Nodding smartweed is a North American native. The genus Polygonum means
“many knees”—a reference to the swollen, jointed sheaths the are found wherever leaf stems meet the vine.
They are found in wet areas, such as swamps.
Plants: 24-60" (60-152 cm) high, usually upright,
sometimes sprawling (the stems aren’t very strong), with green or deep red hairless stems.
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. Where each
leaf stem meets the main stem, there is a thick sheathlike joint (ocrea that is brownish,
tubular, ½-1" (1.5-3 cm) long, with few or no hairs.
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.
Fruits: Egg-shaped, less than ¹/₁₆" (2 mm) long, brown to
black, glossy, containing a single seed.
Some similar species:
9/4/2020 · Wilson’s Cove Trail, Harpswell Heritage Land Trust, Harpswell, Maine ≈ 2½ × 3½" (7.3 × 9.4 cm)
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.
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.
ID is uncertain 
