Japanese knotweed

My usual postings on plants are about plants that are ornamental or useful or both, but occasionally I look at invasives: recently, on privets and tumbleweed, and a bit earlier on monstrously invasive vines —  kudzu and mile-a-minute. Today, thanks to a piece in the 7/5/14 New Scientist (“Let them eat weeds” by Stephanie Pain), I turn to a dreadful pest, the Japanese knotweed. The plant will push aggressively through concrete, survive volcanic eruptions, and more.

From Wikipedia on the species:

Fallopia japonica, commonly known as Japanese knotweed, is a large, herbaceous perennial plant of the family Polygonaceae, native to Eastern Asia in Japan, China and Korea. In North America and Europe the species is very successful and has been classified as an invasive species in several countries. Japanese knotweed has hollow stems with distinct raised nodes that give it the appearance of bamboo, though it is not closely related.

And on the genus:

Fallopia is a genus of about 12–15 species of flowering plants in the family Polygonaceae, often included in a wider treatment of the related genus Polygonum in the past. The genus is native to temperate and subtropical regions of the Northern Hemisphere. The genus includes herbaceous perennial plants, herbaceous vines, and woody vines.

Several species are serious invasive weeds, notably Japanese knotweed in Europe and North America …

The genus is named after Gabriello Fallopio, or Fallopius, who was the superintendent of the botanical garden at Padua. He was also an acclaimed anatomist, being considered a founder of modern anatomy along with Vesalius and Eustachius.

(Yes, the Fallopio or (Latinized) Fallopius for whom the Fallopian tubes are named.)

The polygonums (knotweeds) and their near relatives include some ornamental vines, a number of familiar minor-league weeds, and some spectacular invasives, like mile-a-minute and Japanese knotweed.

From New Scientist:

The knotweed invasion can be traced back to the 1840s when doctor and botanist Philipp von Siebold acquired a specimen of Fallopia japonica from Japan. By 1848, his horticultural firm in the Netherlands was supplying cuttings to buyers in Europe and in 1850, von Siebold sent a specimen to the UK’s Royal Botanic Gardens at Kew. It was an immediate hit, admired for its attractive leaves and sprays of creamy flowers – and for its vigour. Cuttings soon found their way to country estates and nurseries across the UK, and later to the US.

… The one thing the invading knotweed cannot do is produce seeds. Remarkably, every plant in Europe and North America is an offshoot of a single female plant, all clones of von Siebold’s original specimen. The secret of knotweed’s astonishing spread is that new plants can grow from the tiniest fragments of stem or rhizome, all too easily carried by floodwaters or by human activities such as the dumping of garden waste. What’s more, pieces of rhizome can remain dormant for years before sprouting.

The plant, looking very bamboo-like:


And in luxuriant bloom, providing some explanation of why people would have planted it intentionally:


(Many years ago, back in Ohio, I was the victim of an unscrupulouis nursery that sold Fallopia japonica as “easy-to-grow Japanese bamboo”. It took years of extravagant countermeasures to eliminate the thing.)

As the New Scientist piece reports, the latest control on the plant is biological: the psyllid  Aphalara itadori. (Psyllids — or “jumping plant lice” (ordinary “plant lice” are aphids, which creep rather than jump) — are small plant-feeding insects.)

Adult psyllids lay their eggs on knotweed, but it is the emerging nymphs that do the damage. “Once the nymphs start feeding, the plants won’t get any taller,” says [entomologist Dick] Shaw. The rhizomes of psyllid-infested plants start to shrink within a year.

The psyllids don’t kill Japanese knotweed, but they very much limit its scope. Insects to the ready!

2 Responses to “Japanese knotweed”

  1. Robert Coren Says:

    There’s knotweed around our driveway in Cambridge, and I used to have it at my old apartment in North Cambridge. It does indeed grow through asphalt. Haven’t seen any on the Gloucester lot, fortunately (we’ve got enough nasty invasives as it is, TYVM). One takeaway from that description is that remnants should not be put in compost piles.

  2. arnold zwicky Says:

    Now, a long and wonderful piece in the May 2015 Harper’s: “The Day of the Knotweed: Battling Britain’s most destructive invasive plant” by Sam Knight, here

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