Natural History

Geographical Origin and History

Japanese knotweed is native to eastern China, Japan, and parts of Korea and Taiwan (Beerling et al. 1994), though the variety of Japanese knotweed that has been introduced to the west (Fallopia japonica var. japonica) is native only to Japan (Shaw and Seiger 2002). More recently another variety of Japanese knotweed, dwarf knotweed (F. japonica var. compacta) has been introduced by the horticultural trade. Giant knotweed is native to northern Japan and Sakhalin Island (Ohwi 1965 in Conolly 1977), and so has a more northerly native distribution than Japanese knotweed. Bohemian knotweed, the hybrid between Japanese and giant knotweed, has been widely disseminated through cultivation, also spreading vegetatively to natural habitats (Zika and Jacobson 2003).

Japanese knotweed was introduced to Britain in the 1840s by Phillipe von Siebold, who imported it for sale at his commercial nursery in Leiden, The Netherlands (Beerling et al 1994). The plant was soon being sold for high prices to buyers in Britain and continental Europe. North America's plants were then introduced from Britain (Seiger 1997). Herbarium records show that Japanese knotweed was being grown in the US by 1877 (Forman and Kesseli 2003). Giant knotweed was being grown in Britain by the 1860s (Conolly 1977), and was introduced to the US in 1894 as an ornamental and forage plant (Bailey 1933 in Patterson et al. 1977).

In recent years the knotweeds have been recognized as major invasives in Europe, Canada, and particularly Britain. Japanese knotweed is considered Britain's worst weed, and the 1981 Wildlife and Countryside Act prohibits planting or spreading the species there (Mabey 1998 in Shaw and Seiger 2002). It is also increasingly being reported as invasive in New Zealand and Australia (JKA 1999). Japanese knotweed (and probably Bohemian knotweed) is becoming increasingly invasive in North America also, particularly in the northeastern US and in the Pacific Northwest. It is established in scattered sites in Michigan and Wisconsin (Voss 1985, WIS 2006).

Past (and present) confusion concerning the classification of Japanese knotweed and its close relatives is due in large part to Japanese knotweed's history of "discovery" by western botanists. Japanese knotweed was first described by Houttuyn in 1777, who named it Reynoutria japonica (Beerling et al 1994). It was rediscovered around 1840 by Siebold and Zuccarini, who named it Polygonum cuspidatum in 1846. Siebold and Zuccarini's error was not discovered until 1901. Recent taxonomic work (Bailey and Stace 1992) has supported Meissner’s 1856 classification of Japanese knotweed in the genus Fallopia.

Common names for these knotweeds include Japanese bamboo, Japanese fleece flower, Mexican bamboo, donkey rhubarb, Sally rhubarb, German sausage, and pea-shooter plant. In the Midwest they are often simply referred to as "bamboo". (True bamboos are a group of grasses native to east Asia.)

Characteristics

Japanese, giant, and Bohemian knotweeds are all vigorously rhizomatous perennials. Well-established plants develop a woody crown and deep central taproot, that can penetrate as deep as 2 m (6.6 ft) into the soil (Soll 2004). Stout rhizomes grow horizontally from these crowns, forming a network that may eventually extend as much as 15-20 m (49-66 ft) from the crown (Soll 2004).

The stems of these knotweeds are hollow, round, jointed, and glaucus (having a thin waxy coating), and resemble bamboo. They are typically green with pinkish or reddish streaks and spots and reddish nodes, though some stems (especially smaller, immature ones) may be reddish purple. Like all members of the Polygonaceae, the stems have a membranous sheath surrounding each join (node) and petiole (leaf stalk). The arching branches are usually simple or little-branched, with leaves alternately arranged along most of their length. Japanese knotweed typically reaches about 2.5 m (8.2 ft) tall, while giant knotweed may reach over 5 m (16.4 ft) tall. The hybrid is intermediate between these two in height. Knotweed plants begin growth in early spring, achieving full height by early summer (Seiger 1997).

An abundance of small creamy-white (Japanese knotweed) or greenish-white (giant knotweed) flowers are produced along the stalks, from the axils of the leaves. [The dwarf variety of Japanese knotweed has red stems and petioles, and pink flowers.] According to Seiger (1997), the flower clusters of male plants extend upward, with the tip of the cluster higher than the base, while those of the female plants droop, with the tip often lower than the base. The outer tepals (undifferentiated sepals and petals) of all these knotweeds are broadly winged at maturity. All of them typically flower during July and August in the upper Midwest.

Japanese, giant, and Bohemian knotweeds can most easily be distinguished from each other by their leaves (Zika and Jacobson 2003). The leaves along the middle portion of the stems are the most distinctive. Japanese knotweed has relatively thick leaf blades that are generally less than 18 cm (7 in) long and often nearly as wide, with a truncate (flat) base and acuminate (concavely tapering) tip. The leaf blades of giant knotweed are thinner and cordate (heart-shaped) at the base, with an acute or blunt (but convexly tapering) tip, and often reach more than 30 cm (12 in) long. The hybrid between these two species, Bohemian knotweed, is intermediate, with the leaves often approaching either of the parents in size and shape. (The leaf blades along the middle of the stem are the most distinctive.)

