Rumex is an almost cosmopolitan genus of ca. 200 species. It is mostly distributed in the temperate regions. Eleven species are native in Belgium as well (see Lambinon & al. 2004). Plants with mature valves are usually required for the identification of Rumex-species. Outline and dimensions of tepals (ultimately valves) considerably change after flowering and in some species tubercle(s) are only developed in a late stage. In the key beneath only hybrids with at least one non-native parental species are included, plus the widespread native Rumex xpratensis.
1. Leaves hastate or sagittate at base, acid tasting. Flowers unisexual. Valves without or with an indistinct tubercle === 2
1. Leaves tapered, cuneate, truncate or cordate at base, never hastate or sagittate, nor acid tasting. Flowers bisexual. Valves with or without tubercles === 6
2. All leaves petiolate, leaf blade about as long as wide. Stem becoming woody at base (native) === Rumex scutatus
2. Upper leaves more or less sessile, leaf blade much longer than wide. Stem entirely herbaceous === 3
3. Valves ca. 1 mm long, as long as the achene. Upper leaves not stem clasping, hastate at base. Small plants usually not exceeding 50 cm. Pedicels articulated near base of tepals (native) === R. acetosella
3. Valves 2,5-5 mm long, many times longer than the achene at maturity. Upper leaves stem clasping, sagittate at base. Plants often exceeding 50 cm. Pedicels articulated near the middle or towards base === 4
4. Inflorescence lax with simple branches. Tepals ca. 3,3-5,2 mm long (native) === R. acetosa
4. Inflorescence much-branched. Tepals ca. 2,4-3,5 mm long === 5
5. Leaves 2-3x as long as wide. Flowering in May-June === Rumex rugosus
5. Leaves 4-10x as long as wide. Flowering in July-August === R. thyrsiflorus
6. All valves entire and without obvious tubercles === 7
6. At least one valve with a well-developed tubercle, valves dentate or entire === 10
7. Most leaves (at least basal and lower cauline) about as long as wide (at most 1,5x as long as wide), rounded at apex. Valves 4,5-6 mm long. Plant 50-100 cm === R. alpinus
7. Leaves longer, 1,5-15x as long as wide, acute at apex. Valves 6-8,5 mm long. Plant 60-200 cm === 8
8. Lowermost leaves with distinctly undulate margins, 3-15 times as long as wide. Plant 60-120 cm tall. Nut 2,5-3 mm long === 9
8. Lowermost leaves with flat margins, 1,5-3x as long as wide. Robust plant, 80-200 cm tall. Nut 3-4 mm long (native) ===R. aquaticus
9. Valves 4,5-6 mm long. Leaves 3-7x as long as wide, 3-10 cm wide === R. longifolius
9. Valves 3-5 mm long. Leaves 8-15x as long as wide, usually not wider than 3 cm === R. pseudonatronatus
10. Plant without basal rosette. Stem with axillary shoots, flowering later than main stem. Lower cauline leaves linear-lanceolate, 5-6x as long as wide. Valves entire === R. triangulivalvis
10. Plant with basal rosette (sometimes withered at maturity, especially in annual species!). Stem without axillary shoots. Lower cauline leaves variable. Valves dentate or entire === 11
11. All valves with distinct teeth (teeth evidently longer than broad, >0,3 mm long) === 12
11. All valves entire or at most crenate (teeth triangular, at most as long as wide, <0,2 mm long) === 18
12. Valves with all teeth subulate-filiform. Leaves narrowly lanceolate-linear, 3-10x as long as wide === 13
12. Valves not with all teeth subulate-filiform. Leaves much wider === 15
13. Teeth of valves ca. as long as width of valve. Valves ca. 3-4 mm long. Anthers more than 1 mm long. Pedicels up to twice as long as valves (native) === R. palustris
13. Teeth of valves 1,5-3x as long as valve. Valves 2,5-3 mm long. Anthers less than 1 mm long. Pedicels 3-7x as long as valves === 14
14. Leaves (at least basal ones) with distinctly undulate margins and truncate to cordate base. Entire plant densely papillose (mainly lower leaf surface and inflorescence branches). Plant turning dark brown at maturity. At least some valves with three teeth on each or one side. Anthers 0,7-0,9 mm long === R. fueginus
14. Leaf margins either flat or slightly undulate, tapering (cuneate) at base. Plant not or very slightly papillose, turning yellowish (golden) or more rarely brown at maturity. All valves with two teeth on each side. Anthers 0,5-0,7 (- 0,9) mm long (native) === R. maritimus
15. Tubercles on valves distinctly verrucose. Valves and pedicels very thick, surface of valves often very reticulate-veined. Leaves often panduriform (violin-shaped) === R. pulcher p.p.
