UTSG is under invasion by an unusual suspect — a thoroughly aggressive plant.
The culprit is Japanese knotweed, also known as Reynoutria japonica, which is a flowering bamboo-like species that has spread across Ontario and the rest of Canada.
Its population threatens infrastructure and native plant life, as the plant can penetrate concrete and rapidly overtake other plants in the race for nutrients and sunlight.
Knotweed, a plant native to East Asia, is thought to have been introduced to the rest of Canada through Nova Scotia in the 1880s. By 1901, it was grown in Niagara, Ontario for ornamental purposes. Its reach extends widely — an online map lists 63 sightings in Toronto alone.
Its large leaves can block sunlight from plants that grow closer to the ground, starving them of an essential resource for survival. Layers of decomposing stems and leaves shed by the plant can obstruct growth of native species. The space it takes up can reduce wildlife habitats too.
Knotweed also poses a threat to infrastructure because its roots can penetrate concrete. In one case documented in England, the plant grew through a wall and into a couple’s home — slashing the property’s retail value by over $400,000.
Now, it has invaded UTSG as well.
The Varsity confirmed two sightings of knotweed on campus. At the time of its survey in July, a very large plant was photographed overtaking sections of a garden attached to a student residence at St. Michael’s College, and a smaller plant was photographed growing on Sussex Avenue behind Robarts Library.
Two other clumps of knotweed were documented as dotting the western perimeter of UTSG on either side of Spadina Avenue. One was photographed at 698 Spadina Avenue, which is the site for a planned student residence building finalized earlier this year.
Controlling the spread of knotweed can be very difficult. The plants primarily grow offshoots through an underground plant stem, or ‘rhizome,’ which can reach a length of up to 18 metres.
The rhizome can be found as deep as two metres underground — meaning that overturning the entire top layer of soil is the best way to prevent the plants from spreading.
St. Michael’s may be home to the largest knotweed plant on campus
The knotweed patch at St. Michael’s College was photographed in the alley between McCorkell House and Sullivan House at 2 Elmsley Place and 96 St. Joseph Street, and Maritain House and Gilson House at 6 and 8 Elmsley Place. All are student residences. It was also visible from the quad behind Teefy Hall.
Knotweed plants have a distinctive appearance. Their hollow stems resemble bamboo once they mature, reaching up to three metres in height.
They have broad leaves shaped like a heart with a flattened base, which grow off the stem in a zig-zag pattern. The node where the leaf meets a stem is reddish-purple.
The plants blossom in late summer or early autumn, growing tufts of small white flowers.
At St. Michael’s College, the largest plant was photographed at over two metres tall and spreads out over about five metres, and pressed onto the side of a building.
Knotweed was observed growing on both sides of the paved alley, clearly demonstrating how its roots can push through and around concrete obstacles.
Three other locations nearby
The other Japanese knotweed plant on UTSG was photographed growing at the privately-owned house at 16 Sussex Avenue and near other homes. This is close to the Robarts Library and the Sussex Clubhouse, which houses many student organizations such as The Varsity and the Sexual Education Centre.
Another privately-owned property at 15 Glen Morris Street was photographed with knotweed plants growing in its backyard. The plants were spotted sandwiched between Graduate House, a residence building for graduate students, and the Early Learning Centre, which provides services for children of U of T students, staff, and faculty.
The final location was photographed just off-campus at 698 Spadina Avenue, beside the now-closed Ten Editions bookstore. It will be the site of a new student residence.
How The Varsity identified the plants as knotweed
To initially identify the plants as knotweed, The Varsity followed identification guidelines prepared by advocacy groups. The visual identifiers — of having heart-shaped leaves, a hollow stem, and reddish-purple nodes — were previously referred to in the article.
Tyler Jollimore, a Master’s student at Dalhousie University’s Department of Plant, Food, and Environmental Sciences, also confirmed that the sighted plants are knotweed in written correspondence with The Varsity.
He concluded the plants’ identity by examining photographs of the plants taken at the four sites listed in the article, which are included in the photo gallery above.
He added that well-informed observers can reliably identify the plant, even without his expertise as a Japanese knotweed researcher.
