This key is a work in progress and I cannot promise that I have taken account of all variaiton within each taxon, especially those occurring in South America.                                                                   

1. Stigma penicillate, style short or absent, bractioles unequal in length, central and eastern South America. Sec. Dilopha Moq.

2. Stem tawney-sericeous to villous, restricted to the northern Brazilian states of Piaui and Paraíba.

                                                                       F. sericea (Hoffmanns. ex Röm. et Schult.) Moq.

2. Stem tomentose with adpresssed hairs; central, northern, and eastern South America.

3. Inflorescence stout, with (4)8-12 pairs of horizontally opposed spikelets, leaves lanceolate to linear-lanceolate, 5-10 times long as broad, eastern and southern Brazil, eastern Paraguay, northeast Argentina.

                                                                                               F. procera (Seub.) Pedersen

3. Inflorescence generally lax, with 3-6 pairs of opposite spikelets, leaves ovate to obovate or oblanceolate, eastern Venezuela, northeastern and central Brazil.

                                                                                   F. humboldtiana (Röm. et Schult.) Seub.

1. Stigma capitate or minutely bilobed, style short to elongate, bractioles sub-equal in length. North America, western to central South America. Sec. Hoplotheca (Nutt.) Moq.

4. Plants woody, stems broomlike; Galápagos Islands.

5. Infloresences elongate spikes, the spikes acute at apex, shrub ca 1 m tall. Islas Isabela and Santa Cruz.

6. Fruiting perianth without lateral wings, or with thick narrow wings ca 0.5 mm wide, perianth-tube glabrous or nearly so. Isla Isabela.

                                                                            F. juncea subsp. juncea B.L. Rob. & Greenm.

6. Fruiting perianth with broad lateral wings ca 1 mm wide, perianth tube often tomentulose. Isla Santa Cruz

                                                                                           F. juncea subsp. alata J.T. Howell

5. Inflorescences rounded heads or cylindrical spikes, the spikes obtuse at apex, shrub 3-5 dm tall. Islas Santa María, San Salvador, Pinzón, Fernandina, Isabela, and San Cristóbal.

7. Fruiting perianth without laterial wings or with minute ridges, perianth 3-3.5 mm long. Islas Fernandina and Isabela.

                                                                   F. nudicaulis subsp. lanigera (Andersson) Eliasson

7. Fruiting perianth with broad lateral wings ca 1 mm wide, perianth 4-5 mm long

8. Spikes oblong, upper part of stem glabrous or subglabrous. Isla Santa María, San Salvador, San Cristóbal.

                                                                                     F. nudicaulis subsp. nudicaulis Hook.f.

8. Spikes rounded, capitate, upper part of stem subglabrous or silvery-lanate. Isla Pinzón.

                                                                                       F. nudicaulis subsp. curta J.T. Howell

4. Plants herbaceous, stems erect or decumbent; North America and mainland South America.

9. Mature (fruiting) perianth with narrow to broad undivided lateral wings, face of mature perianth naked, lacking a basal tubercle or spine; plants perennial; spikelets loosely flowered, arranged in a 3-ranked spiral; southern Sonoran and Chihuahuan Deserts south.

10. Leaf margins generally wavy to crisped; leaves and stems densely short tomentose; stems ascending or often prostrate and matforming; Cape Region of Baja California

                                                                                              F. xantii R.A. McCauley

10. Leaf margins entire; leaves and stems tomentose; stems ascending to scandent or erect; mainland Mexico south to western Nicaragua, Jamaica and South America

11. Leaf blades narrowly ovate to linear; central Paraguay.

                                                                                              F. paraguyensis Chodat

11. Leaf blades (at least the principal ones) ovate-orbicular or oblanceolate.

12. Leaf blades oblanceolate; Gran Chaco of Paraguay, Bolivia, and northern Argentina.

                                                                                               F. chacoensis Chodat

12. Leaf blades ovate-orbicualar.

13. Plants densly pubescent, much-branched and ascending from base; southern Brazil, northeastern Argentina, Uruguay.

                                                                                              F. tomentosa (Mart.) Moq.

13. Plants sparcely to moderatly pubescent, little-branched from base, stems erect; Mexico south to northern Venezuela, northern Colombia, Ecuador, Peru, Bolivia.

