Giant Hogweed +
Like poison ivy there are other plants that should be avoided and not touched. You may encounter plants that are somewhat similar. However, once you know them, it is hard to mistake a mature Queen Anne's Lace with a mature Giant Hogweed. They are Angelica (Angelica spp.), Cow Parsnip (Heracleum maximum), Giant Hogweed (Heracleum mantegazzianum), Wild Parsnip (Pastinaca sativa) and Queen Anne's Lace/Wild Carrot (Daucus carota). The first three are fairly common in flood planes near the Grand River, Speed River, Conestogo River, Nith River and creeks (like Martin Creek in Woolwich Township). Learn to recognize the first three of them and don't touch them. The invasive Giant Hogweed is being removed by professionals when spotted and reported. If you are not familiar with these species, learn about them from fellow hikers.
Giant Hogweed (Heracleum mantegazzianum)
Although this video is from British Columbia the content is relative to what you may encounter on trails in southern Ontario.
More About this Plant
Giant Hogweed (Heracleum mantegazzianum) is a member of the parsley or carrot family, Apiaceae (Umbelliferae). It is characterized by its size and may grow to 5 to 7 metres in height. It is similar to Cow-Parsnip, (Heraculeum maximum or Heracleum lanatum). It has a stout dark reddish-purple stem and spotted leaf stalks. The stalks and stem produce sturdy pustulate bristles. The stem and stalks are hollow and the stems may be 5 to 10 cm in diameter. The compound leaves of giant hogweed may reach breadths of over one metre. Each leaflet is deeply incised. The inflorescence (flower cluster) is a broad flat-topped umbel composed of many small white florets. Each inflorescence/umbel may attain a diameter of 0.7 m.
Giant Hogweed is native to the Caucasus Mountains and southwestern Asia. It was introduced to the gardens of Europe, the United Kingdom, Canada and the United States. It readily escaped and has naturalized in many of the places where it was first introduced. So you may see it growing along streams in Ontario. However, distinguishing between Giant Hogweed and its similar species takes practice. The bottom line - Don't touch any part of the plant or its look-alikes.
Giant hogweed often grows in wet areas but may colonize a wide variety of habitats such as along roadsides, other rights-of-way, vacant lots, streams and rivers.
Other Online Resources
Ontario's Invading Species Awareness Program - Giant Hogweed
Ontario Giant Hogweed Fact Sheet (PDF)
Ontario Weeds: Giant Hogweed
Wikipedia: Giant Hogweed
Giant Hogweed - Grand River Conservation Authority
Compare Similar Species - Durham Region
Field Guide to Noxious and Other Selected Weeds of British Columbia
Google Images of Giant Hogweed
Giant Hogweed in Connecticut