Geranium is a genus of about 260 species in the temperate regions of the world. The genus is exceedingly popular in horticulture. Yeo (1997, 2001) cites more than 60 species and hybrids that are cultivated as ornamentals in Europe. Selection and hybridisation have considerably blurred specific boundaries in some species groups. Also as a result of its popularity in horticulture the original distributional area of numerous species of Geranium has become quite uncertain. In Belgium, for instance, the residence status of Geranium pratense L. and G. palustre L. is very controversial. They are sometimes accepted as locally native whereas other authors suspect they are merely (old) introductions. Likewise, Geranium phaeum is present in our areas since at least the 16th century and native status has often been suggested but this was recently definitively rejected (Van den Bremt 2006). More or less doubtlessly native in Belgium are (at least in some localities): Geranium aequale (Bab.) Aedo, G. columbinum L., G. dissectum L., G. lucidum L., G. molle L., G. pusillum L., G. robertianum L., G. rotundifolium L., *G. sanguineum L. and *G. sylvaticum L. At least those preceeded by an * are also frequently cultivated as ornamentals and probably more common as an escape than in the wild in Belgium. Some others (especially Geranium columbinum, G. lucidum and G. rotundifolium) were initially almost restricted to more or less specialised habitats in Wallonia (often damp rocky slopes) but have recently considerably extended their area towards the north. They now also occur in man-made habitats in Flanders, mainly by railway tracks, on coalmine heaps or in urban areas.
Geranium is surely imperfectly known in Belgium. Especially garden outcasts require further study but some poorly known taxa are possibly also overlooked. Native Geranium aequale, for instance, was only recently recognised (Lawalrée 2000, Veldkamp 2008). Among those worthwhile looking for is Geranium brutium Gasp. (syn.: G. molle subsp. brutium (Gasp.) P.H. Davis) (see Clement & Foster 1994, Yeo 1997, Sell & Murrell 2009). It looks like a large-flowered Geranium molle (petals ca. 8-12 mm long) and might pass unrecorded (confusion with G. molle or G. pyrenaicum). According to Sell & Murrell (2009) it is often introduced in wild flower seed.
All current information on the genus Geranium is stored in "The Geranium Taxonomic Information System" (available online at: http://www.geranium.es).
1. Stamens at least twice as long as petals, long exserted === Geranium macrorrhizum (incl. G. xcantabrigiense)
1. Stamens hardly or not exserted, at most 1,5x as long as petals === 2
2. Petals abruptly narrowed at base to form a distinct claw about half or more the length of the petal === 3
2. Petals gradually narrowed towards base without a claw or sometimes with a short claw less than half the length of the petal === 5
3. Leaves divided ½-2/3 to base, nearly glabrous and shiny. Sepals distinctly keeled, glabrous (native) === G. lucidum
3. Leaves divided nearly to base, usually hairy and rather dull. Sepals not keeled, glandular hairy === 4
4. Mericarps with dense wrinkles in the basal half and 2-3(-4) deep collar-like ridges towards apex. Petals 5-9 mm long. Anthers yellow === G. purpureum
4. Mericarps with sparse, shallow ribs in the basal half and with 0-1(-2) deep collar-like ridges towards apex. Petals 8-14 mm long. Anthers pinkish-orange to purplish (native) === G. robertianum
5. Annuals, biennials or perennials (if perennial than without a distinct rhizome). Petals up to 10 mm long (often much shorter) === 6
5. Perennials with stout rhizomes. Petals usually much longer (if shorter, then blackish) === 14
6. Fertile stamens 5 (outer whorl of stamens with filaments lacking anthers) (native) === G. pusillum
6. Fertile stamens 10 (all filaments bearing anthers) === 7
7. Mericarps reticulate (i.e. transversely ridged) === 8
7. Mericarps smooth === 9
8. Leaves angular in outline. Mericarps separating without stylar beak (fruit discharge mechanism not operative). Sepals distinctly mucronate (mucro ca. 1 mm long). Rostrum 5-8 mm long === G. divaricatum
8. Leaves rotund in outline. Mericarps separating with long stylar beak (fruit discharge mechanism operative). Sepals indistinctly mucronate (mucro ca. 0,1-0,2 mm long). Rostrum 6-11 mm long (native) === G. molle
9. Mericarps glabrous (native) === G. aequale
9. Mericarps hairy (at least in part but often throughout) === 10
10. Leaves divided ½-2/3 to base === 11
10. Leaves divided nearly to base === 12
11. Perennial. Petals 7-10 mm long, notched at apex === G. pyrenaicum
11. Annual. Petals 5-7 mm long, rounded to truncate at apex, not notched (native) === G. rotundifolium
12. Entire plant eglandular hairy. Peduncle slender, usually longer than 25 mm, longer than the subtending leaves. Petals bright pink, rounded at apex, 8-10 mm long (native) === G. columbinum
12. Plant glandular hairy (especially peduncles). Peduncle stouter, usually shorter than 25 mm, shorter than the subtending leaves. Petals reddish purple or pale lilac, 4-6 mm long === 13
13. Petals pale lilac, entire at apex. Mericarp blackish at maturity, 3-4,5 mm long. Rostrum of fruit 16-21 mm long === G. carolinianum
13. Petals reddish purple, notched at apex. Mericarps brownish at maturity, 2-2,5 mm long. Rostrum of fruit 7-10 mm long (native) === G. dissectum
14. All flowers solitary (native) === G. sanguineum
14. Most flowers 2(-3) per peduncle === 15
15. Petals magenta with black base and veins === G. psilostemon
15. Petals entirely pinkish, purplish, reddish, whitish or blackish, never with a black base === 16
16. Petals very dark purplish to nearly black, apiculate at apex. Mericarps pointed at base === G. phaeum
16. Petals pinkish, purplish, reddish or sometimes whitish but never blackish. Mericarps rounded at base === 17
17. Petals deeply notched at apex. Leaves with 3-5 lobes, leaf lobes shallowly crenate === G. nodosum
17. Petals rounded to slightly emarginate at apex, never deeply notched. Leaf lobes deeply dentate === 18
18. Pedicels densely glandular hairy. Inflorescence compact (pedicels at most 3x as long as sepals) === 19
18. Pedicels eglandular or with some sparse glandular hairs. Inflorescence lax (pedicels at least 4x as long as sepals) === 20
19. Petals rounded at apex. Mericarps with tuft of bristles at base (native) === G. pratense and G. sylvaticum
20. Pedicels hirsute-hairy with coarse retrorse hairs. Fruiting pedicels curved near apex. Stamens with filaments shortly ciliate at base (native) === G. palustre
20. Pedicels with slender, patent eglandular hairs intermixed with numerous very short, more or less glandular ones. Fruiting pedicels erect at apex. Stamens with filaments long ciliate throughout (except in upper 1/3) === G. endressii (incl. G. xoxonianum)
Geranium bohemicum L., G. collinum Steph. ex Willd. and G. rivulare Vill. [syn.: G. sylvaticum subsp. rivulare (Vill.) Rouy] are claimed as aliens in Lambinon & al. (2004). However, these citations could not be confirmed by voucher specimens (Verloove & Lambinon 2008). All were formerly cited by Mullenders (1967) in his flora but apparently without any source. Finally, Geranium versicolor L. was reported from Belgium by Andriessen & al. (1985) but this record was in error for G. xoxonianum (Verloove & Lambinon 2008).
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