The hairs on the lower leaf surface (particularly the midvein) of these plants can greatly aid in identification (Zika and Jacobson 2003). Japanese knotweed has very short hairs, appearing as low, broad bumps on the lower leaf surface. Giant knotweed has narrow-based, multicellular hairs. The hybrid is intermediate, with broad-based, elongate bumps. These hairs can sometimes be distinguished by bending the leaf surface over and holding it against the light, though they are most easily viewed with a 10x-20x hand lense.

The shoots of all these knotweeds are sensitive to frost. The tough, woody stems are killed by the first hard frost, but often remain standing into the next growing season.

Similar Species

Once one becomes even casually familiar with these introduced knotweeds, it is hard to mistake them for anything else. The closest relatives of these knotweeds in North America tend to grow as vines. Japanese knotweed hybridizes with the introduced silver lace vine, Fallopia baldschuanica (Regel) Holub, though the offspring are sterile (Beerling et al. 1994). Other relatives include the introduced black bindweed, Fallopia convolvulus (L.) Á. Löve, and the native fringed black bindweed, Fallopia cilinodis (Michx.) Holub, both of which are rather delicate, herbaceous, sprawling or climbing vines.

Reproduction and Dispersal

Japanese, giant, and Bohemian knotweed are all perennial and vigorously rhizomatous, readily spreading vegetatively in suitable habitats. Seedlings (rare outside their native range) begin forming a rhizome their first year (Forman and Kesseli 2003). Established patches form a large, expanding network of course rhizomes that may extend 15-20 m from the originating plant. New shoots may arise anywhere along the length of the rhizomes.

New patches are usually established vegetatively, through transport of plant fragments to new sites. Rhizome fragments as short as 8 cm (3 in) can produce new plants (Beerling 1990a in Beerling et al. 1994). Stem pieces are also capable of rooting and starting new colonies (Beerling et al. 1994). Plant fragments are even capable of surviving for a significant period of time in seawater (Beerling et al. 1994).

Japanese knotweed is generally considered to be functionally dioecious, with some clones producing only male-fertile flowers, and others producing only female (male-sterile) flowers. The flowers of male plants have fully-developed stamens that produce fertile pollen, but rudimentary pistils, while those of female plants have fully developed pistils, but only rudimentary stamens that do not produce pollen. Both male and female flowers are visited by an abundance of bees and other insects, which carry the pollen from one clone to the next. Occasionally "male" plants of Japanese (or perhaps Bohemian) knotweed have been found to produce seed (Forman and Kesseli 2003). Unfertilized females produce empty seeds (Seiger 1997). Giant knotweed produces (depending on the source) perfect (Gleason and Cronquist 1991), functionally dioecious (JKA 1999), or "bisexual or pistillate" (Freeman and Hinds 2005) flowers. Most authors consider giant knotweed (Conolly 1977) and Bohemian knotweed (Zika and Jacobson 2003) to also be functionally dioecious, with individual clones producing either male or female-fertile flowers.

Even today there is much uncertainly and confusion regarding the sexual reproduction and extent of hybridization between knotweed taxa, especially outside their native ranges. Until recently, reproduction and dispersal in North America was believed to be due almost exclusively to vegetative spread. Two recent investigations have found evidence that Japanese knotweed (and/or Bohemian knotweed) can reproduce by seed, however. Forman and Kesseli (2003) found that populations throughout New England often have both male and female-fertile plants. Bram and McNair (2004) found that populations of Japanese knotweed (or Bohemian knotweed) in Pennsylvania produced large numbers of viable seeds. It should be noted, though, that while both these studies acknowledged the existence of the hybrid, neither clearly differentiated between Japanese knotweed and the hybrid in their field experiments. Bohemian knotweed has often been mistaken for Japanese knotweed in Britain (Bailey 1990 in Seiger 1991) and in North America as well.

To understand the possible knotweed hybrids that may arise, and the implications for their spread in North America, it helps to know just a bit about the genetics of these plants. This group of knotweeds has chromosome numbers in multiples of 11 (JNA 1999). The typical form of Japanese knotweed has 8 sets of chromosomes, so 2n = 88, with "n" (here 44 chromosomes, or 4 sets of chromosomes) being the normal number of chromosomes present in a male or female sex cell (the pollen grain or ovule, respectively). Giant knotweed and dwarf knotweed (a variety of Japanese knotweed) have 4 pairs of chromosomes (2n = 44). Normally each parent contributes half its chromosomes to its offspring. Thus giant and dwarf knotweed can freely interbreed, producing fertile Bohemian knotweed offspring (2n = 44). Hybrids between giant and typical Japanese knotweed produce the most common type of Bohemian knotweed, with 2n = 66. These Bohemian knotweed offspring are only partly fertile, producing aberrant pollen having between 30 and 66 chromosomes. But if Bohemian knotweed pollen with 66 chromosomes were to pollinate giant knotweed with its 44 chromosomes, the resulting "backcrossed" offspring would have 66 + 44/2 = 88 pairs of chromosomes. This offspring could then fill the role of the missing male Japanese knotweed plants, and freely cross with "typical", 2n = 88 female Japanese knotweed. At any rate, any large knotweed that is male-fertile and not giant knotweed is probably Bohemian knotweed (JNA 1999). The British-based Japanese Knotweed Alliance has more details as well as handy diagrams on knotweeds and their possible hybrids on their web page (see JNA 1999).

