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A CPIE Notebook Project – Grasses of Hawai‘i and Guam Digiteria –Page 7
Tribe Paniceae
Key to Genus Digitaria Haller
D. ciliaris habit

Grasses of the genus Digitaria are commonly called "crabgrasses" and may be tufted (bunching) or spreading by stolons or rhizomes. Culms can be erect or decumbant (declining but with terminal part bending upwards). The inflorescence comprises many thin racemes usually angled upwards off the axis, arranged digitately (arising close together) 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; the upper surface is flattened, the lower more rounded. 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) although Faccenda (2022, 2023) has added an additional 5 species (he provides a key). 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). On Guam, in contrast, nine Digitaria species are listed by Raulerson (2016), with five regarded as indigenous and one as endemic (D. gaudichaudii).

Fig. 7B. Spikelet magnified
Figure 7B. Digiteria ciliaris spikelet in anthesis, showing small, triangular lower glume and 1st lemma. Note that next spikelets above (on right) are paired, one is undeveloped (whitish, under stigma, atypical) on a short pedicel, the other on a long pedicel, both arising at a common point near the base of a wavy midrib that runs down the middle of the rachis. This midrib makes the rachis trigonus in cross-section and separates alternating pairs (in D. ciliaris) of spikelets.

Unfortunately, whereas most crabgrasses stand out as members of the genus, Digitaria, differentiating between them requires very careful inspection of the structure of the spikelets, something not achievable without good magnification (see Figure 7B). Although we would like to make this key more general-user friendly, the approach must follow published keys and descriptions (Clayton et al., 2006; Faccenda, 2023) to maintain accuracy in the identifications. Papers by Faccenda (2022, 2023) represent a significant revision of the genus Digiteria in the Hawaiian Islands and our key is based on Faccenda's 2023 key.

Under Construction Sign
[GENUS DIGITERIA]

  
35a (To previous couplet46) Fig. 7C. Sourgrass inflorescence 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 (see Fig. 7C, left). { Very common, medium-size perennial, clumping grass in dry to mesic, disturbed sites. Sourgrass [HAW - NAT] [GUM - 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 or not. { Spreading or clumping, annual or perennial grass

[36]
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
NOTE: THREE CHOICES HERE:
36a (35
  &
To previous couplet46)

Magnifying Lens Spikelets in clusters of 3 in lower to middle part of the raceme. Spikelets elliptic to lanceolate in outline, less than 2.5 mm long. First (lower) glume minute or absent. Rachis narrowly winged and with an angular midrib, or ribbon-like, midrib rounded .. .

[56]
36b Spikelets in pairs (look in middle part of raceme), one on a short pedicel, the other on a long pedicel (see Fig. 7B, above). Spikelet shape elliptic to lanceolate, with tips blunt or pointed (acute); size variable; glumes variable. Rachis winged or not (then triangular in cross-section) .. . [37]
36c

All spikelets single on the rachis; first glume absent, second glume short or absent. Requires searching lower to middle parts of raceme, as most Digiteria have areas where only single spikelets occur or the pedicellate spikelet is incompletely formed (see Fig. 7B). Rachis narrowly winged with an angular midrib .. .

[60]
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
37a (36) Clumping perennial, culms erect. Plant without rhizomes or stolons .. . [38]
37b (36)

Creeping perennial or annual. Plants with rhizomes or stolons and/or culms decumbant and rooting at the nodes .. .

[40]
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
38a (36) Magnifying LensSpikelets less than 2 mm long; lower glume absent. {
    Digitaria orbata Hughes.
38b

Spikelets closer to 3 mm long; lower glume present .. .

[39]
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
39a (38) Racemes with basal 0.5 in (and up to several inches) without spikelets; spikelets above spaced apart and mostly not overlapping. { Perennial. [HAW - NAT]
    Digitaria divaricatissima (R. Br.) Highes
39b Racemes with spikelets not as above; spikelets close and overlapping. Lower spikelet of each pair, hairy, oblong, 2.2-3.5 mm in length. Both glumes about as long as spikelet or second a little less. 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 [HAW - NAT] (see also [52a])
    Digitaria eriantha Steud.
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
40a (37) Magnifying Lens Rachis triangular but lacking wings (no green tissue on rachis edges) .. . [41]
40b

Rachis winged with green tissue along all three edges .. .

[45]
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
41a (40) Magnifying Lens Spikelets without hairs (glabrous). Lower glume present or absent .. . [42]
41b

Spikelets with hairs. Lower glume distinct or absent .. .

[43]
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
42a (36) Magnifying LensLeaves wide, 5-12 mm (1/4-1/2 in) across, hairy. Spikelets 1.2 to 2 mm long, elliptic in outline; lower glume 0.1 mm long, lacking a hyaline margin. { Mat-forming grass spreading by rhizomes. USDA noxious weed. [HAW - NAT]
    Digitaria abyssinica (Hochst ex A. Rich.) Stapf.
42b (36) Leaves narrow (3-6 mm [up to 1/4 in] across), with only sparse hairs. Rhizomes well-developed, with vertical offshoots much branched at the base. Ligule 1–3 mm. tall, truncate, erose or subentire. Spikelets 1.6 to 2.4 mm long. lower glume variable to 1/4 length of spiklelet, hyaline. { Distinctive, forming very dense mats (monotypic stands), spreading by rhizomes. USDA noxious weed. [HAW - NAT]
    Digitaria scalarum (Schweinf.) Chiov.
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
43a (42) Inflorescence branches 2–4; leaves narrow, under 6 mm (<1/4 in) across, often bluish. { Densely stoloniferous and/or rhizomatous, forming a tightly matted turf; lawn weed. Blue Couch [HAW - NAT]
    Digitaria didactyla Willd.
 [42]
43b

Inflorescence branches more than 4; leaves vary from narrow up to 17 mm (0.7 in) across. .. .

