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Native Plant Trust: Go Botany Discover thousands of New England plants

Polygonum buxiforme — prairie knotweed

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Facts

Prairie knotweed is a native, mat-forming perennial that inhabits shorelines, beaches, salt marshes and brackish marshes on or near the Atlantic coast. Its species name, "buxiforme" means "box-like" and may refer to the flowers, with are baggy and squarish; plants bloom from July to November. The leaves are quite blue- or gray-green; this coloration may help them stay cool in the hot, exposed conditions in which they grow. This hardy species is widespread in North America, but listed as endangered in New York.

Habitat

Coastal beaches (sea beaches), marshes

New England distribution

Adapted from BONAP data

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North America distribution

Adapted from BONAP data

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Characteristics

Habitat
wetlands
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 (i.e., 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 sepals and petals
the petals or the sepals are fused into a cup or tube
Stamen number
  • 7
  • 8
Fruit type (general)
the fruit is dry but does not split open when ripe
Fruit length
1.8–3 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 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 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 description
    the flower has a superior ovary, and lacks a hypanthium
    Flower petal color
    • green to brown
    • pink to red
    • white
    Flower reproductive parts
    the flower has both pollen- and 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
    Form of style
    the flower has two or more completely separate styles
    Fused stamen clusters
    NA
    Fusion of sepals and petals
    the petals or the sepals are fused into a cup or tube
    Horns in hoods (Asclepias)
    NA
    Hypanthium
    • the flower does not have a hypanthium
    • the flower has a hypanthium
    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
    Nectar spur
    the flower has no nectar spurs
    Number of pistils
    1
    Number of sepals, petals or tepals
    there are five petals, sepals, or tepals in the flower
    Number of styles
    2–3
    Ovary position
    the ovary is above the point of petal and/or sepal attachment
    Petal and sepal arrangement
    the flower includes two cycles of petal- or sepal-like structures
    Petal and sepal colors
    • green to brown
    • pink to red
    • white
    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
    • the petals of the flower have folds or plaits on them
    Petal hairs (Viola)
    NA
    Petal number
    5
    Petal tips (Cuscuta)
    NA
    Reproductive system
    all the flowers have both carpels and stamens (synoecious)
    Scales inside corolla
    no
    Sepal and petal color
    the petals and sepals are similar
    Sepal appearance
    the sepals resemble petals in color and texture
    Sepal appendages
    the sepals do not have appendages on them
    Sepal appendages (Oenothera)
    NA
    Sepal color
    • green to brown
    • pink to red
    • white
    Sepals fused only to sepals
    the sepals are fused to each other (not other flower parts), at least near their bases
    Stamen morphology
    the stamens within a cycle differ in length or width
    Stamen number
    • 7
    • 8
    Stamen position relative to petals
    NA
    Stamens fused
    the stamens are not attached to one another
    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
    Achene surface (Polygonum)
    • the achene has irregular bumps on it that makes it rough
    • the achene has tiny rounded bumps or projections on it as well as parallel lines or ridges (striate-papillose)
    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
    Fruit (pyxis) dehiscence
    NA
    Fruit features (Brassicaceae)
    NA
    Fruit length
    1.8–3 mm
    Fruit locules
    one
    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)
    Legumes (Fabaceae)
    NA
    Mericarp length
    0 mm
    Mericarp segment shape (Desmodium)
    NA
    Other markings on berry
    NA
    Ovary stipe
    the ovary or fruit does not have a stipe
    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
    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 only a single year or less
    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

