Equisetum L. is a small genus of ca. 15 species. Seven species are native in Belgium: Equisetum arvense L., E. fluviatile L., E. hyemale L., E. palustre L., E. sylvaticum L., E. telmateia Ehrh. and E. variegatum Schleich. Several of these are cultivated as well and might occur in places where they do not occur naturally (for instance Equisetum telmateia).
At least two non-native taxa are increasingly cultivated and have recently been reported as escapes from (or relics of) cultivation: Equisetum hyemale subsp. affine and E. scirpoides. Both are here opposed against their native lookalikes, respectively E. hyemale subsp. hyemale and E. variegatum:
1 Aerial shoots (stem) up to 250 cm high and 15 mm thick. Teeth persistent to falling. Ridges convex to roughened by two rows of tubercles === subsp. affine
Aerial shoots rarely exceeding 100 cm, up to 8 mm thick. Teeth soon falling. Ridges roughened by two rows of tubercles (native) === subsp. hyemale
1 Leaf sheaths with three teeth. Stem with six ridges, ca. 0,5-1 mm thick, usually inclined and tortuous. Subterranean stems and rhizomes strongly papillose and dull === Equisetum scirpoides
Leaf sheaths with 3-14 teeth (often more than five). Stem with 4-12 ridges, 1-3 mm thick, usually erect and straight. Subterranean stems and rhizomes smooth and often glossy (native) === E. variegatum
Literature
Christenhusz M.J.M., Bangiolo L., Chase M.W., Fay M.F., Husby C., Witkus M. & Viruel J. (2019) Phylogenetics, classification and typification of extant horsetails (Equisetum, Equisetaceae. Botanical Journal of the Linnean Society 189: 311–352.
Hauke R.L. (1993) Equisetaceae. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 2. Oxford University Press, New York-Oxford: 76-84.
Jermy A.C., Page C.N. & Acock P.J. (1998) Equisetum. In: Rich T.C.G. & Jermy A.C. (eds.), Plant crib: 8-14. BSBI, London.
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.
Page C.N. & Bennell F.M. (1986) Equisetum. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 1: 5-6. Cambridge University Press, Cambridge.