The scourge of urban Japanese knotweed

You may have observed Japanese knotweed around town or on your property. While the stem looks remarkably like bamboo (which is actually a grass), the leaves are completely different.

The plant grows extremely fast in one season and, once established, is extremely difficult to eradicate.

In 2019, the Environment and Sustainability Committee was made aware of the challenges faced by homeowners who attempt to control Japanese knotweed on their property, and recommendations were suggested to inform and educate residents along with a request to introduce a by-law that would require property owners to remove noxious and invasive weeds.

Until the City passes a noxious/invasive weed by-law, residents are left to either ignore the plant pest or try to remove it with perhaps not the best results.

Now is a good time of the year for property owners to make a positive identification so they can be prepared to start controlling the spread and growth of this environmental nuisance next spring.

About

Japanese knotweed (Fallopia japonica) is found not only on the PEI Invasive Species Council list, but also on the Canadian and every provincial Invasive Species Council lists. It is among the world’s worst invasive alien species.

The Canadian Council on Invasive Species explains that a very limited number of ‘alien, non-native’ plants are identified as ‘high risk’ and pose a direct threat to human health and safety, the environment, and the economy. These plants are known as invasive due to their ability to aggressively take over the landscape.

The reason Japanese knotweed is so aggressive and hard to control is because the roots (rhizomes) grow in all directions at various depths and even sprout shoots underground. The tough shoots can break through gravel, tarmac, and concrete. Prolific rhizome and shoot growth can damage foundations, walls, pavements, drainage works, and flood prevention structures.

➤ Rhizomes can extend 7 metres (about 23 ft) horizontally and 3 metres (about 10 ft) deep, with shoots even sprouting underground! The plant we see above-ground is really the “tip of the iceberg”.

Controlling the spread of Japanese knotweed

The best time to tackle Japanese knotweed is in spring, when the previous year’s dried-up stalks can be more easily removed and before too many new shoots sprout from the rhizomes.

Controlling Japanese knotweed depends on the size of the infestation, but whichever method is used, it must be continued for at least 5 years to ensure that the rhizomes are weakened sufficiently to prevent new growth.

For urban property owners, one of the more common — and inexpensive — control methods is tarping.

Tarping refers to covering an invasive plant population with a dark material to block sunlight and “cook” the root system.

FIRST YEAR:
To tarp an area, first cut Japanese knotweed stems, taking care not to spread any plant pieces. Even the smallest fragment of stem or rhizome can produce new plants.
Carefully place the plant pieces in a thick clear plastic bag and tightly seal the bags.
Write “Japanese knotweed”or “invasive plant” on the bag.
Place it in your waste cart; you can also have up to 2 excess bags beside your cart [source: Island Waste Management].

Next, smooth out the targeted area, remove rocks and any other debris that may damage the covering. Cover the infested area with thick or multiple layers of a dark coloured tarp or heavy material, extending up to 2 metres beyond plant growth, and weigh down with stones, bricks, planks, soil, etc.
How-to video

SUBSEQUENT YEARS:
Monitor the perimeter for new shoots, especially in spring, when growth is strongest. Pull out the shoots as you find them, bag carefully, and follow the disposal method described above. Cover new area and continue monitoring the perimeter for new shoots.

Check the tarp/plastic cover every spring/early summer for damage. Use duct tape to repair tears, or add or replace tarp/plastic as required.

Do not check under the tarp to remove new shoots (they will be white from lack of sun), as this will only provoke the plant to produce even more shoots.


  • It takes time and patience to bring the knotweed under control (up to eight years).
  • If space allows, plant shade-producing native species around the tarp/wrap.
  • KEY MESSAGE FROM EXPERTS: Have a plan and keep at it.

Call to action

Japanese Knotweed is a serious problem, which needs to be taken seriously, especially by city councils. Because Charlottetown has no noxious or invasive weed by-law, home-owners with Japanese knotweed on their property are not legally required to control it or to prevent it from spreading onto neighbouring land.

  1. Contact or write to your councillor to ask for an amendment to the Municipal Property Standards By-law. A Canadian municipality can pass a property standards by-law under the Building Code Act to address the presence of weeds deemed noxious or a threat to the environment or human health and safety.
  2. Contact or write to your councillor and your MLA to urge the PEI Legislature to
    (a) raise greater awareness of the Prince Edward Island Weed Control Act,
    (b) include all alien invasive plant species as noxious weeds in the Act and,
    (c) introduce regulations to control the spread of Japanese knotweed, as the province did with purple loosestrife in 2004.