“So long as individuals have done their research, identification of Japanese knotweed is possible by anybody,” he wrote. “[The] appearance of Japanese knotweed makes it quite easy to identify as it tends to stand significantly taller than other herbaceous plants.”
The difficult task of eradicating knotweed
Knotweed spreads through stem cuttings and can quickly grow to colonize a new area, especially in a moist environment.
Once entrenched, the plant is very difficult to remove. Weather is no impediment, as it can endure harsh conditions and even survive floods.
The plant is especially effective at reproduction when growing alongside rivers. Its roots can break off and travel downstream, enabling it to start new growth in a different location. This is one of the ways it can spread quickly in a city near bodies of water.
Controlling knotweed populations with herbicides can take three to five years.
Physical removal alone is an ineffective strategy
One of the more common ways to eradicate invasive plant species is to cut the plant to reduce its height and diameter.
However, this physical method is not lethal to knotweed, as it can grow back again the following year. It can also make the problem worse. “Cutting could increase the chance of knotweed spreading,” noted Jollimore.
“People (more specifically children) may pick it up and play with [the plant remains], resulting in it being moved to another area,” he wrote to The Varsity.
Ecologists are currently studying effective means of knotweed control. Jollimore noted that according to the findings of a recent study he was involved in, “injections of glyphosate — a [herbicide] — can provide significant reduction in knotweed stem density in the year following the treatment.”
In this approach, every stem of the plant needs to be injected for the treatment to work. This can be labour-intensive, especially if an area contains a high density of knotweed.
“Using cutting and then spraying a herbicide to [prevent] regrowth one month later,” wrote Jollimore, could effectively reduce knotweed populations. Any removed portions of the plant must be disposed of to reduce the risk of spread via the cuttings.
Glyphosate is controversial as a likely human carcinogen. Ontario law has prohibited the use of glyphosate for plant eradication conducted for aesthetic purposes since 2009.
But the Ontario Invasive Plant Council, an advocacy group, wrote that the ban does not apply to glyphosate usage for knotweed removal. The plant’s eradication is motivated by the need to prevent damage to infrastructure and biodiversity, wrote the Council, which qualifies the plant as an exception to the ban.
As “glyphosate-based herbicides are significantly better than all other herbicide groups currently used for knotweed control,” according to Swansea University scientists, the herbicide’s application may still be the best approach for knotweed removal.
Invasive plants at U of T “physically removed” if discovered
Every treatment method against knotweed will require time and careful planning. Most treatments will damage the stems after the first year of treatment.
To prevent the spread of knotweed in the first place requires vigilance by local residents and plant owners.
“Be careful [when] accepting fill soil,” wrote Jollimore, adding that people should “use a keen eye when purchasing plants, as occasionally knotweed may be labeled as something else such as bamboo or Chinese rhubarb.”
Knotweed growth is hardly restrained by “unorganized, poorly thought out management strategies,” he continued. “You want to work smart, then hard — not the other way around.”
Mark Simpson, University of Toronto Director of Building Services, Grounds and Trades responded to The Varsity’s inquiry about sightings of Japanese knotweed on campus.
“The grounds department does, from time to time, find invasive plant species such as Japanese knotweed on University of Toronto property,” he wrote. “Such plant species are physically removed whenever they appear.”
A U of T spokesperson added that the knotweed sighting at St. Michael’s College was not on U of T property. The removal of knotweed at this location on campus is therefore not the responsibility of the grounds department, as the property of St. Michael’s College, which is a federated college, is not managed by U of T.
It’s important to report any sightings of the Japanese knotweed online, as earlier recognition of this invasive plant can help suppress its further growth.
Correction (August 13, 12:02 pm): A previous version of this article implied that U of T is responsible for removing knotweed at St. Michael’s College. A U of T spokesperson has written that the university is not responsible for this removal, as the property of St. Michael’s College is not under U of T’s management. The Varsity regrets this error.
Editor’s Note (August 15, 12:23 am): Additional photographs have been added to the article to provide visual documentation for all four knotweed sightings described in the article. The Varsity has also included further comments from Jollimore, which confirm that the photographs contain knotweed.
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