Scirpus, as currently (narrowly) circumscribed, encloses ca. 20 species (ca. 35 according to Whittemore & Schuyler 2002 but these authors have applied a very narrow species concept, for instance in the Scirpus atrovirens group). Previously all scirpoid-looking species have been accommodated in a broadly circumscribed genus Scirpus.
One species of Scirpus s.str. is native in Belgium, S. sylvaticus L. Two additional species have been recorded as aliens.
1. Perianth bristles much longer than achene, projecting beyond scales. Spikelets ca. 4-8 mm long. Inflorescence conspicuously drooping at maturity === Scirpus cyperinus
1. Perianth bristles either absent or shorter than to slightly longer than achene, not projecting beyond scales. Spikelets ca. 2-4 mm long. Inflorescence not or only slightly drooping at maturity === 2
2. Inflorescence compact, brownish-red. Spikelets in fascicles of 8-20, at the apices of ultimate rays. Perianth bristles 0-3, slender, smooth below, retrorsely barbed above, shorter than the achene === S. atrovirens subsp. georgianus
2. Inflorescence lax, green or greenish-brown. Spikelets in fascicles of 2-5(-8), at the apices of ultimate rays. Perianth bristles 5-6, stiff, retrorsely barbed throughout, as long as to slightly longer than the achene (native) === S. sylvaticus
References
DeFilipps R.A. (1980) Scirpus. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 5. Cambridge University Press, Cambridge: 277-280.
Lambinon J., Delvosalle L., Duvigneaud J. (avec coll. Geerinck D., Lebeau J., Schumacker R. & Vannerom H. (2004) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Cinquième édition. Jardin botanique national de Belgique, Meise: CXXX + 1167 p.
Pignotti L. (2003) Scirpus L. and related genera (Cyperaceae) in Italy. Webbia 58(2): 281-400.
Whittemore A.T. & Schuyler A.E. (2002) Scirpus. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 23. Oxford University Press, New York-Oxford: 8-21.