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Polygonum amphibium L.

Water smartweed, longroot smartweed, water knotweed, amphibious bistort

KingdomPlantaePlants, but not fungi, lichens, or algae
SubkingdomTracheobiontaVascular plants—plants with a “circulatory system” for delivering water and nutrients
DivisionMagnoliophytaFlowering plants, also known as angiosperms
ClassMagnoliopsidaDicotyledons—plants with two initial seed leaves
SubclassCaryophyllidaeCacti, many other succulents, carnivorous plants, and leadworts
OrderCaryophyllalesIncludes cacti, carnations, amaranths, ice plants, and many carnivorous plants
FamilyPolygonaceaeKnotweed or smartweed family
GenusPolygonumGreek poly, “many,” and gonu, “knee,” for the swollen jointed stems
SpeciesamphibiumAmphibious, suited for or adapted to growing on land or in the water

About plant names...

Water smartweed is native to much of North America, as well as Asia, Europe, and parts of Africa. Introduced elsewhere as well, it is sometimes considered a noxious weed. It is found floating in ponds, streams, and marshes. I first spied it while tubing down the Delaware river.

Plants: Water smartweed roots extend thick stems which float or grow horizontally until they find a moist area, into which they extend new roots. Stems reach up to 9½' (3 m) long and ⅜" (1 cm) around.

Leaves: Leaves are alternate, and often lanceolate, but they take on many other shapes. They may be floating or upright, with papery sheaths at leaf bases. They are ¾-8" (2-20 cm) × ⅛-1¾" (5-50 mm), with petioles ¹/₃₂-1" (1-30 mm). Leaves are not mottled. Ocrea are ⅛-1¾" (5-50 mm), with the upper part fringed with small hairs ¹/₃₂-⅛" () long.

Flowers: Flowers are pink, appearing in a thick oval cluster of tiny five-lobed flowers. The cluster is ¾-6" (2-15 cm) long. Each individual flower is ⅛-³/₁₆" (4-5 mm) in diameter. Flowers appear June to September.

Fruits: Tiny brown rounded achenes ¹/₁₆-⅛" (2.5-3 mm) long. Each achene contains one seed. They appear from July to September.

Edibility: Young shoots are edible if used sparingly. Like many plants, they contain oxalic acid, which contributes bitterness and is toxic if eaten often or in large quantities.

 

Polygonum amphibium (water smartweed, longroot smartweed, water knotweed, amphibious bistort)

9/14/2013 · Whitewater Rafting Trip, Barryville to Pond Eddy Leg, Delaware River, Barryville, Penn­syl­vania
≈ 4 × 6" (11 × 16 cm)

Polygonum amphibium (water smartweed, longroot smartweed, water knotweed, amphibious bistort)

8/16/2016 · John Tinker Trail, Nashua River, Groton, Mass­a­chu­setts

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Polygonum amphibium

Polygonum cespitosum

Polygonum hydropiperoides
Common Name

water smartweed

Oriental lady’s thumb

mild water-pepper
Plant Up to 9½' (3 m) × ⅜" (1 cm), sprawling or floating. Up to 3' (91 cm) in size, sprawling or erect. 6-39" (15-100 cm) long, but slender and with upper portions usually erect, erect to sprawling. Stems are green to red. (Compare to P. amphibium, with stems up to ⅜" (1 cm) around, and more often sprawled along the surface of the water).
Flowers Pink, appearing in a thick oval cluster of tiny five-lobed flowers. The cluster is ¾-6" (2-15 cm) long. Each individual flower is ⅛-³/₁₆" (4-5 mm) in diameter. Flowers appear June to September. Pink/white flower spikes are ¾-1½" (1.9-3.8 cm) tall × ¹/₁₆-³/₁₆" (3-6 mm) thick, densely filled with individual flowers less than ¼" (6.3 mm) in size. (By contrast, P. persicaria has flower stalks ⅛-⅜" (6-12 mm) thick.) Flowers appear from June to October. Greenish, pinkish, or white flowers form loose, elongated spikes 1-3" (3-8 cm) long and ¹/₁₆-⅛" (2-5 mm) around. The spikes are on peduncles ⅜-1" (1-3 cm) long.
Leaves Alternate, and often lanceolate, but they take on many other shapes. They may be floating or upright, with papery sheaths at leaf bases. They are ¾-8" (2-20 cm) × ⅛-1¾" (5-50 mm), with petioles ¹/₃₂-1" (1-30 mm). They are not mottled. Alternate, elliptic to lanceolate, ¾-3" (2-7.5 cm) × ⅜-1¼" (1-3.5 cm), and pointed at the base and tip. Petioles are ¹/₃₂-⅛" (1-5 mm). Leaves are not mottled. Alternate, lanceolate, bristly, with smooth edges. Floating or upright, up to 1¾-6" (5-15 cm) × ⅜-⅞" (1-2.5 cm), tapering to a sharp point. Petioles are less than ⅛" (5 mm) long. Leaves are not mottled.
Stem Ocrea are ⅛-1¾" (5-50 mm), with the upper part fringed with small hairs 0.5-4.5 mm long. Ocrea encompassing the joints are long-bristly, and ⅛-⅜" (4-12 mm) long,, vs. less than ⅛" (5 mm) for P. persicaria. Ocrea are brown, cylindric, and ⅛-⅞" (5-23 mm) long, with an inflated base, topped with hairs ¹/₁₆-⅜" (2-10 mm).
Fruit Tiny brown rounded achenes ¹/₁₆-⅛" (2.5-3 mm) long. Each achene contains one seed. They appear from July to September. A black achene with a triangular cross section. Dry, hard, smooth, shiny achenes each contain a single seed. They are ¹/₃₂-⅛" (1.5-3 mm) long, and three-sided.
Range/ Zones

