Polygonum amphibium (Water smartweed)

Polygonum amphibium L.

Water smartweed, longroot smartweed, water knotweed, amphibious bistort

Kingdom	Plantae	Plants, but not fungi, lichens, or algae
Subkingdom	Tracheobionta	Vascular plants—plants with a “circulatory system” for delivering water and nutrients
Division	Magnoliophyta	Flowering plants, also known as angiosperms
Class	Magnoliopsida	Dicotyledons—plants with two initial seed leaves
Subclass	Caryophyllidae	Cacti, many other succulents, carnivorous plants, and leadworts
Order	Caryophyllales	Includes cacti, carnations, amaranths, ice plants, and many carnivorous plants
Family	Polygonaceae	Knotweed or smartweed family
Genus	Polygonum	Greek poly, “many,” and gonu, “knee,” for the swollen jointed stems
Species	amphibium	Amphibious, 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.

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

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

	You are here 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

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

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.

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

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

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

Range:

About this map...