Skip to main content

Japanese Knotweed Facts and Problems: A Lovely Enemy

Linda Crampton is a writer and former science teacher with an honors degree in biology. She enjoys writing about science and nature.

Japanese knotweed stems and bluebells

Japanese knotweed stems and bluebells

An Interesting and Problematic Plant

Japanese knotweed is an intriguing plant. I have a love/hate relationship with the species. Its features are attractive and interesting. It’s thought that the plant was deliberately introduced to North America because of its lovely appearance, but in my area (and in many others) it’s now considered to be a pest. The plant spreads rapidly in these places, causes multiple problems, and can be very hard to eradicate.

The species is native to Asia and is sometimes thought to have medicinal value there. Like many plants that grow outside their natural habitat, Japanese knotweed has become a nuisance in other parts of the world. It has the ability to regenerate from a small part of the plant left in the soil after the above-ground part has been removed. This ability is impressive biologically, but it contributes to the plant's bad reputation.

The straight or slightly angled base of typical Japanese knotweed leaves can be seen in this photo.

The straight or slightly angled base of typical Japanese knotweed leaves can be seen in this photo.

I live in southwestern British Columbia, where knotweed is a problem in some areas. It's also a problem in other parts of Canada, in the United States, and in additional countries.

Knotweed Species in British Columbia

Four types of knotweed are found in British Columbia: Japanese, Bohemian, giant, and Himalayan. As mentioned below, the Japanese knotweed has multiple scientific names, including Fallopia japonica. So do the other types of knotweed in my part of the world. Bohemian knotweed (Fallopia x bohemica) is a hybrid between the Japanese species and the giant one. It was once thought that the Japanese knotweed was the most abundant species here, but it has recently been discovered that the Bohemian knotweed is the most common type.

The Japanese knotweed has spade-shaped leaves and ovate ones. The giant knotweed (Fallopia sachalinensis) has huge leaves with crenated borders and lobed bases. The Himalayan knotweed (Persicaria wallichii) has long and narrow leaves. The leaves of all of the species and the hybrid have a pointed tip.

If the leaves are present, the species are easily identified. Hybrids may not be as easy to identify, though I think I have discovered a Bohemian knotweed near my home. It's located in a different area from the Japanese species shown in my first three photos.

Japanese knotweed in May with bluebells in the background

Japanese knotweed in May with bluebells in the background

A Japanese Knotweed Habitat Near My Home

A shallow strip of wild land can be accessed at the bottom of the road where I live. One part of this strip contains tall trees, shrubs, herbaceous plants, grass, mushrooms at the right time of year, and—in one section—a creek. Ducks and great blue herons are attracted to the creek.

In another part of the wild strip, there’s a more open and drier (but not dry) area containing a trail between the sides of houses and the next road. The trail is bordered by relatively low plants, including the Japanese knotweed and the bluebells shown in my photos above. The bottom leaves in the first photo appear to have been chewed by animals, but the leaves higher up have the typical appearance of a Japanese knotweed, as shown in the second and third photo.

The knotweed clump in the area has been in the same spot for several years. It's increasing in width, but relatively slowly. Knotweed can form new aerial parts from its underground rhizomes. In many places, the species spreads far more rapidly. Although the plant does grow in drier areas, as in its habitat in my neighbourhood, it grows better in moister ones. It can often be a serious problem.

A couple of years ago, my city put up a warning sign by the treed area about upcoming pesticide use for Japanese knotweed. I thought that was odd because I had never seen knotweed there. Whatever they did, it hasn't removed the plant in the open area.

Knotweed on a grass verge; the features of the leaves make me suspect that this is a hybrid and that it's a Bohemian knotweed

Knotweed on a grass verge; the features of the leaves make me suspect that this is a hybrid and that it's a Bohemian knotweed

I recently discovered the knotweed above on a grass verge in my neighbourhood that is located between houses and a road. The plant is low in height but quite large in area. I doubt whether it will be on the verge for long. The petals on the leaves in the photo are from the beautiful cherry trees bordering the road.

A close-up view of the knotweed on the grass verge that shows the membranous sheath above a joint

A close-up view of the knotweed on the grass verge that shows the membranous sheath above a joint

Japanese or Asian Knotweed Features

Japanese or Asian knotweed (Fallopia japonica, Reynoutria japonica, or Polygonum cuspidatum) is a herbaceous perennial in the buckwheat family, or the Polygonaceae. The plant is sometimes known as Japanese bamboo. Perhaps in the future botanists will decide that the different scientific names represent different species, but for now any of the three scientific names above means Japanese knotweed.

The species was introduced to North America as an ornamental plant in the late nineteenth century. It has rhizomes, or horizontal underground stems, and can spread widely. The hidden and widespread rhizomes are one reason why it’s hard to eliminate the plant. Just a short section of a rhizome can produce roots and shoots.

The plant’s large and hollow above-ground stems may remind some people of bamboo, though the stems aren’t hollow at the nodes where the leaves are attached. One of the plant’s alternate names is Japanese bamboo. The surface of the stems is green in colour but is speckled with red. They appear to be segmented by the red rings that are present at regular intervals. The leaf stalks are connected to this ring, which is not hollow.

