Fallopia japonica (Houtt.) R. Decr.

Japanese knotweed

New England Distribution

Adapted from BONAP data

Where native and non-native distributions co-occur in a county, only the native distribution is shown.

North America Distribution

Adapted from BONAP data

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Facts About

One of the most invasive weeds in the world, Japanese knotweed is native to Asia, where it is regarded as having medicinal value. It was introduced to North America in the 1870s as an ornamental and forage plant. It is difficult to exaggerate how aggressive this species can be: it has been observed growing through two inches of concrete, and it will regenerate from as little as 5g of stem or root tissue. Control of Japanese knotweed is laborious and expensive.

Habitat

Anthropogenic (man-made or disturbed habitats), floodplain (river or stream floodplains), forest edges, meadows and fields, shores of rivers or lakes

Characteristics

Habitat
terrestrial
New England state
  • Connecticut
  • Maine
  • Massachusetts
  • New Hampshire
  • Rhode Island
  • Vermont
Flower petal color
  • green to brown
  • pink to red
  • white
Leaf type
the leaves are simple (lobed or unlobed but not separated into leaflets)
Leaf arrangement
alternate: there is one leaf per node along the stem
Leaf blade edges
the edge of the leaf blade is entire (has no teeth or lobes)
Flower symmetry
there are two or more ways to evenly divide the flower (the flower is radially symmetrical)
Number of sepals, petals or tepals
there are five petals, sepals or tepals in the flower
Fusion of petals
the petals are fused into a corolla tube
Stamen number
8
Fruit type (general)
the fruit is dry but does not split open when ripe
Fruit length
2.3–3.6 mm
Show All Characteristics
  • Clonal plantlets
    Bulbils
    the plant does not appear to have bulbils
    Bulblets replace flowers
    there are no bulblets where the flowers are located
  • Flowers
    Anther color
    the anthers show no hint of a pink, reddish or purplish tint
    Anther opening
    the anthers have narrow slits or furrows that run lengthwise along the anthers
    Anther spurs
    the anthers do not have spurs on them
    Calyx growth after flowering
    the calyx grows to cover or partially cover the fruit
    Calyx symmetry
    there are two or more ways to evenly divide the calyx (the calyx is radially symmetrical)
    Carpels fused
    the carpels are fused to one another
    Corolla palate
    no
    Corona lobe length
    0 mm
    Epicalyx
    the flower does not have an epicalyx
    Epicalyx number of parts
    0
    Filament surface
    the filament is smooth, with no hairs or scales
    Flower petal color
    • green to brown
    • pink to red
    • white
    Flower reproductive parts
    • the flower has both pollen- and seed-producing parts
    • the flower has either only pollen- or only seed-producing parts
    Flower symmetry
    there are two or more ways to evenly divide the flower (the flower is radially symmetrical)
    Flowers sunken into stem
    no
    Fused stamen clusters
    NA
    Fusion of petals
    the petals are fused into a corolla tube
    Hairs on flower stalk
    the flower stalk has no hairs on it
    Hairs on inflorescence
    the axis of the inflorescence has hairs entirely without glands
    Horns in hoods (Asclepias)
    NA
    Hypanthium
    • the flower does not have a hypanthium
    • the flower has a hypanthium
    Inflorescence length
    40–120 mm
    Inflorescence one-sided
    the flowers are arrayed in a spiral around the inflorescence axis or branches, or occur singly, or in several ranks
    Inner tepals (Rumex)
    NA
    Length of flower stalk
    3–5 mm
    Length of peduncle
    0–2.5 mm
    Marks on petals
    there are no noticeable marks on the petals
    Nectar spur
    the flower has no nectar spurs
    Number of branches in umbel
    0
    Number of carpels
    3
    Number of pistils
    1
    Number of sepals, petals or tepals
    there are five petals, sepals or tepals in the flower
    Number of styles
    3
    Ovary position
    the ovary is above the point of petal and/or sepal attachment
    Perianth shape
    the perianth is campanulate (bell-shaped, with a tube about as long as wide, flaring at the mouth)
    Petal and sepal arrangement
    the flower includes two cycles of petal- or sepal-like structures
    Petal appearance
    the petals are thin and delicate, and pigmented (colored other than green or brown)
    Petal folds or pleats
    the petals of the flower do not have folds or plaits
    Petal hairs (Viola)
    NA
    Petal hairs on inner/upper surface
    there are no hairs on the inner/upper petal surface
    Petal length
    4–6 mm
    Petal nectaries
    the petals do not have nectaries
    Petal number
    2
    Petal shape
    • the petal outline is elliptic (shaped like an ellipse; widest near the middle and tapering at both ends)
    • the petal outline is obovate (roughly egg-shaped, but with the widest point above the middle of the leaf blade)
