CDFA Plant Health

ENCLYCLOWEEDIA: Noxious Weed Ratings

Bohemian knotweed Fallopia x bohemica (Chrtek & Chrtková) J. P. Bailey (F. sachalinensis x F. japonica) [CDFA Rating: A ]

Japanese knotweed [Fallopia japonica (Houtt.) Ronse Decraene][POLCU] [CDFA Rating: A] Photographs Map of Distribution

Himalayan knotweed [Persicaria wallichii Greuter & Burdet][Bayer code: none][CDFA list: B] Photographs Map of Distribution

Sakhalin knotweed or Giant knotweed [Fallopia sachalinensis (F. Schmidt ex Maxim.) Ronse Decraene][REYSA] [CDFA Rating: A] Photographs Map of Distribution

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  • Japanese knotweed: Japanese bamboo, Mexican bamboo, Japanese fleeceflower, Polygonum cuspidatum Siebold & Zucc., Reynoutria japonica Houtt., Polygonum zuccarinii Small, Pleuropterus cuspidatus (Sieb. & Zucc.) Moldenke, Pleuropterus zuccarinii (Small) Small
  • Himalayan knotweed: bell-shaped knotweed, Polygonum polystachyum Meissner, Persicaria polystachya (Wall. ex Meissn.) Gross, Aconogonum polystachyum (Wallich ex Meisn.) Haraldson
  • Sakhalin knotweed: giant knotweed, sacaline, Polygonum sachalinense Maxim., Reynoutria sachalinensis (Houtt.) Friedr. S. Petrop.

GENERAL DESCRIPTION:Noxious clumping perennials with large leaves, hollow stems, and long creeping rhizomes. Plants grow vigorously and create dense colonies that exclude other vegetation. Established colonies are extremely difficult to eradicate. All three were introduced as garden ornamentals, but have widely escaped cultivation and become invasive in moist, disturbed places.

  • Japanese knotweed: To 3 m tall. Colonies are especially problematic in the Eastern U.S. Young shoots, stems, and rhizomes are edible. Plants are used medicinally in China and Japan. Introduced from the mountainous regions of Japan, Taiwan, Korea, and eastern China.
  • Himalayan knotweed: To 2 m tall. Leaves are consumed as a pot herb in its native range. Introduced from the Himalayan region of south central Asia.
  • Sakhalin knotweed: To 3(4) m tall. Closely related to and can hybridize with Japanese knotweed. Plants appear to have allelopathic properties. Introduced from mountains of Japan and Sakhalin Islands.

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SEEDLINGS:Rarely encountered. Japanese knotweed: Cotyledons narrowly elliptic-lanceolate, ~ 10 mm long, on stalks 2-4 mm long. Stalk bases fused and sheathing. Subsequent leaves alternate, ovate, ~ 1-1.5 cm long, with fused, membranous, sheathing stipules (ocrea).

MATURE PLANT:Main stems erect, often arched near top, simple to minimally branched, grooved, thick, hollow, weakly woody, swollen at nodes, usually reddish-brown at maturity. Twigs often zigzag slightly from node to node. Leaves alternate, leathery, on stalks ~ 2-3 cm long. Tips acute to acuminate. Stems and leaves +/- glabrous except where noted. Ocrea (specialized stipules) fused, membranous, sheathing stem above each node, usually fringed at the top.

  • Japanese knotweed: Leaves broadly ovate, ~ 10-15 cm long, sometimes wider than long. Bases truncate to tapered. Ocrea deciduous, short (~ 4-6 mm long), glabrous, sometimes reddish.
  • Himalayan knotweed: Stems sometimes pubescent, especially new growth. Leaves lanceolate, 10-20 cm long, often with short hairs on veins, margins, and lower surfaces. Bases slightly heart-shaped to tapered. Tips tapered, acute to acuminate. Ocrea persistent, long (> 6 mm), obliquely angled at the top, hairy near the base, usually not reddish.
  • Sakhalin knotweed: Leaves broadly lanceolate, 15-35 cm long. Bases heart-shaped. Ocrea persistent, long (> 6 mm), glabrous, often reddish.

