an invasive plant hunt and we found one! It just happened not to be the one we were looking for!