    Petal tip shape
    • the petal tip is acute (sharply pointed)
    • the petal tip is obtuse (bluntly pointed)
    Petal tips (Cuscuta)
    NA
    Raceme attachment (Veronica)
    NA
    Reproductive system
    • all the flowers have both carpels and stamens (synoecious)
    • all the flowers on each plant have only carpels or only stamens, with only one type being present on each plant (dioecious)
    Sepal and petal color
    the petals and sepals are similar
    Sepal appearance
    the sepals resemble petals in color and texture
    Sepal appendages (Oenothera)
    NA
    Sepal color
    • green to brown
    • pink to red
    • white
    Sepal features
    one or more sepals have a prominent keel (a ridge that runs along the midline from base to tip)
    Sepal length
    4–6 mm
    Sepal number
    3
    Sepal shape
    • the sepal outline is eliiptic (widest near the middle and tapering at both ends)
    • the sepal outline is obovate (egg-shaped, but with the widest point above the middle of the leaf blade)
    Sepal texture
    the sepals are either very thin but flexible, like a membrane, or they are leaf-like in texture
    Sepal tip shape
    • the sepal tip is acute (is sharply pointed)
    • the sepal tip is obtuse (is bluntly pointed)
    Sepal uniformity
    all the sepals are about the same size
    Sepals fused only to sepals
    the sepals are fused to each other (not other flower parts), at least near their bases
    Spur length
    0 mm
    Spur number
    NA
    Stamen attachment
    the stamens are not attached to the petals or tepals
    Stamen morphology
    the stamens within a cycle differ in length or width
    Stamen number
    8
    Stamen relative length
    anything
    Stamens fused
    the stamens are not attached to one another
    Staminodes
    there are no staminodes on the flower
    Style petal-like
    the styles are not petal-like
    Umbel flower reproductive parts
    NA
    Upper lip of bilabiate corolla
    NA
  • Fruits or seeds
    Achene relative orientation
    the achenes are perpendicular to the plane of the perianth (vertical)
    Achene shape
    • the achenes are another shape in outline
    • the achenes are oval in outline
    Achene surface (Polygonum)
    NA
    Achene type
    the fruit is an achene (dry, indehiscent, and usually one-seeded)
    Berry color
    NA
    Capsule color (Viola)
    NA
    Capsule ribs
    NA
    Capsule splitting
    NA
    Carpel beak length
    0 mm
    Fruit (pyxis) dehiscence
    NA
    Fruit cross-section
    the fruit is roughly triangular in cross-section
    Fruit features (Brassicaceae)
    NA
    Fruit length
    2.3–3.6 mm
    Fruit length relative to sepals
    the fruit is shorter than its associated sepals
    Fruit locules
    one
    Fruit shape
    • the fruit is ellipsoid (widest in the middle and tapering to each end)
    • the fruit is ovoid (egg-shaped)
    Fruit stalk orientation
    the fruits point upward or spread or curve outward
    Fruit type (general)
    the fruit is dry but does not split open when ripe
    Fruit type (specific)
    the fruit is an achene (dry, usually one-seeded, does not separate or split open at maturity)
    Fruit width
    1.4–1.9 mm
    Hair type on fruit
    NA
    Hairs on fruit
    the fruits are not hairy
    Legumes (Fabaceae)
    NA
    Mericarp length
    0 mm
    Mericarp segment shape (Desmodium)
    NA
    Other markings on berry
    NA
    Placenta arrangement
    the plant has basal placentation, where one or a few ovules develop at the base of a simple or compound ovary
    Rows of seeds in fruit (Brassicaceae)
    NA
    Schizocarpic fruit compression
    NA
    Schizocarpic fruit segments
    0
    Seed number
    1
    Septum in fruit (Brassicaceae)
    NA
    Wings on fruit
    the fruit does not have wings on it
    prickles on fruits
    the fruits do not have thorn-like defensive structures
  • Glands or sap
    Glands on leaf blade
    the leaf blades do not have glandular dots or scales
    Sap
    the sap is clear and watery
    Sap color
    the sap is clear
  • Growth form
    Growth form
    the plant is an herb (it has self-supporting stems)
    Lifespan
    the plant lives more than two years
    Parasitism
    the plant is not parasitic
    Plant color
    the leaves or young stems of the plant are green
    Plants darken when dry
    no
    Spines on plant
    the plant has no spines
    Underground organs
    the plant has a rhizome (a horizontal underground stem with roots growing from it)
  • Leaves
    Bracteole number (Apiaceae)
    0
    Bracts in plantain (Plantago)
    NA
    Final leaf segment length (compound lvs only)
    0 mm
    Final leaf segment length to width ratio (compound lvs only)
    0
    Final leaf segment width (compound lvs only)
    0 mm
    Floral bracts
    the flower does not have bracts
    Hairs on leaf stalk
    the petiole has no hairs on it
    Hairs on underside of leaf
    the underside of the leaf is fuzzy or hairy
    Hairs on upper side of leaf
