Japanese knotweed [Polygonum
cuspidatum Siebold & Zucc.][POLCU][CDFA list: B] Photographs
Map of Distribution
Himalayan knotweed [Polygonum
polystachyum Meissner][Bayer code: none][CDFA list: B] Photographs
Map of Distribution
Sakhalin knotweed or Giant
knotweed [Polygonum sachalinense Maxim.][REYSA][CDFA list: B] Photographs
Map of Distribution
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.
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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).
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.
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.
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.
and SEEDS:Outer 3 sepals (inner 3 in Himalayan
knotweed) persistent, enclose and disperse with achene. Achenes +/- ovoid,
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
DISTRIBUTION:Uncommon in California. To 500 m (1600 ft), except where noted.
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
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.
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
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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 Polygonum cuspidatum 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 Polygonum cuspidatum. 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