The overall relationship of genera within Cyperaceae is still controversial. Ongoing molecular phylogenetic investigations will probably clarify the generic limits and assess the number of genera. The most recent classifications of the Cyperaceae were established by Bruhl (1995) and Goetghebeur (1998). The current treatment roughly follows Goetghebeur l.c.
Bolboschoenus maritimus (L.) Palla (syn.: Scirpus maritimus L.) is here treated as a collective species. Recent studies distinguish five species in Europe (Hroudová & al. 2007). In addition to Bolboschoenus maritimus s.str. (primarily confined to saline habitats) at least one additional taxon occurs in Belgium, B. laticarpus Marhold, Hroudová, Zákravský & Duchácek. The latter is mainly an inland species, distributed along rivers. Its residence status is unclear but Bolboschoenus laticarpus most likely is a neglected native species. Other species of the Bolboschoenus maritimus-group possibly occur in Belgium (as overlooked natives or as non-natives) but additional research will be required. A Belgian record of Bolboschoenus yagara (Ohwi) A.E. Kozhevn. (Browning & al. 1996) in fact turned out to belong to B. laticarpus. See Hroudová & al. (2007) for the distinction of the five European representatives of Bolboschoenus.
Rhynchospora Vahl counts two native species in Belgium, both confined to marshy acidic soils. At least one, Rhynchospora fusca (L.) Ait., was recently recorded in a nature reserve in Wingene, outside its current native distribution range (probably introduced accidentally by naturalists; see Stieperaere 1997).
Schoenoplectus (Reichenb.) Palla counts four native species in Belgium. The two commonest, Schoenoplectus lacustris (L.) Palla (syn.: Scirpus lacustris L.) and S. tabernaemontani (C.C. Gmel.) Palla (syn.: Scirpus tabernaemontani C.C. Gmel.), are often introduced deliberately (as ornamentals, for sewage treatment or land reclamation). Schoenoplectus pungens (Vahl) Palla was formerly believed to be an American xenophyte but it is native in western Europe (including Belgium) as well.
Scirpoides holoschoenus (L.) Soják (syn.: Scirpus holoschoenus L.) is a very rare native species. Two more or less distinct subspecies have been recorded. The usual, doubtlessly native taxon is subsp. holoschoenus. A second taxon, subsp. australis (L.) Soják, was formerly recorded in man-made habitats (levelled soil) near Antwerpen. Its residence status is somewhat unclear (see also Desfayes 2004).
Pycreus, in this account still given generic rank, is better included in a broadly circumscribed genus Cyperus (Laridon & al. 2013).
1. All flowers unisexual. Flowers and achenes (at least partially) enclosed in a perigynium (utricle) === Carex
1. All flowers bisexual. Flowers and achenes not enclosed in a perigynium === 2
2. Stem hollow. Leaves harshly scabrid, with saw-edged margins (native) === Cladium
2. Stem solid (often with soft pith). Leaf margins not strongly saw-edged === 3
3. Perianth with numerous bristles (>6) that are at least 10 mm long, transforming into a conspicuous cottony head at maturity (native) === Eriophorum
3. Perianth not transforming into a cottony head (if spikelets more or less woolly in appearance, due to protruding perianth bristles, then these 6 in number and less than 10 mm long) === 4
4. Inflorescence of one terminal spikelet, without bract(s) at base === 5
4. Inflorescence of at least two spikelets or, if inflorescence of one spikelet, then with a stem- or leaf-like bract at base (spikelet apparently lateral) === 7
5. Plant very leafy with grass-like leaves, usually floating in water. Stem branched (native) === Isolepis p.p. (incl. Eleogiton)
5. Plant without grass-like leaves (most leaves reduced to sheaths). Stem not branched === 6
6. Uppermost leaf sheath with a short blade. Perianth bristles antrorsely barbed (native) === Trichophorum
6. Uppermost leaf sheath without a blade. Perianth bristles retrorsely barbed === Eleocharis
7. Inflorescence a compact, flattened head. Spikelets on two opposite sides of main axis (native) === Blysmus
7. Inflorescence variable, if compact and flattened then spikelets not exclusively on two opposite sides of main axis === 8
8. Glumes of spikelets on two opposite sides of axis (distichously arranged) === 9
8. Glumes spirally arranged === 11
9. Spikelets with 1-4 fertile florets. Inflorescence a compact, flattened blackish head. Inflorescence bracts 1(-2) (native) === Schoenus
9. Spikelets many-flowered (>4). Inflorescence variable (usually a simple or compound umbel or an umbel-like raceme). Inflorescence bracts several === 10
10. Achenes trigonous. Stigmas three === Cyperus
10. Achenes biconvex. Stigmas two (native) === Pycreus
11. Inflorescence bracts 2-4. Stem trigonous === 12
11. Inflorescence bracts 0-1. Stem terete or trigonous === 13
12. Spikelets few, 10-40 mm long. Achenes brownish, 1,5-3 mm. Stem sharply trigonous (native) === Bolboschoenus
12. Spikelets numerous, 2-8 mm long. Achenes whitish, 0.5-1 mm. Stem bluntly trigonous === Scirpus
13. Spikelets with 2-3 fertile flowers. Inflorescence clearly terminal (native) === Rhynchospora
13. Spikelets with numerous fertile florets. Inflorescence (pseudo-) lateral, with bract as a continuation of the stem === 14
14. Small, slender annual (rarely perennial), less than 20 cm tall. Stamens 1-2 (native) === Isolepis p.p.
14. Relatively robust perennials (rarely annual), more than 20 cm tall. Stamens 3 === 15
15. Spikes spherical to globose. Perianth bristles absent (native) === Scirpoides
15. Spikes ovoid to oblong. Perianth bristles often present, 0-6 === 16
16. Glumes clearly emarginated. Rhizomatous perennial. Achenes always smooth at maturity. Perianth bristles always present (native) === Schoenoplectus
16. Glumes entire at apex. Tufted annual or perennial. Achenes smooth to rugulose at maturity. Perianth bristles present or absent === Schoenoplectiella
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