Ecological Impacts
Knotweeds typically grow as dense stands that shade out any plants below. Shoots can grow as fast as 8 cm (3.1 in) per day in early spring (Locandro 1973 in Seiger 1997). This rapid growth allows the plants to quickly overtop competitors, depriving them of sunlight. Dense patches of knotweed can displace diverse riparian plant communities along streams and rivers (Reeder and Eick 2001). In Britain only a handful of early-flowering, shade-tolerant herbs are able to persist within dense Japanese knotweed stands (Beerling et al. 1994). The dead stalks and leaf litter are resistant to decay and build up below the patches, further inhibiting competition from other plants. These decay-resistant stems may affect nutrient availability for other plants also (Seiger 1997).
Dense stands of Japanese knotweed have been shown to reduce diversity of plant-feeding insects, though large numbers of nectar-feeding insects are attracted to the flowers (Beerling and Dawah 1993). Glands at the bases of the leaf petioles function as extra nectaries, also attracting insects (Beerling and Dawah 1993). Japanese knotweed is of little use to most wildlife (Shaw and Seiger 2002).
Japanese knotweed has sometimes been planted for erosion control. But despite their extensive rhizome system, knotweeds do a poor job of controlling erosion, and can even increase erosion by displacing grasses and other plants (Soll 2004). Increased streambank erosion can lead to siltation of spawning beds and reduced fish habitat. Dense stands along streambanks can also reduce stream flow and may represent a flood hazard (Beerling et al. 1994).
Economic costs of knotweeds have included repair of flood control structures and replacement of cracked paving and asphalt through which the plant has grown (Shaw and Seiger 2002). The rhizomes are capable of emerging from 1 m (3 ft) below ground, and have been observed breaking through up to 5 cm (2 in) of pavement (Locandro 1973 in Seiger 1991). Dense stands of knotweed also restrict access to streams and rivers, reducing recreational opportunities. Fishel (1999) even lists Japanese knotweed as a "weed of field crops and pastures" in Missouri. The costs of Japanese knotweed mitigation and control in the United Kingdom is estimated to be tens of millions of dollars per year (Shaw and Seiger 2002).
Once knotweeds become common on the landscape, control becomes expensive. The cost of herbicidal treatment in the United Kingdom, excluding any revegetation costs, is estimated at around $1.60 per square meter for a year of repeated spraying of glyphosate (Hathaway 1999 in Shaw and Seiger 2002). The cost of funding an effective nationwide control effort against knotweed in Britain has been estimated at £1.56 billion (or just over $900 million) (CABI 2005).
As the frost-free season in the northern portion of North America continues to lengthen due to global warming, seedling survival and vigor of established plants can be expected to increase (Forman and Kesseli 2003). Any warming trend in Britain is predicted to increase the range of Japanese knotweed there (Beerling and Woodward 1994).
Status in Area
Japanese knotweed is officially listed as a noxious weed in Alabama, California, Oregon, Vermont, and Washington (Freeman and Hinds 2005). Giant knotweed is listed as noxious in California, Oregon, and Washington. Japanese knotweed is widely established in the upper Great Lakes region, and is considered invasive in Minnesota, Wisconsin, and Michigan. The often overlooked Bohemian knotweed is probably also widespread across the region. Giant knotweed has been recorded sporadically from Michigan and Wisconsin, including in northeastern Bayfield County, Wisconsin (Voss 1985, WIS 2006).