    the upper side of the leaf is not hairy, or it has very few hairs
    Hooked hairs on underside of leaf
    no
    Inflated hairs on leaf
    the leaf blade does not have inflated hairs on it
    Leaf arrangement
    alternate: there is one leaf per node along the stem
    Leaf blade base
    the leaf has a distinct leaf stalk (petiole)
    Leaf blade base shape
    • the base of the leaf blade is cuneate (wedge-shaped, tapers to the base with relatively straight, converging edges), or narrow
    • the base of the leaf blade is truncate (ends abruptly in a more or less straight line as though cut off)
    Leaf blade base symmetry
    the leaf blade base is symmetrical
    Leaf blade bloom
    there is a noticeable powdery or waxy bloom on the underside of the leaf
    Leaf blade edges
    the edge of the leaf blade is entire (has no teeth or lobes)
    Leaf blade flatness
    the leaf is flat (planar) at the edges
    Leaf blade length
    50–150 mm
    Leaf blade primary vein pattern
    the secondary veins branch off at intervals from the primary vein
    Leaf blade shape
    the leaf blade is ovate (widest below the middle and broadly tapering at both ends)
    Leaf blade surface colors
    the upper side of the leaf blade is relatively uniform in color
    Leaf blade texture
    the leaf blade is herbaceous (has a leafy texture)
    Leaf blade vein pattern
    the major veins of the leaf blade branch, but do not rejoin
    Leaf blade veins
    the leaf blade has one main vein running from the base towards the tip (it may or may not have secondary veins)
    Leaf blade width
    20–100 mm
    Leaf duration
    the leaves drop off in winter (or they whither but persist on the plant)
    Leaf form
    the leaves are green, with an expanded blade and a leaf-like texture
    Leaf shiny
    the upper side of the leaf is dull or slightly shiny
    Leaf spines
    there are no spines on the leaf edges
    Leaf stalk
    the leaves have leaf stalks
    Leaf stalk attachment to leaf
    the petiole attaches at the basal margin of the leaf blade
    Leaf stalk base
    • the petiole base clasps the stem or sheathes the stem
    • the petiole base is narrow where it attaches to the stem
    Leaf stalk length
    10–30 mm
    Leaf teeth and lobes
    the edge of the leaf blade is entire (has no teeth or lobes)
    Leaf tip
    • the tip of the leaf blade is acuminate (tapers to a long, thin point)
    • the tip of the leaf blade is acute (sharply pointed)
    Leaf type
    the leaves are simple (lobed or unlobed but not separated into leaflets)
    Leaf types
    NA
    Leaf variation
    the leaves are nearly similar in size, prominence of teeth, and length of stalks throughout the stem
    Leaflet number
    0
    Leaflet petiolules
    NA
    Leaves per node
    there is one leaf per node along the stem
    Pinnately compound leaf type
    NA
    Specific leaf type
    the leaves are simple (lobed or unlobed but not separated into leaflets)
    Stipels
    NA
    Stipule features
    • glabrous on the surface
    • the stipules are hairy with hairs standing out at a 45 to 90 degree angle, or pressed to the stipule surface
    • the stipules are straight (or somewhat slanted) across the top
    • the stipules do not have tiny hairs (cilia) at their tips
    Stipule length
    4–10 mm
    Stipule shape
    the stipules are tubular (cylindrical and hollow)
    Stipules
    the plant has stipules
    Teeth per side of leaf blade
    0
  • Place
    Habitat
    terrestrial
    New England state
    • Connecticut
    • Maine
    • Massachusetts
    • New Hampshire
    • Rhode Island
    • Vermont
    Specific habitat
    • edges of forests
    • man-made or disturbed habitats
    • meadows or fields
    • river or stream floodplains
    • shores of rivers or lakes
  • Scent
    Plant odor
    the plant does not have much of a smell
  • Stem, shoot, branch
    Branched tendrils
    NA
    Direction of stem hairs
    NA
    Flowering stem cross-section
    the flowering stem is circular, or with lots of small angles so that it is roughly circular
    Hair between stem nodes
    the stem has no hairs between the nodes
    Hairs between stem nodes
    the stem has no hairs between the nodes
    Hooked hairs on stem between nodes
    no
    Leaves on stem
    there is at least one full leaf above the base of the flowering stem
    Length of hairs between stem nodes
    0 mm
    Plant height
    150–300 cm
    Stem bloom
    there is no powdery or waxy film on the stem
    Stem hair distribution
    NA
    Stem nodes swollen
    the stem is swollen at the nodes
    Stem orientation
    the stems are upright or angled outwards
    Stem roughness between nodes
    the stem does not feel rough
    Stem spacing
    the plant is growing in tufts, or compact clusters with closely spaced stems, or it is densely matted together in clumps, cushionlike
    Stem succulence
    the stems are not succulent
    Tendril origin
    NA
    Tendrils
    the plant does not have tendrils
    Wings on stem
    the stem does not have wings on it