ROOTS and UNDERGROUND STRUCTURES:Rhizomes thick, extensive, store large quantities of carbohydrates, and spread aggressively. Fragments can produce new plants.Japanese knotweed: Rhizomes often 5-6 m long, but documented to 20 m long. Rhizomes can penetrate 2 inches of asphalt. Rhizomes buried to soil depths of 1 m can regenerate, but fragments regenerate best from just below the soil surface.

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FLOWERS:August-October. Panicles branched, open, lax, with numerous flowers. Sepals 5, petal-like, +/- fused at the base. Petals lacking. Plants with unisexual flowers have male flowers with vestigial ovaries and female flowers with infertile stamens (staminodes). Insect-pollinated.

  • Japanese knotweed: Dioecious (unisexual flowers, male and female flowers on separate plants). Panicles axillary on upper stems, to 15 cm long (longer in fruit), with small cup-like bracts at each node. Sepals of male and female flowers +/- white, 2-8 mm long, outer 3 keeled (narrowly winged in fruit).
  • Himalayan knotweed: Flowers bisexual, fragrant. Panicles terminal and on axillary branch tips, typically 20-35 cm long. Sepals white to pinkish, 5-7 mm long, outer 2 narrower than inner 3, not keeled or winged in flower or fruit.
  • Sakhalin knotweed: Dioecious or monoecious (unisexual flowers, male and female flowers on same plant). Panicles axillary on upper stems, ~ 10 cm long. Sepals of male flowers greenish-white, 6-7 mm long. Sepals of female flowers +/- white, 12-15 mm long, outer 3 broadly winged.

FRUITS and SEEDS:Outer 3 sepals (inner 3 in Himalayan knotweed) persistent, enclose and disperse with achene. Achenes +/- ovoid, 3-sided.

  • Japanese knotweed: Fruits with sepals 2-8 mm long. Sepals narrowly winged. Achenes shiny (dark) brown, ~ 2-4 mm long.
  • Himalayan knotweed: Fruits with sepals 5-7 mm long. Sepals lack wings. Achenes pale, ~ 5-7 mm long, typically do not develop in California.
  • Sakhalin knotweed: Fruits with sepals 12-15 mm long. Sepals broadly winged. Achenes shiny (dark) brown, narrow, ~ 2-4 mm long.

POSTSENESCENCE CHARACTERISTICS:Above ground parts die during the cold season, but the red-brown, often arched, grooved, hollow stems with zigzag twigs can persist through winter. Sometimes a few fruits cling to twigs.

HABITAT:Disturbed moist sites, roadsides, riparian and wetland areas. Plants typically grow in open, sunny areas on moist soils in cool temperate climates.

  • Japanese knotweed: Grows on a wide range of soil types. Tolerates some soil dryness. Colonizes bare volcanic soils, including those high in sulfur (pH < 4), in its native range.
  • Himalayan knotweed: Grows in wet meadows and marshes of forested areas 2500-4000 m (8200-13000 ft) in its native range, but can be found to sea level in California.

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DISTRIBUTION:Uncommon in California. To 500 m (1600 ft), except where noted.

  • Japanese knotweed: Northern California, especially southern North Coast Ranges and San Francisco Bay region, excluding Modoc Plateau; to Alaska; Eastern U.S.; Eastern Canada. To 1000 m (3300 ft).
  • Himalayan knotweed: North and Central Coast; to Canada; Massachusetts.
  • Sakhalin knotweed: Northern California, except Modoc Plateau; to Canada, Idaho; Eastern U.S.

PROPAGATION/PHENOLOGY:Reproduces vegetatively from rhizomes and by seed. Rhizome fragments disperse with water currents or flooding and with natural or human facilitated soil movement. Fruits (achenes enclosed by sepals) disperse primarily with wind.

  • Japanese knotweed: In the U.S. and U.K., reproduction by seed appears less important than vegetative reproduction by rhizomes. Plants produce abundant seed, but a large proportion is non-viable when fertile male plants are rare. Seeds develop ~ 2 weeks after flowering. Seedling survival is usually low, but plants are persistent once established. Seedlings rarely establish under shady conditions. Growth of plants shaded after establishment is greatly reduced. Plants tolerate high levels of sulfur dioxide.
  • Himalayan knotweed: No information available.
  • Sakhalin knotweed: Under experimental conditions, plants do not adjust photosynthetic rates to low light levels and appear unable to adapt to shaded conditions.