Scroll to Continue

The plant appears to die in autumn but in general doesn't completely disappear. Stalks that are apparently dead may be visible in winter. The underground parts of the plant are almost certainly alive at this time and become active in spring.

Flowers and leaves of Japanese knotweed

Flowers and leaves of Japanese knotweed

Leaves, Flowers, Fruits, and Seeds

Leaf Features

The leaves of Japanese knotweed have a broad base and a pointed tip. They are sometimes described as “arrow-shaped.” Their margins may be smooth or slightly wavy, but they aren't lobed. The leaves are attached to the stem in an alternate arrangement. The stalk that attaches the leaves to the main stem forms a zigzag shape.

The giant and Himalayan knotweeds have hairs on the lower surface of their leaves under the veins. The Japanese knotweed has small bumps known as sabres instead. These give the undersurface of a leaf a rough feel.

Flower Facts

The plant produces small white flowers arranged in a highly branched formation known as a panicle. The sexes are separate. Each flower has five petals. The petals are technically known as tepals because there are no differences between the petals and the sepals. The flowers attract honey bees, which reportedly produce a tasty honey from the nectar. Some companies deliberately grow the plant for this reason.

Fruits and Seeds

The fruits are winged, as shown in the photo below. They are technically known as achenes and are roughly triangular in shape. An achene is a small and dry fruit that contains only one seed and is indehiscent. Indehiscent fruits don't open when they are mature. They depend on another factor to release the seed into the soil, such as decomposition or digestion of the fruit inside an animal. The digestion doesn't destroy the seed, which is able to leave the animal's body in its feces. The knotweed seed is dark brown.

It is difficult to exaggerate how aggressive this species can be: it has been observed growing through two inches of concrete, and it will regenerate from as little as 5g of stem or root tissue.

— Native Plant Trust Go Botany (in reference to the Japanese knotweed)

A Noxious Weed

Japanese knotweed is often referred to as a “noxious” weed. This is understandable due to its harmful effects and the difficulty in removing it. The plant can regenerate from just a small piece of stem or root.

Dense Growth

The plant often forms dense groups and can spread widely. Its density prevents other plants from growing in the area, thereby reducing biodiversity. It blocks light from smaller plants, preventing them from carrying out photosynthesis. In addition, thick mats of dead knotweed parts can form in the area, smothering other plants and blocking sunlight from the ones that aren't smothered. There is a suspicion that the roots of the knotweed exude harmful substances for other plants.

Slope Erosion

Though the above-ground parts of the plant are dense, the roots are not, although they are deep. This means that a slope colonized by the knotweed can erode. This may not be a problem for the knotweed, which has a good ability to regenerate, but it may be a problem for the landscape and other plants.

Effects on Animals and Humans

The dense plant growth can also affect the lives of the animals that live and feed in the area by interfering with their movement and their foraging or hunting. In addition, it can interfere with certain human activities, especially those requiring the presence of water near the plant's habitat. Sources that should be reliable say that the plant can degrade the quality of nearby water. It can also force its way through concrete and asphalt and damage roads and bridge foundations with its underground parts.

Safety Concerns

In some places, knotweed is said to form such large clumps besides roads that traffic signs or oncoming traffic may be blocked from a person's view. The clumps may hide people as well.

The key to getting rid of knotweed is to get rid of the underground rhizomes. Since the rhizomes are stems, they produce shoots and roots. Removing the above-ground parts of the plant may be an ineffective control mechanism, unless the process is done repeatedly in order to starve the rhizomes. It's important to dispose of any cut sections of the plant safely so that they don't get a chance to produce a new individual.

Observing the Plant

I understand the need to remove knotweed, especially in some areas. Before the plant disappears from my neighbourhood, however, I will continue to observe it. It’s an interesting plant, despite the problems that it can cause. Other plants that are not inhibited by the plant's presence–like bluebells–are also interesting to see.

I admit that in my immediate neighbourhood the plant is not a major problem. If I lived somewhere where knotweed interfered with my life, the life of other people, or the life of animals, I would almost certainly have strongly negative feelings about it.

Though knotweed needs to be removed from areas where it's causing problems, it's good that it lives peacefully in some parts of the world. Scientists often find useful chemicals inside plants, including medicinal ones. If knotweed or any other plant that is sometimes a pest disappears, we will lose the ability to benefit from future discoveries about its life, its effects on the natural world, and its biochemistry. That would be a great shame.

References

  • Japanese knotweed facts from PennState Extension (Note: This article contains some good facts, but in one place it calls a rhizome a horizontal root and in another refers to it as a horizontal stem. The second statement is correct.)
  • Information about the plant from the Invasive Species Centre in Canada
  • Fallopia japonica features from Native Plant Trust Go Botany (which is based in New England)
  • An Ontario weed from the Ontario Ministry of Agriculture, Food and Rural Affairs
  • Reynoutria japonica facts from North Carolina State University
  • Bohemian knotweed information from the Invasive Species Council of BC
  • Best management practices for knotweed species in the Metro Vancouver region (PDF document) from the Metro Vancouver Regional District
  • Knotweed species in King County from its official website

© 2022 Linda Crampton

Related Articles