14. Habit decumbent and scandent on adjacent vegetation; stems slender; bracteoles glabrous or rarely minutely pubescent distally; Mexico along the western slope of the Sierra Madre del Sur from Jalisco south to Oaxaca

                                                                              F. interrupta var. colimensis R.A. McCauley

14. Habit erect; stems stout

15. Bracteoles glabrous distally; leaf margins generally darkened (principally in North America, more vairiable in South America); central plateau of Mexico south

                                                                                         F. interrupta var. interrupta (L.) Moq.

15. Bracteoles pubescent distally; leaf margins never darkened; Mexico, southern Sonoran Desert.

                                                                    F. interrupta var. alata (S. Watson) R.A. McCauley

9. Mature (fruiting) perianth with variously divided to dentate lateral wings; face of mature perianth with one to three basal tubercles or spines; plants perennial or annual; spikelets loosely to densely flowered in 3- or 5-ranked spirals; central and southern United States, northern and Gulf coastal Mexico south through Veracruz.

16. Mature perianth with one basal tubercle or spine, rarely absent; leaf blades (at least the distal ones) ovate-orbicular or short oblanceolate; spikelets dense, flowers arranged in a 5-ranked spiral (viewed from apex); plants perennial; extreme southern Texas and Gulf coast south to central Veracruz

                                                                                              F. texana Coulter & Fisher

16. Mature perianth with two or three basal tubercles or spines, rarely one; leaf blades lanceolate to oblanceolate and elliptic; spikelets loose or dense, flowers arranged in a 3- or 5-ranked spiral; plants perennial or annual; central and southern United States, northern Mexico.

17. Mature perianth slightly oblique apically, with irregularly and deeply cut (''spiny'') lateral wings; flowers 2.4-3.8 mm long; stems slender, branched from base (except in depauperate specimens with only 1 slender stalk); floral spikes 3-ranked; bracteoles glabrous, not pubescent distally; stem pubescence grayish-white; widespread and generally weedy

                                                                                              F. gracilis (Hook) Moq.

17. Mature perianth symmetrical, with irregularly dentate to crenulate lateral wings; flowers 3.5-6 mm long; stems stout with one or more erect to decumbent branches from ground level; floral spikes 5-ranked (rarely 3-ranked); bracteoles glabrous to sparsely or densely pubescent distally; stem pubescence white to grayish-white or brownish.

18. Stem pubescence bright white; pubescence of mature flowers dense, bright white; bracteoles glabrous; plants perennial; root woody and irregularly shaped; flowers (3.5-) 4-5.5 mm long; leaf blades broadly linear to lanceolate or oblanceolate; fruiting perianth with irregularly dentate lateral wings; southern Arizona, southern New Mexico, southwest Texas, northeastern Sonora, Chihuahua, Coahuila, Nuevo León.

                                                                                           F. arizonica Thornber ex Standley

18. Stem pubescence grayish-white or brownish; bracteoles pubescent distally; plants annual; root semi-woody and tapered.

19. Interstaminal lobes blunt, brown-tinted (often observed as darkened flower tips), 0.5-0.7mm long, usually not recurved; bracteoles sparsely hairy distally; flowers 3.5-5.4 mm long; south-central Texas

                                                                                            F. drummondii Moq.

19. Interstaminal lobes acute, light colored or pinkish, 0.5-1.3 mm long, usually recurved; bracteoles with small tufts of distinct pubescence distally; flowers 4-6 mm long; central, southern, and southeastern United States.

20. Leaf blades orbicular to broadly elliptic, abaxial surface of leaves densely wooly; southern Texas, principally coastal

                                                                                             F. latifolia R.A. McCauley

20. Leaf blades linear to lanceolate to oblanceolate or oblong, abaxial surface of leaves silky; central, southern, and southeastern United States

                                                                                               F. floridana (Nutt.) Moq.