15. Tubercles on valves smooth or nearly so, never verrucose. Valves and pedicels not thick. Leaves not panduriform === 16
16. Annual. Basal leaves up to 12 cm long, usually truncate at base. Usually all tepals with a well-developed tubercle === R. dentatus
16. Perennial. Basal leaves up to 40 cm long, cordate at base. Often only one valve with a well-developed tubercle === 17
17. Leaves undulate-crispate at margins. Fertility often reduced. Valves variable in outline (entire or crenate) and development (at least some shriveled) (native) === R. xpratensis p.p.
17. Leaves flat at margins or only slightly undulate. Plant fertile. Valves all uniform in outline and development === R. obtusifolius p.p.
18. Valves lingulate, much longer than wide, 3-4 mm long. Tubercles occupying nearly whole width of valve. Inflorescence lax, with remote whorls (native) === R. conglomeratus and R. sanguineus
18. Valves rounded to triangular, often longer. Tubercles much narrower than width of valve. Inflorescence usually dense === 19
19. All valves entire to the naked eye === 20
19. At least some valves crenate === 23
20. Valves 3-5,5 mm wide. Leaf margins strongly or slightly undulate === 21
20. Valves 5-8 mm wide. Leaf margins (nearly) flat, without undulate margins === 22
21. Leaves tapered at base, (usually narrowly) lanceolate, margins always undulate-crispate (weakly or strongly so). Plant 30-100 cm, rarely more. Fully fertile === R. crispus (incl. alien infraspecific taxa)
21. Leaves slightly cordate to truncate at base, wider with margins slightly undulate. Plant up to 200 cm tall. Mostly sterile, nuts shrivelled (although many flowers full-sized!) === R. xconfusus
22. Valves ovate-triangular, acute at apex, each with a well-developed tubercle (native) === R. hydrolapathum
22. Valves orbicular, rounded at apex, usually only one with a well-developed tubercle (tubercle developing late, only visible at maturity!) === R. patientia
23. Leaves narrowly lanceolate, margins strongly undulate-crispate, tapered at base. All valves with a well-developed tubercle. Plant fertile === R. stenophyllus
23. Leaves broadly lanceolate, margins flat or undulate, cordate at base. Tubercles 1-3. Plant fertile or partly sterile === 24
24. Tubercles distinctly verrucose. Leaves often panduriform, basal leaves < 15 cm long. Valves thick, usually distinctly reticulate-veined === R. pulcher p.p.
24. Tubercles smooth or nearly so. Leaves not panduriform, basal leaves up to 40 cm long. Valves not thick and distinctly reticulate-veined === 25
25. Plant mostly sterile, nuts shrivelled (although many flowers full-sized!) === 26
25. Plant fully fertile, all nuts normally developing === 27
26. Plant usually tall, in general habit like Rumex patientia, usually exceeding 100 cm. Leaf margins never undulate-crispate === R. xerubescens
26. Plant smaller, rarely exceeding 100 cm. Leaf margins undulate-crispate (native) === R. xpratensis p.p.
27. Valves ovate-triangular (longer than wide), with several prominent teeth. Lower leaves more than 3x as long as wide === R. obtusifolius p.p.
27. Valves nearly orbicular (ca. as long as wide), with slightly crenate margins. Lower leaves often large, up to 3x as long as wide === 28
28. Lower side of leaves distinctly papillose on the veins. Valves usually with a single tubercle. Plant rarely exceeding 100 cm. Ripe fruits dark brown === R. kerneri
28. Lower side of leaves smooth on the veins. Valves usually with three unequal tubercles. Plant usually exceeding 100 cm. Ripe fruits reddish brown === R. cristatus
Additional aliens: Rumex altissimus Wood (N-Am., wool alien), R. brownii Campd. (Aus., wool alien), R. bucephalophorusL. (Medit., grain alien), R. confertus Willd. (E-Eur., temp. As., garden escape) and R. obovatus Danser (S-Am., wool and grain alien).
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