PPDL Picture of the Week for
October 31, 2011

Japanese Knotweed

Glenn Nice, Weed Diagnostician, Department of Botany & Plant Pathology

Japanese knotweed (Polygonum cuspidatumSiebold & Zucc.) is one of those plants that, like kudzu, was not a very good idea.  It was first introduced as an ornamental plant and has since been planted on purpose as a groundcover or hedge.  The problem is, if you put it in, it is very difficult to get rid of.

Japanese knotweed is a colony forming perennial that can grow as tall as 10 feet.  At first glance many think it is bamboo, but it is in the same family as the smartweeds.  There are male and female plants.  I have only seen female plants in Indiana, but have heard accounts of possible male plants.  When both sexes are not present the plants can still propagate through vegetative means.  Colonies will expand and the movement of soil with rootstock can spread Japanese knotweed to other locations.  In England, it is considered a very problematic plant and is against the law to plant or grow (The Wildlife and Countryside Act 1981.)[1].

As property changes hands, previously planted Japanese knotweed can be a challenge for the new owners.  In one such extreme case in England, a couple have Japanese knotweed invading their house[2].  A colony off the property has moved under the house and is now pushing up through the house.  The plant is invading the house.  That is truly an extreme case of an invasive plant taking over a house.

1. Noxious Weeds.  Ian Willoughby.  1996.  Research Information Note 274.  Issued by the Research Division of the Forestry Commission. [www.forestry.gov.uk/pdf/rin274.pdf/$FILE/rin274.pdf]

2. “Couple are forced to demolish their £300K four-bed home after it was invaded by Japanese Knotweed.”  Valerie Elliott.  2011.  Mail Online News.  (Accessed October 24, 2011)  [http://www.dailymail.co.uk]

Click image to enlarge

Japanese knotweed coming through the walls of a house in England. 

Japanese knotweed

Japanese knotweed flowers

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service