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A CPIE Notebook Project – Grasses and Sedges of Hawai‘i Page 7

Key to Genus Digiteria Haller

D. ciliaris habit

Grasses of the genus Digitaria are commonly called "crabgrasses" and are fairly distinguishable in the field: tufted or spreading grasses with two to many thin racemes angled upwards off the axis, arranged digitately in some species. The racemes have 2-flowered spikelets (first floret reduced to a sterile lemma) on pedicels. Spikelets are elliptic to elongate in shape, compressed, and pressed against the rachis. First glume either much reduced or absent in most species. Ligule typically short, scarious or membranous. Sourgrass (D. insularis), being plumose (paniculate with numerous branches), is distinctive but has a gestalt not readily associated with other Digitaria in Hawai‘i.

Eleven species of Digitaria are known to be present in Hawai‘i (Clayton & Snow, 2010) and many are commonly encountered as weeds in lawns and disturbed sites, as well as pasture grasses. All Digitaria species in Hawai‘i are naturalized species (non-native), except D. setigera (kūkaepua‘a)—a southern Asia species distributed across the Pacific Islands—is possibly indigenous or a very early Polynesian introduction (Wagner, Herbst, & Sohmer, 1990).

Unfortunately, while most Digitaria species stand out as members of the genus, differentiating between them usually requires careful inspection of the structure of the spikelet, something not achievable without magnification under a dissecting microscope. Although in time we will attempt to make this key more general-user friendly, the approach must follow published keys and descriptions (Clayton et al., 2006 onwards) to maintain accuracy in the identifications. Making the task more difficult is a sometimes poor agreement among published keys.


35a (46) Elongated spikelets silky, covered in numerous long hairs, grouped in twos, threes, or more on long (some branched) pedicels. Rachis narrow, not winged, evenly three-sided. Inflorescence a plume-like panicle, with racemes typically all leaning or nodding in same general direction. { Very common, medium-size perennial, clumping grass in dry to mesic, disturbed sites. Sourgrass. [NAT]
    Digitaria insularis (L.) Mez ex Ekman
35b Spikelets either glabrous (without hairs), sparsely hairy, wooly with short hairs, or having sparse long hairs (ciliate), but not densely silky; rachis winged. { Spreading or clumping, annual or perennial grass

36a (35) (46)

Spikelets in clusters of 3 to 5, at least in the middle part of the raceme. Spikelets elliptic in outline, slightly plump, less than 2 mm long. Second or upper glume as long as or nearly as long as spikelet; first glume minute or absent. Rachis narrowly winged with a rounded midrib

36b Magnifying Lens Spikelets in pairs, one on a short pedicel, the other on a long pedicel; spikelet shape and size variable; glumes variable. Rachis narrowly winged with an angular midrib [37]
37a (36) Magnifying Lens Spikelets without hairs (glabrous), 1.2 to 2 mm long, elliptic in outline; lower glume 0.1 mm long. { Mat-forming grass spreading by rhizomes. [NAT]
    Digitaria abyssinica (Hochst ex A. Rich.) Stapf.

Some spikelets with hairs

38a (37) Racemes with basal 0.5 in (and up to several inches) without spikelets. { Perennial. [NAT]
    Digitaria divaricatissima (R. Br.) Highes

Racemes with spikelets almost down to the base

39a (38) Lower spikelet of each pair, hairy, oblong, 2.2-3.5 mm in length. Ligule fimbriate. Stolons and stems hairy, leaf blades rough (scaberulous) on both sides. { Perennial, densely tufted grass between 1 and 4 ft (35-140 cm) tall. A popular pasture grass on Hawai‘i Island. Pangola grass. [NAT]
    Digitaria eriantha Steud.

Grass otherwise

40a (39) Magnifying Lens Edges of rachis smooth. Racemes usually three in number, the lowest typically reduced in length, or sometimes absent. { A delicate, typically lanky, small grass of lawns and disturbed locations in mesic to wet environments. (Fig. 7C). [NAT]
    Digitaria radicosa (J. Presl.) Miq.