  • 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
    Hairs on underside of leaf
    the underside of the leaf is not hairy, or it has very few hairs
    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
    Leaf blade edges
    the edge of the leaf blade is entire (has no teeth or lobes)
    Leaf blade length
    6–45 mm
    Leaf blade shape
    • the leaf blade is elliptic (widest near the middle and tapering at both ends)
    • the leaf blade is lanceolate (lance-shaped; widest below the middle and tapering at both ends)
    • the leaf blade is oblanceolate (lance-shaped, but with the widest point above the middle of the leaf blade)
    • the leaf blade is obovate (egg-shaped, but with the widest point above the middle of the leaf blade)
    Leaf blade surface colors
    the upper side of the leaf blade is relatively uniform in color
    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 spines
    there are no spines on the leaf edges
    Leaf stalk
    the leaves have leaf stalks
    Leaf teeth and lobes
    the edge of the leaf blade is entire (has no teeth or lobes)
    Leaf type
    the leaves are simple (i.e., lobed or unlobed but not separated into leaflets)
    Leaf variation
    the leaves are nearly similar in size, prominence of teeth, and length of stalks throughout the stem
    Leaflet number
    0
    Leaves per node
    there is one leaf per node along the stem
    Pinnately compound leaf type
    NA
    Specific leaf type
    the leaves are simple (i.e., lobed or unlobed but not separated into leaflets)
    Stipels
    NA
    Stipule features
    • glabrous on the surface
    • the stipules have two lobes at their tips
    Stipule fused to leaf stalk
    the stipules are fused to the petioles for some or most of their length
    Stipules
    the plant has stipules

  • Place

    Habitat
    wetlands
    New England state
    • Connecticut
    • Maine
    • Massachusetts
    • New Hampshire
    • Rhode Island
    • Vermont
    Specific habitat
    • marshes
    • sea beaches

  • Scent

    Plant odor
    the plant does not have much of a smell

  • Stem, shoot, branch

    Branched tendrils
    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
    Plant height
    5–15 cm
    Tendril origin
    NA
    Tendrils
    the plant does not have tendrils

Wetland status

Not classified

In New England

Distribution

Connecticut
present
Maine
present
Massachusetts
present
New Hampshire
present
Rhode Island
present
Vermont
present

Conservation status

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

Massachusetts
fairly widespread (S-rank: S4)

From Flora Novae Angliae dichotomous key

6.  Polygonum buxiforme Small N│E

prairie knotweed. Polygonum aviculare L. var. littorale (Link) Mert.; P. littorale Link • CT, MA, ME, NH, RI, VT. Atlantic coast beaches and shorelines, saline to brackish marshes, rarely inland on shorelines or edges of salted roads and lots. Costea and Tardiff (2003b) chose to ally Polygonum buxiforme to P. aviculare because they noted that the outer tepals of members of the P. aviculare complex are not always flat. Though that is correct, real differences in the tepal morphology still exist between P. aviculare and P. buxiforme (i.e., simply because the tepals of P. aviculare have been poorly described in the literature does not mean they are similar to P. buxiforme). Polygonum buxiforme has outer tepals that are saccate at the base and the perianth is 0.9–1.3 times as long as wide (vs. non-saccate tepals and perianth 1.5–2.9 times as long as wide in P. aviculare). Given that P. buxiforme also differs in distribution (it is native to North America), it is not here considered part of the largely Eurasian P. aviculare. Polygonum buxiforme has been attributed to VT based on two vegetative collections—Goshen, Dutton s.n. (VT!) and Burlington, Dutton s.n. (VT!). They likely represent collections from the P. aviculare complex but are equivocal and cannot stand as vouchers for P. buxiforme. However, a recent collection from Caledonia County and a historical collection from Addison County—27 Aug 1900, Brainerd s.n. (GH)—are this taxon and should likely be considered introduced.

Native to North America?

Yes

Sometimes confused with

Polygonum aviculare:
outer 3 tepals flat or folded, of approximately equal width and length to the inner tepals and not or scarcely concealing them, without a pouch-like protrusion at the base (vs. P. buxiforme, with the outer 3 tepals cucullate, much wider and often longer the inner tepals, partially or completely concealing them, usually with a pouch-like protrustion near the base).
Polygonum fowleri:
achenes uniformly granular-roughened, abruptly tapered to a slender beak, 2.8–4 mm long or up to 5 mm long in late-season collections, and tepals 3–5 mm long (vs. P. buxiforme, with achenes striate-papillose or irregularly roughened, gradually tapering to apex, 2–2.8 mm long, and tepals 2–3 mm long).

Synonyms

  • Polygonum aviculare L. var. littorale (Link) Mert.
  • Polygonum littorale Link

Family

Polygonaceae

Genus

Polygonum