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Key to Ludwigia, Key B: Ludwigia with alternate leaves, 8-14 stamens, 4-7 sepals, and 4-7 petals
(sections Pterocaulon, Macrocarpon, Seminuda, and Jussiaea)

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1 Sepals 4 (or rarely 5); stamens 8 (or rarely 10); seeds in 2-several vertical series in each locule, free of endocarp tissue; annual or perennial herbs or a woody shrub below.

2 Capsule obconic, gradually broadening from base to apex, weakly angled; young stems and both surfaces of leaves shaggy-pubescent with tawny hairs; [section Myrtocarpus]  Ludwigia peruviana

2 Capsule narrowly cylindrical, nearly isodiametric, or, if obconic, sharply 4-angled and usually winged on the angles; young stems and both surfaces of leaves glabrous, sparsely short-pubescent, or sparsely villous.

3 Internodes of the stem not winged, weakly angled, usually villous or strigillose at least proximally (sometimes inconspicuous); capsules nearly terete, narrowly cylindrical, 1.7-5.0 cm long, often curved.

4 Petals (1.5-) 3-5 cm long; sepals ca. 10 mm wide at base  Ludwigia bonariensis

4 Petals 1-2 cm long; sepals 3-5 mm wide at base  Ludwigia octovalvis

3 Internodes of the stem sharply 4-angled or 4-winged, glabrous; capsule sharply 4-angled, obconic, often winged on the angles, 1-2 cm long (rarely longer in L. longifolia); [section Pterocaulon].

5 Leaves linear-lanceolate, 5-35 cm long, 0.4-2.5 cm wide; petals 2-3 cm long; sepals 10-18 mm long; capsules 1.0-4.2 cm long; [rare waif, FL]  Ludwigia longifolia

5 Leaves lanceolate to elliptic, 2-20 cm long, 0.2-5 cm wide; petals 0.4-2 cm long; sepals 3-12 mm long; capsules 1.0-2.0 cm long; [widespread].

Show caption*© Richard & Teresa Ware           6 Leaves sessile; petals 1-2 cm long; sepals 7-12 mm long; stems strongly 4-winged (from decurrent leaf bases)  Ludwigia decurrens

6 Leaves petiolate, petioles 0.2-2.2 cm; petals 0.4-0.5 cm long; sepals 3-6 mm long; stems sharply 4-angled but usually not winged  Ludwigia erecta

1 Sepals 5 (-7); stamens 10 (-14); seeds in 1 vertical series in each locule, loosely embraced or embedded in endocarp tissue.

7 Stems erect; floral tube much longer than the pedicel; seeds loosely embraced by a corky, horseshoe-shaped segment of endocarp; [section Seminudae]  Ludwigia leptocarpa

7 Stems (at least the lower portions) decumbent, creeping, or floating in mats (the flowering stems more-or-less erect in L. grandiflora and L. hexapetala); floral tube much shorter than the pedicel; seeds embedded in the woody endocarp; [section Jussiaea].

8 Flowering stems more-or-less erect; emergent leaves chartaceous, light to medium green, dull, mostly lanceolate or oblanceolate, petioles 0.1-2 (-2.5) cm; stems, sepals, ovaries, and emergent leaves (at least along veins) villous, hairs often viscid; bracteoles obovate, 1-1.8 mm long.

9 Sepals (6-) 8-11 (-14) mm long; emergent leaves 5-8.5 cm long, 7-11 mm wide, mostly linear-lanceolate, usually widest below the middle; petals (1.2-) 1.6-2.0 (-2.6) cm long; style 4.7-6.7 (-8.2) mm long; stems densely villous  Ludwigia grandiflora

9 Sepals (8-) 12-19 mm long; emergent leaves 5.5-13 cm long, 9-18 mm wide, mostly oblanceolate, usually widest above the middle; petals (1.5-) 2.0-2.9 (-3) cm long; style (5.8-) 6-10 mm long; stems sparsely to densely villous (rarely glabrous)  Ludwigia hexapetala

8 Flowering stems decumbent, floating, or creeping; stem and leaves glabrous or glabrescent; petals mostly 1-1.5 cm long; anthers 1-1.7 mm long.

10 Stems glabrous; emergent leaves with petioles 0.6-6 cm, blades (2-) 4-10cm, eglandular-mucronate, alternate; stamens subequal, anthers all 1.2-1.4 mm, stigmas well-exserted at anthesis; pedicels 3.5-9 cm, ovaries 14-20 mm; seeds 16-18 per locule; [widespread]  Ludwigia peploides var. glabrescens

10 Stems glabrous or sparsely to densely villous; emergent leaves with petioles 0.3-2.8 cm, blades (0.4-) 0.8-6 (-9.5) cm, often glandular-mucronate, alternate or fascicled; stamens unequal, filaments of 2 different lengths and anthers of 2 different sizes, 0.5-2.2 mm, surrounding the stigmas at anthesis; pedicels 0.7-3.8 cm, ovaries 6-14 mm; seeds 7-15 per locule; [s. MS westward and southward].

11 Stems densely, rarely sparsely villous; ovaries and sepals densely hirtellous; capsules (2-) 2.4-3.2 cm long; [native of South America, rare waif in LA]  Ludwigia peploides var. montevidensis

11 Stems sparsely villous to glabrous; ovaries and sepals glabrous or with a few scattered hairs; capsules 1.0-1.7 (-2.5) cm long; [native, more widespread]  Ludwigia peploides var. peploides