Wetland Status

Occurs most often in non-wetlands, but rarely in wetlands. (Wetland indicator code: FACU-)

New England Distribution and Conservation Status

Distribution

Connecticut
present, invasive, prohibited
Maine
present, invasive, prohibited
Massachusetts
present, invasive, prohibited
New Hampshire
present, invasive, prohibited
Rhode Island
present, invasive
Vermont
present, invasive, prohibited

Conservation Status

Exact status definitions can vary from state to state. For details, please check with your state.

Connecticut
not applicable (S-rank: SNA)
Massachusetts
unranked (S-rank: SNR)

Native to North America?

No

Sometimes Confused With

Fallopia sachalinensis:
leaf blades with sparse hairs on the lower surface, 7-20 cm wideleaf blades without hairs on the lower surface, 2-10 cm wide (vs. F. japonica, with leaf blades without hairs on the lower surface, 2-10 cm wide).

Synonyms

  • Pleuropterus cuspidatus (Sieb. & Zucc.) Moldenke
  • Polygonum cuspidatum Sieb. & Zucc.
  • Reynoutria japonica Houtt.

Family

Polygonaceae

Genus

Fallopia

Notes on Subspecies and Varieties in New England

Our variety is Fallopia japonica (Houtt.) R. Decr. var. japonica.

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Information from Dichotomous Key of Flora Novae Angliae

6.  Fallopia japonica (Houtt.) R. Decr. var. japonica E

Japanese knotweed. Pleuropterus cuspidatus (Sieb. & Zucc.) Moldenke; Polygonum cuspidatum Sieb. & Zucc.; Reynoutria japonica Houtt. • CT, MA, ME, NH, RI, VT. Fields, roadsides, waste areas, forest edges, river shores and banks.

6×7. Fallopia japonica × Fallopia sachalinensis Fallopia ×‌bohemica (Chrtek & Chrtková) J.P. Bailey is an uncommon knotweed hybrid known from CT, MA, ME, RI, VT. It is becoming increasingly more frequent on the landscape. Its leaf morphology is intermediate between F. japonica and F. sachalinensis (i.e., they tend to be large and are truncate to cordate at the base). The best discriminating character is the minute, but firm, stoutly conical, 1- or 2-celled hairs found on the abaxial leaf surface (this type of hair not found in the parental taxa; view at 20 × or higher) 
[Fig. 788, M].