Invasive Plants

Japanese Knotweed

Japanese Knotweed, Polygonum cuspidatum

Who will win?

Often called bamboo, Japanese knotweed is found across PEI. It is a very invasive plant “from away” that out-competes other plants, creating a monoculture.  This reduction in plant biodiversity reduces the quality of wildlife habitat and food sources, resulting in fewer wildlife species being present.

Japanese knotweed is a non-woody, perennial plant that dies back to the ground each year and can grow up to a height of 3m annually.  It spreads mainly by underground stems (rhizomes).  These rhizomes can grow 60 feet horizontally and 10 feet down.  They cause issues with infrastructure and can even come up through pavement!

At Victoria Park in Charlottetown, we have been experimenting with different ways to get rid of “our” knotweed patches. We first tried cutting the knotweed on a weekly basis (to deplete the supply of food in its rhizomes). After one season of cutting, the knotweed was still growing fast and furious… we needed to find a better way.

We then tried laying down different colors of tarps over the knotweed patch and putting soil on top of some of the tarps. The blue tarps let in too much light allowing the knotweed to grow almost unrestricted. The black tarps created a lot of heat which “cooked” the tips of the shoots but still allowed the plants to grow. The tarp that was covered with the soil created dense shade (which the knotweed does not like) and prevented and water from getting to the knotweed plants.  There was no green growth under the tarp/soil combination.  

In 2007, we decided to cover the entire patch of knotweed with a tarp, put 8” of soil over the tarp, seed this area to grass and mow it.  We ensured the tarps overlapped a significant amount to prevent the knotweed from pushing its way up between the tarps.   Since we were stressing the knotweed plants, they put a lot of effort into growing out beyond the edges of the tarp.  City summer students pulled the knotweed on the outer edges of the tarps weekly. 

When we initially started this project, it took a couple of students up to 4 hours to pull the knotweed around the perimeter of the tarped area.  In 2017, students found it very difficult to find any knotweed sprouting around the edges of the tarp.  We rolled back a section of tarp to see if any knotweed was still alive under the tarps and none grew.

I believe we have conquered this patch of knotweed!