Municipalities with a noxious weed by-law:

Posted: Sep 28, 2021, 8:04 AM AT | Last Updated: September 30, 2021, 12:50 PM AT

III. Walkable urban places — Environmental benefits

Concern for the environment may be one of the earliest and most straightforward drivers for increased walking and active mobility.

From climate change to air pollution, loss of biodiversity to green infrastructure, walking provides an active means for people to mitigate and address local and global environmental concerns.1

“The recovery of sprawl to vibrant places is literally our generation’s greatest challenge.”

Steve Mouzon, Architect and New Urbanist

Robert Steuteville, editor of Public Square (a CNU Journal), has been publishing a series of articles based on the 2016 Cities Alive: Towards a walking world report, which shows the benefits of walkable cities – social, economic, environmental and political – and sets out measures for improving walkability.

This post itemizes the list from Steuteville’s article (link embedded in heading below), and includes additional details from the report.

Ten environmental benefits of walkable places

  1. Reduces greenhouse gas emissions
    Fewer cars, fewer emissions.
  2. Improves urban microclimates
    The Urban Heat Island (UHI) effect, where denser urban areas are significantly warmer than surrounding rural areas, is a major issue facing cities today. Increased urban vegetation and mature tree canopies contribute to the mitigation of the urban heat island effect by reducing the air temperature and provide pleasant strolling temperatures.
  3. Optimises land use
    Soil is not a renewable resource and is essential to nurture plants and animals. In addition, it is vulnerable to impacts from vehicular traffic, industry and construction. Walkability improvements can help reduce the amount of land required for transport facilities (roads and parking), encouraging denser land use patterns.
  4. Reduces air pollution
    When walkable environments incorporate more trees and vegetation, they will inherently clean the air: 17 trees can absorb enough CO2 annually to offest nearly 42,000 km of driving.
  5. Improves water management
    Reallocating investment from motor vehicle infrastructure into parks and pedestrian environments improves the overall health of city ecosystems and help divert millions of litres of stormwater runoff.
  6. Promotes alternative transportation
    Public transit users are pedestrians or a combination of pedestrian/cyclist. Purposeful investment in walking and active transportation networks encourages increased pedestrian and cycling activity.
  7. Makes cities more beautiful
    Streets that offer a robust, attractive experience can accommodate a variety of diverse uses such as outdoor dining, seating and gathering areas. Additionally, beautification through landscaping, public art, and wayfinding becomes an important feature.
  8. Increases active use of space
    Pedestrian improvements can provide people with more pleasant spaces to stay, and lead to an increase in the active use of public space and the facilities it contains, such as benches, playgrounds, water bubblers, public gyms and skate areas.
  9. Cuts ambient noise
    Plant leaves have been shown to tone down noise by reflecting, diverting and absorbing acoustic energy. Trees with abundant foliage are especially effective at minimizing noise levels.
  10. It makes better use of space
    Street designs that restructure the street network better serve pedestrians and cyclists, often repurposing space reclaimed from vehicle travel lanes and on-street parking to accommodate cycle and pedestrian infrastructure.

“The pedestrian is an extremely fragile species, the canary in the coal mine of urban livability.”

Smart Growth America, nationwide coalition promoting a better way to growth

1 Excerpts from Cities Alive: Towards a walking world, a report published by Arup in 2016.


More on this topic:

What is permeable, pervious, or porous pavement?

Permeable, pervious or porous surfaces are types of pavement with a high porosity that allow rainwater to pass through into the ground below.

Permeable paving can be one part of building green parking lots, which can also include rain gardens, art, trees, solar covers, and other creative elements.

According to the Environmental Protection Agency, permeable pavements can also help reduce flooding of building foundations and ponding of water on driveways, sidewalks and patios.

Ontario-based Random Acts of Green, a women-led social enterprise, recently published a blog post entitled “6 Ways Permeable Pavement Benefits the Earth”.


More on this topic:

Are Pervious, Permeable, and Porous Pavers Really the Same?

Excellent video (includes paid promotion) that explains and shows examples of how permeable pavement works (ends at 8:32)