Habitats Floating in ponds, streams, and marshes Common to invasive in disturbed habitats such as pastures, roadsides, and lawns. It is partial to wet areas. Often forming mats along lake or pond margins.
Type Wild Wild Wild

 

 
Polygonum lapathifolium

Polygonum persicaria
Common Name

nodding smartweed

lady’s thumb
Plant 24-60" (60-152 cm) high, usually upright, some­times sprawling, with green or deep red hairless stems. Up to 3' (91 cm) in height. Stems are openly branched and often reddish.
Flowers Greenish-white to pink, in columns about ¼-⅜" (6.3-9.5 mm) in diameter and 1-3" (2.5-7.6 cm) long. Numerous clusters may be upright, arched, or drooping. The tiny, individual flowers never actually open. Tiny pink and white flowers form groups of spikes ½-1½" (1.3-3.8 cm) long. Flowers appear from June to July.
Leaves Alternate, typically lanceolate but variable in shape, fatter at the base and tapering to a sharp tip, 1¾-10" (5-25 cm) × ½-1¾" (1.5-5 cm). Petioles are ⅜-¾" (1-2 cm). Leaves have tiny, hooklike hairs that make them feel rough. They are not mottled. Alternate, lanceolate to elliptic, 1-8" (2.5-20 cm) long × ¼-2" (6.3-50 mm) wide. Petioles are ⅛-¼" (5-8 mm). Leaves often have a dark purplish triangular or lunate patch on them.
Stem The ocreas are brownish, tubular, ½-1" (1.5-3 cm) long, with few or no hairs. Ocreas have hairs that extend up to ¹/₁₆" (2 mm) from the top.
Fruit Egg-shaped, less than ¹/₁₆" (2 mm) long, brown to black, glossy, containing a single seed. Brown to black, glossy, with one seed. They are roughly egg-shaped, but somewhat angular. Each fruit is less than ¹/₁₆" (2 mm) long. Fruits appear from July to September.
Range/ Zones

USDA Zones: 4-8
Habitats Margins of water bodies, pastures, orchards, irrigated crop fields, and rice fields. Disturbed soils that are wet or seasonally wet, such as ditches, pastures, grain fields, and water edges.
Type Wild Wild

 

Online References:

Polygonum amphibium at the Washington State Department of Ecology

Polygonum amphibium on Wikipedia

Polygonum amphibium on Plants for a Future, a resource and information centre for edible and otherwise useful plants

Polygonum amphibium at the Lady Bird Johnson Wildflower Center

Polygonum amphibium on eFloras

References:

Clemants, Steven; Gracie, Carol, Wildflowers in the Field and Forest, Oxford University Press, 2006, p. 76

Weakley, Alan S.; Ludwig, J. Christopher; Townsend, John F.; Crowder, Bland (Ed.), Flora of Virginia, Botanical Research Institute of Texas Press, 2013, p. 811

Polygonum amphibium (water smartweed, longroot smartweed, water knotweed, amphibious bistort)

9/14/2013 · Whitewater Rafting Trip, Barryville to Pond Eddy Leg, Delaware River, Barryville, Penn­syl­vania
≈ 4 × 6" (10 × 14 cm)

Polygonum amphibium (water smartweed, longroot smartweed, water knotweed, amphibious bistort)

9/14/2013 · Whitewater Rafting Trip, Barryville to Pond Eddy Leg, Delaware River, Barryville, Penn­syl­vania
≈ 2½ × 3½" (7.3 × 9 cm)

 

Polygonum amphibium description by Thomas H. Kent, last updated 8 Sep 2021.

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Polygonum amphibium (water smartweed, longroot smartweed, water knotweed, amphibious bistort)

8/16/2016 · John Tinker Trail, Nashua River, Groton, Mass­a­chu­setts

Polygonum amphibium (water smartweed, longroot smartweed, water knotweed, amphibious bistort)

9/14/2013 · Whitewater Rafting Trip, Barryville to Pond Eddy Leg, Delaware River, Barryville, Penn­syl­vania
≈ 3½ × 4" (9.2 × 10 cm)

Polygonum amphibium (water smartweed, longroot smartweed, water knotweed, amphibious bistort)

8/16/2016 · John Tinker Trail, Nashua River, Groton, Mass­a­chu­setts

Range:

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