Edges of rachis scabrid (rough, due to very tiny teeth along edges)

41a (40) Magnifying Lens Upper glume short, less than 1/4 length of spikelet; lower glume short (<0.3 mm) or absent. Racemes 3 to 15 in number, sometimes digitate. { Small or medium size, annual clumping grass of mostly disturbed sites. Kūkaepua‘a, mau‘u kūkaepua‘a, itchy crabgrass. [IND?]
    Digitaria setigera Roth

Upper glume longer, at least 1/3 length of spikelet to 4/5 length of spikelet

42a (41) Magnifying Lens Spikelets 2 - 2.5 mm long (0.1 inch or less). Upper glume between 1/3 and 1/2 length of spikelet; lower glume minute or obscure. [NAT]
    Digitaria horizontalis Willd.

Spikelets usually greater than 2.5 mm long (>0.1 inch). Upper glume greater than 1/2 length of spikelet, typically closer to 2/3 length of spikelet; lower glume triangular, 0.2-0.5 mm long

43a (41) Magnifying Lens Racemes digitate or subdigitate (arising whorled from one or two nodes), 6 in (15 cm) long or longer. Spikelets nearly 3 mm long (>0.1 inch). Ligule a membrane with a thin, fimbriate margin. { A fairly robust perennial forming dense tussocks or spreading by stolons; grows in wetter areas; popular pasture grass on Hawai‘i Island. Pangola grass. [NAT]
    Digitaria eriantha Steud.

Annual grass not so robust (but culms may reach 1 m), spreading by decumbant culms rooting at nodes. Racemes more or less digitate. Ligule membranous, erose.

44a (43) Racemes 2 to 4 in number, stiff, arising from a common point (or paired). Upper glume surface glabrous, margins cliate to setose. First lemma of lower spikelet with veins ribbed, spaced equally apart. { Annual, rooting at lower nodes. A weed in turfgrasses. Tropical or Asian crabgrass. [NAT]
    Digitaria bicornis (Lam.) Roem. & Schult.

Racemes 2 to 12, stiff or flexible, arranged somewhat digitately or with one or more up to 1 cm below others. Hairs on spikelets variable from ciliate (long stiff hairs) to appressed puberulent (having dense, short, somewhat curly hairs), these mostly or only on margins of upper glume and first lemma. Veins of first lemma usually widely spaced flanking the midvein. Upper glume surface pubescent between three veins, margins cliate.{ Highly variable, typically small grass, spreading to form untidy patches in lawns and pastures. Henry's crabgrass, kūkaepua‘a. (Figs. 7A & 7B. [NAT]

    Digitaria ciliaris (Retz) Koeler
45a (36) Magnifying Lens Spikelets entirely without hairs (glabrous), and 1.2-1.6 mm long; fruit inside pale or light brown; Lemma with (3) or 5 veins; upper glume as long as spikelet. Racemes usually two or three, digitate (attached close together). { small, creeping annual forming mats in disturbed (mesic) locations. Creeping kūkaepua‘a [NAT]
    Digitaria fuscescens (J. Presl.) Henrard

Spikelets with hairs although these variable, sometimes sparse, occasionally a spikelet with rows of stiff hairs beside the veins; spikelets (1.2) 1.4 to 1.7 mm long; fruit inside dark. Upper glume just short of (>75% ) spikelet length. Lemma with 5 or 7 veins, middle three close and evenly spaced. Racemes typically 3 or more, spread along culm axis. { Fairly common, small annual or perennial clumping grass in mesic environments. Smooth or violet crabgrass, kūkaepua‘a uka [NAT]

    Digitaria violascens Link


Species of Digitaria reported to be in Hawai‘i and either not covered
  in this key or included under a more recent name (= accepted name):

    D. adscendens (Kunth) Henr. (=D. ciliaris)
    D. decumbens Stent (=D. eriantha)
    D. pentzii Stent (=D. eriantha)
    D. pruriens (Trin.) Buse (=D. setigera)
    D. sanguinalis (L.) Scop. (=D. ciliaris)
    Tricachne insularis (L.) Nees (=D. insularis)


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Fig. 7B. Scanned parts of Digitaria ciliaris (Henry's crabgrass) showing fairly typical inflorescence of a Digitaria (1.1 MB).

Fig. 7C. Scanned parts of Digitaria radicosa showing lanky growth form (habit) and typical Digitaria inflorescence (4.0 MB).

© 2012-17 AECOS, Inc. [FILE: GrassKey_Digiteria.html] Genus Digitaria – Page 7
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