MANAGEMENT FAVORING/DISCOURAGING SURVIVAL:Carefully digging out small clumps when discovered can prevent new colonies from establishing. However, rhizomes and fragments left in the ground or nearby can regenerate and spread infestations. Repeated cutting of stems (4 or more times per season), especially in conjunction with shading by black plastic or heavy shade cloth, depletes rhizome energy reserves and can help control infestations.

SIMILAR SPECIES:Unlike the knotweeds, ladysthumb, marshpepper smartweed, pale smartweed, swamp smartweed, and Pennsylvania smartweed are typically smaller (to ~ 2 m tall) and have non-woody stems and flowers in spike-like racemes. Refer to the table Comparison of Polygonum species for more information.

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Prevention: These Polygonum species are aggressive perennials with tough creeping rhizomes. Japanese, Himalayan, and Sakhalin knotweeds were originally introduced as garden ornamentals from south central Asia or Japan. They have subsequently escaped and invade open wetlands, irrigation ditches and riparian areas. They form dense colonies and strongly compete with native plants for resources. Once established, these species are extremely difficult to eradicate.
Swamp smartweed is native to California and occurs along waterways, riparian areas, and wetlands. It is highly variable with aquatic and terrestrial forms that may change with changing environmental conditions. Although a native plant, it can become a serious problem in irrigated fields and rice growing areas, and may clog irrigation ditches and canals. In natural areas, swamp smartweed is not considered a problem.
The areas these plants invade often make prevention a difficult task. Seedlings are rarely seen and are not believed to be a serious method of invasion. However, rhizome fragments may break off, spread by water, and be deposited in new areas downstream. The key to prevention is rapid detection and removal of new clumps. Infested areas along waterways are very likely to be sources for new infestations downstream.

Mechanical: Grubbing is effective for small populations. The entire root system must be removed, since resprouting can occur from long rhizomes. A pulaski is useful for digging out mature clumps, while hand pulling works well for small plants in moist areas. The plant material should be removed, dried and burned if possible. Mowing or cutting plant shoots is ineffective alone. However, mowing followed by herbicide treatments will provide some control.

Biological: There are currently no registered biological control agents for use on any of these Polygonum species. Grazing may be an effective strategy to prevent establishment. It has been observed that Fallopia japonica will not establish where grazing pressure is high. However, heavy grazing may also select for other undesirable weedy species. Any grazing strategy should be carefully controlled to prevent damage in riparian areas.

Chemical: Glyphosate (2% v/v) and triclopyr (2% v/v) may be applied as a foliar treatment to young, actively growing shoots. Imazapyr (1.0-1.5 lb ae/A) will provide effective control of Fallopia japonica. Dicamba (0.25 lb ae applied in 1 gallon of water per 400 ft2) is also effective for spot treatments. Glyphosate is the only effective treatment registered for aquatic use. However, triclopyr and imazapyr may be registered for aquatic weed control in the near future. The best herbicide strategy is an integrated strategy with mowing or cutting. There are two basic methods: 1) Cut stalks to a two inch height and immediately apply a 25% solution of glyphosate or triclopyr to the cross section of the stems. 2) Cut or mow infestations when the plants reach the early bud stage in the late spring or summer and treat the regrowth in the fall with glyphosate or triclopyr. Glyphosate may also be selectively applied with a rope wick applicator when desirable vegetation is around infestations. These herbicides may be injurious to other plants and should be used carefully in sensitive areas.

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Mitchell, R. S. 1968. Variation in the Polygonum amphibium complex and its taxonomic significance. Berkeley: University of California Press.
Patterson, D. T., Longstreth, D. J., and Peet, M. M. 1977. Photosynthetic adaptation to light intensity in Sakhalin knotweed (Polygonum sachalinense). Weed Sci. 25:319-323.
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