Polygonum
See list of 13 species in this genusMost species of New England Polygonum produce two types of fruits—summer achenes that are brown to black (rarely otherwise) and frequently with surface ornamentation and late-season achenes that are ± olive, smooth, and significantly larger than summer achenes. Late-season achenes are not provided for in the following key. Some species have leaves along the main axes with considerably larger blades than those on the branches (often 3 or more times as long). Such plants are referred to as heterophyllous. Other species have leaves of approximately equal size (or the leaves of the main axes are only slightly larger). These plants are referred to as homophyllous. In the following key, measurements of stipule length must be made on intact stipules (not on those that have disintegrated into fibers). Also, as the perianth is accrescent, it should be measured in fruit. Note also that the leaves are often shed later in the season, leaving only the bracts, which can be substantially smaller and differ in length to width ratios from the leaves. Polygonum argyrocoleon Stued. ex Kunze was reported from MA by Magee and Ahles (1999) and from VT by Kartesz (1999). The MA record was based on a specimen of P. aviculare s.l.— Ahles 77772 ( MASS!). The VT report was erroneous. References: Löve and Löve (1956), Costea and Tardif (2003b).
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1a. Pedicels articulated near the base; tepal midveins unbranched; flowers with 8 stamens; sheathing stipules truncate to scarcely bilobed at apex
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1b. Pedicels articulated at or near the summit; tepal midveins usually dendritically branched; flowers with (3–) 5 (–8) stamens; sheathing stipules bilobed
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2a. Stems and branches sharply quadrangular, erect; flowers and fruits in the axils of tiny leaves, giving the appearance of a terminal inflorescence [Fig. 798]; achenes black-brown to black, minutely granular-roughened except on the angles (and often with a variably sized smooth region on the interior of each face prior to maturity)
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3a. Pedicels straight, the flowers and fruits erect or ascending; leaf blades pleated with 2 longitudinal folds, minutely serrate on the margin
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3b. Pedicels abruptly recurved, the flowers and fruits soon nodding [Fig. 798]; leaf blades without longitudinal folds, with entire margins
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2b. Stems and branches terete or 8- to 16-angled, prostrate to erect; flowers and fruits in most species in the axils of normal or slightly reduced leaves, giving the appearance of axillary inflorescences; achenes yellow-green, olive, light brown or brown to black, papillose, smooth, or minutely roughened over each face
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4a. Tepals with yellow-green to yellow margins
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5a. Tepals basally connate for 40–55% of their length [Fig. 796]; achenes uniformly papillose; leaf blades blue-green
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5b. Tepals basally connate for 20–38% of their length; achenes striate-papillose, smooth, or irregularly roughened; leaf blades usually bright green to yellow-green (note: color ± retained in drying)
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6a. Leaf blades elliptic to ovate, (8–) 10–30 mm wide; achenes striate-papillose
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6b. Leaf blades linear to narrow-elliptic, 1–8 (–10) mm wide; achenes smooth or irregularly roughened (in part)
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4b. Tepals with white to pink to red margins
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7a. Inflorescence with reduced bracteal leaves usually shorter than to as long as the flowers they subtend (except sometimes the lower bracts, which may be foliaceous)
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8a. Achenes striate-papillose, 1.7–2.8 mm long; tepals 1.9–3 mm long
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8b. Achenes smooth or irregularly roughened, 3–5 mm long; tepals 3–5 mm long
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7b. Inflorescence with conspicuous bracteal leaves that are often several times longer than the flowers they subtend
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9a. Outer 3 tepals flat or folded, of approximately equal width and length to the inner tepals and not or scarcely concealing them [Fig. 797]; plants usually of inland, non-saline habitats such as roadsides, sidewalks, and disturbed lots (rarely coastal)
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9b. Outer 3 tepals cucullate, much wider and often longer the inner tepals, partially or completely concealing them [Fig. 799]; plants usually of brackish and saline habitats such as coastal marshes and dunes (sometimes inland in P. buxiforme and P. ramosissimum)
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10a. Leaf blades pale green to white-green, somewhat to strongly glaucous; tepals loosely ascending and not investing apical portion of achene
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11a. Leaf blades pale green, only somewhat glaucous; sheathing stipules (i.e., ocreae) of lower nodes 4–7 (–8) mm long, with 3–7 nerves; tepals obovate; achenes 3.5–5 mm long (up to 6.5 mm long in late-season collections)
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11b. Leaf blades white-green, distinctly glaucous; sheathing stipules of lower nodes 7–10 mm long, with 8–16 nerves; tepals oval; achenes 3–4 mm long
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10b. Leaf blades green, blue-green, or yellow-green, sometimes tinged with red; tepals ± erect and rather closely investing achene (though achene may be exserted beyond perianth)
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12a. Leaf blades linear to narrow-elliptic, mostly 4–12 times as long as wide; achenes smooth or irregularly roughened; plants heterophyllous or nearly homophyllous; androecium with 3–6 stamens (in part)
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12b. Leaf blades oblanceolate to oblong or elliptic-oblong, mostly 2–4 times as long as wide; achenes striate-papillose, uniformly granular-roughened, or irregularly roughened; plants homophyllous; androecium with 6–8 stamens
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13a. Achenes uniformly granular-roughened, abruptly tapered to a slender beak, 2.8–4 mm long (up to 5 mm long in late-season collections); tepals 3–5 mm long, without a pouch-like swelling near the base; plants prostrate to, more commonly, ascending
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13b. Achenes striate-papillose or irregularly roughened, gradually tapering to apex, 2–2.8 mm long; tepals 2–3 mm long, often with a pouch-like protrusion near the base; plants usually prostrate and mat-forming
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Each photo represents one species in this genus.