In Britain, Japanese knotweed commonly hybridizes with the central Asian silver lace vine as well (Beerling et al. 1994). Apparent hybrids with silver lace vine have also been grown from seed collected in Washington, DC (Seiger 1993 in Seiger 1997). These hybrids have reduced vigor but occasionally survive, at least in Britain (Shaw and Seiger 2002). Backcrossing of these hybrids with Japanese knotweed would produce male-fertile plants. Hybrids with the silver lace vine are apparently still rare in North America, perhaps because (aside from their reduced fitness) silver lace vine is still uncommon here.

Within their native range, both Japanese and giant knotweed reproduce by seed, with seeds being produced in the fall and sprouting in the spring (Maruta 1983). Even there, though, vegetative spread can be critical to a population's survival (Maruta 1983). On Mt. Fuji in Japan, Maruta (1983) found greatly varying rates of seedling survival at different altitudes. At 1400 m (0.87 mi) above sea level, where the growing season was 130 days and the winter low reach -14° C (6.8° F), 63% of naturally-establishing seedlings survived their first winter. At 2500 m (1.6 mi) (the upper limit for the knotweed population on the mountain), where the growing season was only 70 days and the low reached -19° C (-2.2° F), only 3% of first-year seedlings survived the winter. Maruta (1983) concluded that the longer growing season at 1400 m allowed seedlings to grow larger, and that larger seedlings were much more likely to survive the winter.

Knotweed plants are potentially capable of producing vast numbers of seeds. Estimates of the maximum number of seeds that can be produced per stem range from 127,000 (Bram and McNair 2004) to 191,892 (Bailey 1994 in Bram and McNair 2004), if all the flowers produce seed. Seeds rapidly gain viability from early to mid-fall (Bram and McNair 2004). Germination is inhibited in the dark (Seiger 1993 in Seiger 1997).

Regardless of whether knotweeds reproduce by seed in North America, the most common mode of spread is either through dumping of yard waste, deliberate planting, or dispersal of rhizomes, shoots (and perhaps seed) by flowing water. Beavers find the stalks large and woody enough to use for construction, and sometimes move live stalks to new sites. Once established, all these knotweeds are tough, tenacious competitors that spread relentlessly in suitable habitats.

That giant knotweed is less common than Japanese knotweed in North America mirrors its abundance and distribution in Britain (Conolly 1977). Interestingly, Conolly (1977) observes that in Britain, giant knotweed is usually less aggressive in gardens than Japanese knotweed, and therefore is weeded out and discarded less often, resulting in fewer patches establishing in the wild.

Habitat Preference and Tolerance

Japanese knotweed grows best in full sun and tolerates only moderate shade, with shaded plants attaining a smaller size (Beerling et al. 1994). Giant knotweed plants have little ability to adapt physiologically to shade (Patterson et al. 1977), and the same is presumably true for Japanese knotweed. Shoots of knotweeds are sensitive to hard frosts, which limits the plant's growing season (Beerling 1993). In Europe Japanese knotweed occurs in climates with at least 2505 degree-days (using a threshold temperature of 0° C) and absolute minimum winter lows above -30° C (-22° F) (Beerling 1993). It grows as far north as 63° north latitude along the west coast of Norway (Beerling 1993). Comparing biogeographic and climatic data to Japanese knotweed distribution in Europe, Beerling et al. (1994) estimate that Japanese knotweed needs a minimum of 120 frost-free days during the growing season to survive. In Britain it is more frequent in areas of high rainfall (Conolly 1977).

Japanese knotweed is extremely tolerant of soil type and pH. In the US it grows in loam, silt, and sand (Locandro 1973 in Seiger 1997). It does best in nutrient-rich soils, but is tolerant of low levels of nitrogen and other nutrients (Hirose and Tateno 1984). In Britain it has been found growing in pH levels ranging from 3.0 to 8.5 (Grime et al. 1988, Beerling et al. 1994). It tolerates soils with high sulfur dioxide or heavy metal concentrations, and is tolerant of atmospheric contamination by these substances as well (Beerling et al. 1994). In Japan it commonly inhabits hills and mountains, often in areas of high rainfall (Hirose and Tateno 1984), and typically colonizes new lava flows (Maruta 1983). It is most vigorous in moist to seasonally wet soils, and is sensitive to prolonged drought. Its ability to colonize sites with extreme soil conditions has led to its being planted on coal tailings piles (Reeder and Eick 2001).