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Key to Genus Sporobolus R. Br.
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Grasses of the genus Sporobolus are relatively easily spotted in the field when in flower, having an inflorescence that is typically a long, narrow, cylindrical panicle (called a "rattail" in some species) crowded with simple, one-flowered spikelets (see photo at right). In the case of S. virginicus under ideal conditions, the inflorescence can be fairly thick (see Fig. 11C) rather than rattail-like; this native dune grass has been used for verge planting along Mokulele Highway on Maui and is the only Sporobolus in Hawai‘i that spreads by runners (rhizomes). The rest are clumping grasses and many are characterized by narrow (wiry) leaves. Many are difficult to distinguish (identify to species) unless examined closely under magnification, especially species in the S. indicus complex (S. africanus, S. diandrus, S. elongatus, S. fertilis, and S. indicus). However, much needed clarification has been provided by genetic studies undertaken at Bishop Museum and University of Hawai‘i (Faccenda, Yorkton, & Morden, 2024).
The spikelet of a Sporobolus is simple, consisting of a single floret encased in four floral bracts: two glumes, a lemma, and a palea (see Fig. 11B). Glumes are distinguished as first or lower glume and second or upper glume. In some species, the glumes may not be obvious except on very close examination, being short, tattered, and closely adherent against the lemma or palea. When the grain is released, the glumes are left behind; or all the floral bracts remain attached to the pedicel and the bracts left gaping.
Figure 11B. Drawing of a spikelet of Sporobolus indicus copied
from the file: FNA25_P45_Sporobolus_pg_125.jpeg, floraofnorthamerica.org.
Sporobolus diandrus starts out with the culm branches closely appressed to the axis (see Fig. 11A, at right) as in the other species, but the branches spread open as the flowers mature (see Fig. 11C, below; the same is somewhat true for S. pyramidatus). Species in the S. indicus complex were recently reviewed by Faccenda et al. (2024), where it is suggested that "[m]orphological identification of plants in the S. indicus complex... is impossible in the field". Examination of the seed after separating from the pericarp may be necessary.
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[GENUS SPOROBOLUS]
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55a
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(53) |
Clumping grasses. Leaves mostly crowded near the base, lax and/or wirey. { Annual or perennial .. .
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[56] |
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55b
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Figure 11C. Well-developed inflorescences
of ‘aki‘aki (Sporobolus virginicus) growing
under ideal conditions (S. virginicus var. phleoides).
Perennial, spreading grass, producing extensive stiff rhizomes. Leaves stiff, distinctly distichous (appearing or approximating two-ranked) in typical dune habitat. Leaves with scattered, long hairs on upper surface. Ligule a fringe of short, dense hairs. { Grass common along the marine shore and on coastal dunes from just above the high tide mark. Rarely flowers outside the ideal habitat near the ocean (Fig. 11C). ‘aki‘aki, seashore rush grass, beach dropseed [HAW - IND] beach-dropseed, salt-grass, jatopa [GUM - NAT]
Sporobolus virginicus (L.) Kunth
Resembles in habit (distichous leaf arrangement) and habitat, Paspalum vaginatum and Distichlis spicata.
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56a
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(55) |
Small annuals, nearly always under 30 cm (1 ft) in height (S. pyramidatus, sometimes a short-lived perennial, may exceed this a bit) .. .
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[61] |
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56b
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Medium to large perennials, over 30 cm (~1 ft) tall .. .
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[57] |
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57a
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(56) |
Infloresence an open panicle, panicle branches strongly divergent from the culm .. .
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[63] |
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57b
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Panicle either a spike, or spike-like; if branched, branches held tight against the culm .. .
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[58] |
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58a
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(57) |
Leaf sheath pilose (dense with hairs). Inflorescence gray, 6 to 20 cm (2.4 to 8 in) long. Spikelets dense on branches held tight to the axis. Spikelets 2.0-2.5 mm long. Lemma and palea exceeding seed by its own length, gaping open. Fruit maturing red or brown { Medium, perennial, wiry, clumping grass, widespread on disturbed sites and pastures above 500 ft (150 m) elevation. Smutgrass, African dropseed, rattail grass. [HAW - NAT] [GUM (Saipan) - NAT ]Sporobolus africanus (Poir.) Robyns & Tournay
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58b
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Inflorescence green, gray, or yellowish and betwen 13 and 50 cm (5 to 20 in) long. Spike branches either held tight tight against the axis or, if spreading, not speading more than 30°. Spikelets less than 2.0 mm long .. .
