'World's worst plant' sprouts in Ontario this time of year and can destroy your home

Although spring in Ontario means warmer weather, more rain, and budding trees, it also signifies the resurgence of one of the province's most problematic plants, which, if left untreated, can quietly wreak havoc on your property.

The highly invasive perennial, Japanese knotweed (Reynoutria japonica), has been dubbed the "world's worst plant" by some, thanks to its ability to develop massive underground root systems called rhizomes, which can damage foundations, crack pipes, break through concrete, and result in expensive repairs that could take years and thousands of dollars to complete.

As a result, the International Union for Conservation of Nature (IUCN) has even added the species to its list of the 100 worst invasive plants in the past.

The plant is native to eastern Asia, including Japan, Korea, China, and Taiwan, and was introduced to North America in the 1800s as an ornamental plant and to help control erosion. Today, the invasive plant is established across Canada, including in southern and central Ontario, where it often grows along roadsides, in gardens, and near older or abandoned building sites.

According to the biodiversity social network, iNaturalist, several sightings of the species have already been documented in the province over the past few weeks, as the plant usually begins to sprout in late March and April.

Sighting of Japanese knotweed at Rouge National Urban Park on March 1. Photo: 
gary-james.

The species can be identified by its bamboo-like appearance, and can reach anywhere from one to three metres in height. Its stems are round, reddish purple in colour, and smooth, while its leaves are ovate with a flat base, and its flowers are greenish white.

As the aggressive plant grows, its rhizomes, which are typically dark brown with an orange interior, can account for two-thirds of its total mass and extend more than two metres deep. Under the right conditions, these rhizomes can spread outwards at a rate of 50 centimetres per year.

Sighting of Japanese knotweed in Toronto on Feb. 16. Photo: jade_colbert. 

Japanese knotweed also presents issues for other surrounding plants, as its thick layers can make it difficult for other native species to establish and survive.

As a result, the plant is listed as a restricted species under the Ontario Invasive Species Act, making it illegal to import, deposit, breed, grow, buy, sell, or trade it.

Thanks to its nearly indestructible nature, the most effective way of removing the plant is through targeted and repeated applications of herbicides over many years by a professional. If you ever come across the plant, refrain from digging it up or cutting it, which can be both insufficient and result in the spread of the plant.