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CPIE Notebook Project - Key to the Grasses of Hawai‘i Page 11

Key to Genus Sporobolus R. Br.

Sporobolus inflorescences 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 S. africanus) 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. 11B) rather than rattail-like; this native dune grass is being used for verge planting along Mokulele Highway on Maui.

Sporobolus diandrus starts out with the branches closely appressed to the axis as in the other species, but the branches spread open as the flowers mature (see Fig. 11A at right). Sporobolus indicus, S. africanus, and S. fertilis are difficult to distinguish as characteristics of each tend to intergrade (Clayton & Snow, 2010).

55a (53) Perennial, clumping grasses. Leaves mostly crowded near the base, lax and/or wirey [56]

Sporobulus virginicus in flower Perennial, spreading grass, producing extensive stiff rhizomes. Leaves stiff, distinctly distichous (tightly two ranked) in typical dune habitat. See Fig. 11B (left). 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. Vegetative growth resembles Paspalum vaginatum and Distichlis spicata, species which also grow in or near saline water. ‘aki‘aki, seashore rush grass, beach dropseed. (Fig. 11B) [IND]

Sporobolus virginicus (L.) Kunth

Figure 11B. Well developed inflorescences of ‘aki
(Sporobolus virginicus) growing under wetter
than normal conditions.

56a (55) Leaf collar and margins sparsely pilose (hairy); ligule membranous. Panicle narrow, but with branches spreading at maturity (see Fig. 11C, below). Floret with two stamens (best viewed at anthesis). Spikelets somewhat diffuse. { Small, perennial, clumping grass, widespread on disturbed sites, particularly coastal dry sites and limestone substrate. Indian dropseed. (Figs. 11A, 11C) [NAT]
Sporobolus diandrus (Retz.) P. Beauv.

56b Leaf collar and margins pilose or without hairs (glabrous); ligule of fine hairs (ciliate or ciliolate). Panicle narrow, cylindrical, branches held against the axis (appressed) or upright at maturity. Floret with two or three stamens. Spikelets very crowded

57a (56) Spikelets greater than 1.5 mm long. Stamens 3 per floret.


Leaf collar and margins pilose (dense with hairs); ligule ciliolate (short dense hairs). Spikelets elongate, acute, to 2 mm long. Stamens 2 per floret. Glumes short, obtuse. { Small clumping grass found as a weed in lawns and on disturbed sites, including dry coastal sites. Smutgrass. [NAT]

Sporobolus elongatus R. Br.
58a (57)

Spikelets small, under 2 mm long. Fruit brown { Medium, perennial, clumping grass, found on disturbed sites from sea level up to 6200 ft (1890 m) elevation. West Indian dropseed, smutgrass. [NAT]

Sporobolus indicus (L.) R. Br.

58b Spikelets 2 to 3 mm long. Fruit red { Medium, perennial, wiry, clumping grass, widespread on disturbed sites and pastures above 500 ft (150 m) elevation. Smutgrass, African dropseed, rattail grass. [NAT]
Sporobolus africanus (Poir.) Robyns & Tournay
      Under Construction Sign

Species of Sporobolus reported to be in Hawai‘i
  but not covered in this key:

    S. fertilis (Steud.) Clayton - similar to S. indicus and S. africanus
    S. piliferus (Trin.) Kunth - panicle branches whorled
    S. pyramidatus (Lam.) Hitchc. - panicle branches whorled


Sporobolus diandrus

Figure 11C. Inflorescences of Sporobolus diandrus demonstrating progressive spreading of branches upwards from base as the heads mature (0.5 MB).


Table of Contents TABLE OF CONTENTS  To grass inflorescence types INFLORESCENCE TYPES     Grass Key Introduction INTRODUCTION       AECOS AECOS, Inc.

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