Verbena, as currently circumscribed (see for instance Sanders 2001), is a genus of ca. 50 species. It traditionally falls apart into two more or less distinct sections: section Verbena with most representatives in North America and section Pachystachyae Schauer with chiefly South American species. Non-native taxa of both sections occur in Belgium and one species, Verbena officinalis L., is native. Nesom (2010) recently proposed a slightly modified infrageneric classification. Several species of Verbena are cultivated as ornamentals (Wann 2000) and some are reputed environmental or agricultural weeds.
1. Stem prostrate or procumbent to decumbent-ascending. Bracts much longer than the calyx, foliaceous, ca. 8-15 mm long === Verbena bracteata
1. Stem erect. Bracts shorter than or about equaling calyx, ca. 1-3 mm long === 2
2. Inflorescence of compound cymes. Spikes thick and compact, not exceeding 20 cm and scarcely elongating after anthesis. Fruits very closely congested along rachis === 3
2. Inflorescence a solitary spike or a simple cyme. Spikes thin and rather lax, up to 40 cm long and much elongating after anthesis. Fruits often remotely arranged along rachis (except in V. hastata) === 6
3. Leaves narrowly tapering into a cuneate-attenuate base, (sub-) petiolate === V. brasiliensis
3. Leaves sessile, rounded to almost cordate at base, often more or less semi-amplexicaul === 4
4. Bracts shorter than the calyx. Robust plant, up to 200 cm tall. Leaves herbaceous, not very rigid. Inflorescence much branched with comparatively slender spikes === 5
4. Bracts longer than the calyx. Lower plant, rarely exceeding 60 cm. Leaves coriaceous and rigid. Inflorescence usually in subternate groups with thick spikes, (5-) 10-15(-20) mm diameter === V. rigida
5. Corolla showy, more than 4 mm across with tube longer than 5 mm. Spikes rarely exceeding 15 mm at maturity, often as wide as or wider than long (var. conglomerata). Mericarps more than 1.5 mm long. Leaves and calyces stipitate-glandular === V. bonariensis
5. Corolla minute, less than 4 mm across and with tube shorter than 5 mm. Spikes elongating in fruit, more than 15 mm long, much longer than wide. Mericarps less than 15 mm long. Leaves and calyces not stipitate-glandular === V. incompta
6. Lower leaves deeply pinnately lobed (native) === V. officinalis
6. Lower leaves serrate or shallowly lobed === 6
7. Spike stiffly erect, fruits imbricate. Corolla blue-violet. Lower leaves sometimes lobed or hastate === V. hastata
7. Spike very slender, filiform, fruits very remotely arranged. Corolla white. Lower leaves never lobed === V. urticifolia
Additional aliens: Verbena halei Small (incl. V. gaudichaudii (Briq.) P.W. Michael, syn.: V. officinalis L. var. gaudichaudii Briq.) (N-Am., wool alien) and V. supina L. (Medit., wool alien).
Literature
Barber S. (1982) Taxonomic studies in the Verbena stricta complex (Verbenaceae). Syst. Bot. 7: 433-456.
Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.
Lugerbauer K. (2018) Von Luftikussen und Eisenlanzen. Gartenpraxis 02-2018: 6-11.
Moldenke H.N. (1961) Materials towards a monograph of the genus Verbena. Phytologia 8: 108-496.
Munir A.A. (2002) A taxonomic revision of the genus Verbena L. (Verbenaceae) in Australia. J. Adelaide Bot. Gard. 20: 21-103.
Nesom G.L. (2010) Infrageneric classification of Verbena (Verbenaceae). Phytoneuron 2010-11: 1-15.
O’Leary N., Múlgura M.E. & Morrone O. (2007) Revisión taxonómica de las especies del género Verbena (Verbenaceae): serie Pachystachyae. Ann. Missouri Bot. Gard. 94: 571-622.
O’Leary N., Múlgura M.E. & Morrone O. (2010) Revisión taxonómica de las especies del género Verbena (Verbenaceae) II: serie Verbena. Ann. Missouri Bot. Gard. 97: 365-424.
Perry L.M. (1933) A revision of the North American species of Verbena. Ann. Missouri Bot. Gard. 20(2): 239-363.
Sanders R.W. (2001) The genera of Verbenaceae in the Southeastern United States. Harvard Papers Bot. 5(2): 303-358.
Sell P. & Murrell G. (2006) Flora of Great Britain and Ireland. Vol. 4 Campanulaceae – Asteraceae. Cambridge University Press, Cambridge: XXVIII + 624 p.
Wann J.D. (2000) Verbena. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 6. Cambridge University Press, Cambridge: 151-153.
Sell P. & Murrell G. (2006) Flora of Great Britain and Ireland. Vol. 4 Campanulaceae – Asteraceae. Cambridge University Press, Cambridge: XXVIII + 624 p.