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  • Writer's pictureCarl Anderson

Beware of Knotweed - the menace of construction

Updated: Jan 3, 2022

What is knotweed?

There are 4 types of knotweed in British Columbia - Himalayan, Giant, Japanese and Bohemian. It is a highly invasive non-native plant that is quickly taking over sites across the province. Japanese and Giant species are thought to have been originally introduced to Canada in the 1800s as a garden plant after their popularity in English gardens. These two species created an offspring, Bohemian knotweed, which is the most invasive of the species. Himalayan is less prevalent but still an issue.

Knotweeds thrive in moist soil (usually near watercourses and beaches) and sunlight…something BC offers in abundance, especially in the spring when it starts is an annual active season. Knotweed is tough, and we mean crazy tough. It can move through almost anything…including concrete! It is perennial and goes dormant in the winter making it strong enough to survive a good Canadian winter as well. In the spring, in March, it grows new stalks, spreads its roots and it blossoms around late August. The 4 species can cross-pollinate. It grows between 1.5m to 6m tall. All in all an impressive but dangerous plant.

Why is it a Menace?

Knotweed’s ability to penetrate hard surfaces causes damage to buildings, storm sewer, and sanitary services, pavements, and sidewalks. Treating knotweed is complicated and it has the potential to add significant delays and costs to construction projects, especially if dealt with incorrectly. Its mere presence on site has been known to prevent the sale of and/or devalue land.

Just one piece of rhizome, root, or seed can produce a new plant. Soil moving from one site to another is at particular risk of unintentionally risking multiple sites. Its root system is extensive: every stalk you see could have a root ball extending horizontally underground up to 15m-20m and 3m deep!! with new stalks growing new roots, the site (and potentially neighboring sites) can quickly have an underground network extending through huge sections of the property.

As they often grow next to watercourses, they are naturally distributed from site to site easily.

How to identify Japanese knotweed?

Knotweed shoots appear in spring, rapidly growing from a plant that resembles asparagus, into a dense thicket of knotweed 3 meters tall or more by June. It is important that you identify knotweed as early as possible. If you think you may have knotweed on your site, contact an expert to confirm and come up with a plan to deal with it correctly.

How to eradicate – Time or Money.

It is notoriously difficult to kill and you will need the help of a professional. You have a few options but they involve weighing time and money.

  • In the first option, the herbicide is injected directly into stalks, moving through the rhizomes. The plant may look dead but it’s very difficult to kill the underground rhizome system in its entirety, and even more difficult to verify with confidence that eradication has been achieved. Treatment is often unsuccessful unless performed over the course of 2 to 3+ years/seasons. There are inherent risks even after a site has been treated with herbicide. If building within an area where knotweed has been previously treated with herbicide, extreme caution is required. The mere action of disturbing ground during groundworks or landscaping can reinvigorate rhizomes made dormant by herbicide treatment, resulting in considerable spread across a much larger area. Removal is often the way forward for development sites.

  • If waiting 2 to 3+ years is not practical, you should consult an expert well versed in knotweed for options. On a recent project, we were presented with 2 options to expedite construction: Bury or take away.

  • The excavation option involved cutting off the stalks and digging up the surrounding soil (in a minimum 15m wide radius, 3m deep). Once excavated stalks, roots and related soil need to be buried at a minimum of 5m down, wrapped in a special geotextile.

  • Often, physical removal is the answer. Although it is more expensive than herbicide treatment, it’s carried out in days not years, has a better success rate, cuts delays, risks, and, therefore, overall (or potential) cost.

  • In both the removal and the burial options, there is still a huge risk of the site activities inadvertently moving deleterious around the site or just as bad, offsite which will just make a new problem in a few years time somewhere else (and/or maybe right where the knotweed was in the first place).

The moral of the story is, investigate your site regularly as a maintenance procedure and especially before buying land or building on it. If you think it may have knotweed, call an expert to come and have a look. It’s not expensive and it will be money well spent to have enabled you to make sound decisions for your potential property and/or project.


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