Giant Hogweed? It may be a look alike!
By Tys Theysmeyer, Head of Natural Areas, Royal Botanical Gardens
The Invasive ornamental plant Giant Hogweed has several look-alike species flowering at similar times creating identification confusion. These similar native plants grow in wetland areas and are found commonly along two of RBG nature trails. The similar looking species include Cow Parsnip, Angelica, and Common Elderberry, all with large white flowers, but leaves that are different.
RBG staff regularly review the property for hogweed plants to keep the number of hogweed plants present on the property at zero. If you’re out on the trails and you spot a plant that looks suspiciously like Giant Hogweed, chances are it is a look-alike!
What is Giant Hogweed, and Why is it a Problem?
True to its name, Giant Hogweed (Heracleum mantegazzianum) can grow quite large for an herbaceous plant, reaching up to three metres tall. It has large, sharply toothed leaves and a hollow stem. Second year plants produce large umbels of white flowers in mid-summer, which look similar to the flowers of other plants in the carrot family (Apiaceae). It was introduced first to western Europe, and then to North America as an ornamental, due to its impressive size.
Although this Giant Hogweed is awe-inspiring, it is an invasive species. It has been found across Canada, with scattered populations in British Columbia, Alberta, Saskatchewan, Ontario, Quebec, New Brunswick, Nova Scotia, and Newfoundland! While it occurs in only scattered populations in Ontario for now, this plant is capable of producing up to 120,000 seeds which can float on the water’s surface and spread through rivers, creeks and streams. If a population of these monstrous plants establish, they can easily outcompete native plant species for light and resources due to their large size, fast growth, and copious seed production.
Photos 1, 2, 4: Credit Peter Kelly
Giant Hogweed quite toxic. It contains sap which is phototoxic, meaning that chemicals inside the liquid react with the sun, causing severe dermatitis. While several other plant species in the carrot family produce phototoxic sap, the sap from our mystery plant causes (by far) the most severe reactions. Getting the sap on exposed skin on a sunny day can cause blisters and scars which can last for several years.
Identifying Giant Hogweed
From the Ontario Invasive Plants Council: Leaves are prominently spiked with a pronounced jagged appearance. On mature plants, leaves are divided into three equal or almost equal parts which are then divided into a further 3 parts (ternate). Smaller plants may just be deeply lobed. Leaves can grow up to 1 m wide.
From the Ontario Invasive Plants Council: Whitish flowers appear in mid-June and are clustered in umbel shaped heads which can measure up to 1 m across. Umbels are an umbrella-shaped cluster of short-stalked flowers, typical of plants of the carrot family.
From the Ontario Invasive Plants Council: Giant hogweed is a perennial herb which can grow to 5.5 m under ideal conditions, though such sizes are rarely seen. Typically the plant reaches heights of 3-4.5 m across Ontario although that varies based on soil and habitat type.
Staff at Royal Botanical Gardens keep a tight watch on any potential Giant Hogweed on our properties; so far in 2022, no true Giant Hogweed plants have been reported, though you may spot the following look-alikes along RBG trails that border floodplains like Grindstone Marsh, Creekside Walk, and Spencer Creek Trail.
Take a look at the leaf and flower shape of the three plants below, and be assured there’s no reason to panic or report to RBG staff if you spot these along the trails.
Cow Parsnip (Heracleum maximum)
Canada Elderberry (Sambucus canadensis)
American Angelica (Angelica Atropurpurea)
No matter what species you spot in the nature sanctuaries, it is important to stay on-trail and not wander. The nature sanctuaries are full of delicate species, many of which are easily trampled by human (or dog) feet.
Though the plants listed above do not present as dangerous a threat as Giant Hogweed, many plants (including native species) can cause skin irritation, or provide cover for ticks. Yet another reason to stay on the marked trails, wear appropriate gear, and not remove any plants along your hike.
For more hiking tips, vist rbg.ca/onthetrails