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When Pigs Swim

Climate ChangeIssue 1NatureNature Issue 1

Creeping up and down Sixteen Mile Creek this summer is Halton’s largest ecological alien. Giant Hogweed has been colonizing this stretch of Halton for at least forty years. Sixteen Mile creek meanders through the escarpment from Halton Hills, to Milton, and then to Oakville. It touches farms, wetlands, roadways, and passes by our homes. Unlike the benign and dainty blooms of Queen Anne’s Lace, Giant Hogweed towers over grown men, spewing its large florescent heads into the air. Giant Hogweed earns its notoriety from photo-toxicity. Contact with the sap can result in burns to human skin; it’s the ‘anti sunscreen’, and the sap can leave one photo sensitive for several years.

Queen Anne's Lace, pictured here, is very short compared to Giant Hogweed.

Queen Anne’s Lace, pictured here, is very short compared to Giant Hogweed.

The real story of Giant Hogweed isn’t in its alien appearance and sci-fi effects on our skin, it’s based in one of the oldest narrative genres: the invasion of Giant Hogweed is the tale of travel, the journey across water, and its impact on our biodiversity.


Giant Hogweed, native to the Caucasus Mountains in Central Asia, adapts well to moist soils in many situations. In the 19th Century, gardeners thrilled at the ornamental look of the plant and brought it over to the gardens in Britain. It’s speculated that something similar happened for the plant to spread in Canada. Now, the plant has colonized much of Europe, and North America, and is an international scourge.
A single Giant Hogweed plant has been known to produce 10,000 winged seeds per growing season. The seeds can stay cloistered in the top five inches of soil, called the seed bank, and it is known that their viability can last many years. In Ontario, Giant Hogweed prefers to put its roots into moist soil. In Canada, Giant Hogweed is most prevalent in our moist creeks and river banks.

Giant Hogweed grows several feet before it blooms

Giant Hogweed grows several feet before it blooms


When Giant Hogweed disperses its seed, thousands of seeds float downstream until washed up onto a bank. The seeds of Giant Hogweed are hardy: they’ll float for three days, and around three days is when the seed can become waterlogged and sink to the bottom of the waterway. The long-term viability of Giant Hogweed seeds means that the seed can travel miles by creek and rest until conditions are suitable for germination. The seeds must endure two to three months of cold weather before a root will sprout through the hard seed shell.
Due to the sheer number of seeds that Giant Hogweed releases, and the disappearance of local vegetation, Giant Hogweed seeds overwhelm seed banks. A seed bank is the natural storage of dormant seeds in the soil. If viable, a seed could theoretically germinate at anytime, so long as the specific conditions it needs to grow are present. Viable seeds can remain dormant over many years, and grow at a later date. This is why it is difficult to contain Giant Hogweed: it releases thousands of seeds, it travels, and it remains viable for long periods of time. Because of its long term viability, this is why control measures need to occur year after year to exterminate Giant Hogweed: it’s not only the one plant, it’s also the multitude of seeds that will sprout from seeds in the seed bank for years to come.

Giant Hogweed

A technician standing next to Giant Hogweed

Because of its resilient seeds, Giant Hogweed out-competes local vegetation and negatively impacts the environment. Moreover, the large leaves of the Giant Hogweed block the sun from local native species that emerge later than the hogweed does. Local native species in the seed bank are unable to grow because the Giant Hogweed directly impacts their specific growing conditions. Local vegetation cannot compete, and dies off, therefore leaving a monoculture of Giant Hogweed.

Climate Change

With climate change, local creeks will face new stresses and changes in hydrology. A monoculture of Giant Hogweed may make creeks less resilient in the face of climate change. There is evidence that climate change is affecting the volume of spring rains in our area. Excess rain means that Giant Hogweed seeds may travel outside of creeks and streams, and move further onto dry land. Seeds dormant in the seed bank may move with the increased water volume that floods out of the creeks. Giant Hogweed may then colonize dry land and fields and outcompete with the local species in that ecosystem. Wildlife and pollinators move elsewhere if local vegetation is no longer present. The land then becomes a weak monoculture. If local vegetation disappears, we’ll witness the loss of some of our local biodiversity.

To prevent the spread of Giant Hogweed, we need to stem the spread of seed. Colonies can start from one seed, and Giant Hogweed can disperse up to 10,000 from one plant. The sheer size of Giant Hogweed makes it difficult to eradicate the plant. Those who attempt to control it are in danger of coming in contact with the sap that is phototoxic. If they disturb the flowery heads after they have ripened, it’s highly likely the seeds will drop and spread. Mowing is not recommended because the seeds can get stuck in the equipment and can be transported to new location that way. Moreover, there is a ‘splash factor’ when mowing: the blades splash sap around, and walking over the newly cut plants drastically increases the chance of sap-on-skin contact.


Conservation Halton has a program to eliminate Giant Hogweed from its property, but it is a persistent species that is difficult to remove. Options for removal include spraying a herbicide or digging it out. Our staff take safety precautions when dealing with Giant Hogweed. On a Conservation Halton property, such as a park, conservation area, or other signed and marked property, please contact Conservation Halton. On your own property, private land, or municipal park, please contact your local municipal weed inspector.
Climate change deeply affects our waterways. Abrupt, powerful storms and heavy spring rains flood watercourses. We don’t yet fully understand the combined impact of climate change, and Giant Hogweed on the environment, but we can do our best to prevent the possible spread of its seed outside of watercourses, and to exterminate the plants already there. This two-pronged effort will, hopefully, maintain our natural environment. Although the sap is what makes Giant Hogweed infamous, the real quandary is in the loss of our native plants affecting our resilience to climate change.

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Last modified: July 14, 2016

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