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[59] |
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59a
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(58) |
Leaf sheath glabrous at margin below collar. Ligule a minute ciliate fringe. Spikelets small, under 2 mm long. Lemma and palea only a little longer than seed, not gaping. Stamens always 3. Top of seed with broadly rounded corners (remove seed from fruit; see Faccenda et al., 2024, p. 50 for instructions). { Medium, perennial, clumping grass, presently found on disturbed sites on Kaua‘i, O‘ahu, and Moloka‘i. West Indian dropseed, smutgrass. [HAW - NAT] ;
Sporobolus indicus (L.) R. Br.
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59b
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Leaf collar and sheath margins pilose (dense with hairs); ligule ciliolate (short dense hairs) on a low membrane. Spikelets elongate, acute, 1.8 to 2 mm long. Stamens usually 2 per floret. Glumes short, obtuse or erose, the upper about half the length of the spikelet, the lower shorter still. Seed truncate at top, with sharp corners. { Small clumping grass found as a weed in lawns and on disturbed sites, including dry coastal sites. Presently known from Midway and all the major Hawaiian Islands. Smutgrass [HAW - NAT] rat-tail dropseed [GUM - NAT]
Sporobolus elongatus R. Br.
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61a
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(56) |
Lower inflorescence branches whorled. { Typically growing in saline, calcareous soils .. .
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[62] |
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61b
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Lower inflorescence branches not whorled; primary and secondary branches spreading, somewhat inflated spikelets mostly on outer half of branches. Spikelets 0.7-1.1 mm long, lead gray to purplish in color. Lower glume smaller than upper or sometimes missing. { Weedy in plant nurseries and moist disturbed sites. [HAW - NAT]
Sporobolus tenuissimus (Mart. ex Schrank) Kuntze
Somwhat resembles in habit, Eragrostis amabilis.
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62a
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(61) |
Leaf hairs not papillose-based; leaf margins scabrous, not spiny. Inflorescences narrow panicles on culms extending outward (some geniculate) from a somewhat to greatly flattened basal rosette of narrow leaves (see Fig. 11E). Upper glume as long as spikelet (1.8 - 2.5 mm); lower glume very short (0.4 mm). Ligule of short, stiff hairs (ciliate). { found in dry, leeward areas on limestone soils. May grow to 35 cm or more in height. [HAW - NAT]
Sporobolus pyramidatus (Lam.) Hitch.
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62b
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Leaves often with papillose-based hairs and leaf margins conspicuously spiny, often with long bristles. Panicle ovate, 2-7 cm tall; lowest node branches whorled. Spikelets 1-1.7 mm long. { [HAW - NAT] Sporobolus coromandelianus (Retz.) Kunth
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63a
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(57) |
Leaf collar and margins sparsely pilose (hairy); ligule membranous. Panicle narrow, but with branches spreading at maturity (see Fig. 11D, below). Secondary branches lacking florets on lower 1/6–1/4, these branches 1–4 (rarely up to 10) cm long; spikelets 1.2–1.6 mm long, somewhat diffuse. Floret with two stamens (view at anthesis). { Small, perennial, clumping grass, widespread on disturbed sites, particularly coastal dry sites and limestone substrate (Figs. 11A, 11D). Indian dropseed [HAW - NAT] wire grass [GUM - NAT]
Sporobolus diandrus (Retz.) P. Beauv.
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63b
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(57) |
Generally much larger than others in the S. indicus complex, reaching 1.5 m (4+ ft), panicles to 0.5 m and light straw-colored. Secondary inflorescence branches with florets to the base, loosely divergent from the rachis, branches <2 cm long. Spikelets 1.6–2.0 mm long. { Common on most islands in Hawai‘i, but easily confused with S. africanus and others [HAW - NAT] [GUM - NAT]
Sporobolus fertilis (Steud.) Clayton
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Species of Sporobolus reported to be in Hawai‘i but not covered in this key:
S. piliferus (Trin.) Kunth - panicle branches whorled. No consistent
differences with S. pyramidatus; a misidentification (see Imada, 2019).
Species of Sporobolus reported to be in the Marianas but not covered in this key:
S. ferinosus Hosok.
S. pyramidalis P. Beauv.
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SPOROBOLUS PHOTOS
[CLICK ON THUMBNAIL TO OPEN AN ENLARGED IMAGE]
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Figure 11D. Inflorescences of Sporobolus diandrus demon-
strating progressive spreading
open of branches upwards as
the head matures (0.5 MB).
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Figure 11E. Typical habit of
Sporobolus pyramidatus
showing outwardly extended
culms (6.9 MB).
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