[44]
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
44a (43) Annual. Sometimes rooting at nodes but not stoloniferous. Lower racemes often compound at their base. Upper glume 1.5–1.7 mm, 3/4 as long as the spikelet. { Inflorescences with racemes either held nearly perpendicular to the axis or all racemes erect and parallel to the axis. A federal noxious weed, naturalized in up-country Maui. [HAW - NAT] (see couplet [48a])
    Digitaria velutina Mez
44b Perennial, strongly stoloniferous. All racemes simple. Lower spikelet of each pair, hairy, lanceolate or elliptic 2.2-2.9 mm in length. First glume absent or a minute scale; second glume as long as spikelet. Stolons and stems hairy. { . [HAW - NAT]
    Digitaria eriostachya Mez
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
45a (40) Magnifying Lens Sessile and pedicellate spikelets identical with respect to veination (check middle to apex of racemes branches): veins on lower glume spaced equally apart .. . [46]
45b

Racemes 2 to 4 in number. Sessile and pedicellate spikelets dimorphic: first lemma of sessile spikelet with veins ribbed, spaced equally apart; pedicellate spikelet with veins close to the margins; mature pedicellate spikelets may be densely hairy. Spikelets 3 mm long. Lower glume triangular. Upper glume 2/3 length of spikelet. { A weed in turfgrasses. Tropical or Asian crabgrass very similar to D. ciliaris [HAW - NAT] [GUM - NAT] [FACU]

    Digitaria bicornis (Lam.) Roem. & Schult.
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
46a (47) Recemes held vertical or nearly so .. . [47]
46b

Recemes strongly divergent from the axis .. .

[49]
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
47a (46) Magnifying Lens Spikelets 2-3 mm long. Upper glume up to 1/3 length of spikelet; lower glume absent. Racemes 3 to 15 in number, sometimes digitate. Ligule a prominant, erose membrane. { Small or medium size, annual clumping grass of mostly disturbed sites (Fig. 7D). Kūkaepua‘a, mau‘u kūkaepua‘a, itchy crabgrass [HAW - IND?] [GUM - IND]
    Digitaria setigera Roth
47b

Upper glume longer, at least 1/2 length of spikelet .. .

[48]
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
48a (43) Magnifying Lens Annual. Leaf sheaths and blades hairy. Sometimes rooting at nodes but not stoloniferous. Lower racemes often compound at their base. Spikelet 1.5-2.0 mm in length. Upper glume 3/4 as long as the spikelet.{ A federal noxious weed, naturalized in up-country Maui. [HAW - NAT] (see couplet [53a])
    Digitaria velutina Mez
48b Leaf sheaths without hairs. Racemes 2-9 cm long; lower racemes not compound at base; lowest node of panicle without hairs. Lanceolate spikelet 2.1-2.7 mm in length. Lower glume about 0.2 mm long; upper glume 1/2 as long as spikelet. { Widespread at low elevations. Resembles D. ciliaris, but with racemes helds erect. [HAW - NAT]
    Digitaria henryi Rendle
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
49a (47) Magnifying Lens Veins of first (sterile) lemma unequally spaced: wide gap between midvein and adjacent veins, narrow gap between outer vein pairs .. . [50]
49b

Veins of first (sterile) lemma equally spaced .. .

[52]
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
50a (49) Magnifying Lens Lower glume absent, upper glume up to 1/3 length of spikelet [HAW - NAT] (see couplet [47a]).
    Digitaria setigera Roth
50b

Lower glume present or absent, but upper glume greater than 1/2 length of spikelet .. .

[51]
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
51a (39) Magnifying Lens Edges of rachis smooth. Racemes usually three in number, the lowest typically reduced in length, or sometimes absent. Stolons capillary (very narrow in diameter). { A delicate, typically lanky, small grass of lawns and disturbed locations. (see Fig. 7D). [HAW - NAT] [GUM - IND]
    Digitaria radicosa (J. Presl.) Miq.
51b

Hairs at lowest node of the inflorescence up to 1 mm long. Racemes 2 to 12, highly variable in length. Edges of rachis scabrid (rough, due to very tiny teeth along edges). Spikelets 2 - 3.7 mm long; 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; veination and pubescence identical on both spikelets of a pair. { Variable (possibly because of many misidentifications attributed to this species) and common, small crabgrass, spreading to form untidy patches in lawns and pastures (Figs. 7A, 7B, & 7D). Henry's crabgrass, kūkaepua‘a [HAW - NAT] [GUM - IND]

    Digitaria ciliaris (Retz) Koeler
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
(49) 52a Inflorescence branches without cilia. Spikelets greater than 2.5 mm long .. . [53]
52b

Inflorescence branches sometimes with cilia. Spikelets less than 2.5 mm long .. .

[54]
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
53a Magnifying Lens Racemes digitate or subdigitate (arising whorled from one or two nodes), 6 in (15 cm) long or longer. Inflorescence branches (racemes) without cilia. Ligule a membrane with a thin, fimbriate margin. Spikelets 2.8–3.5 mm long. Lower glume 0.5-0.6 mm long; upper glume a little more than 1/2 length of spikelet. { A fairly robust perennial forming dense tussocks or spreading by stolons; grows in wetter areas; popular pasture grass on Hawai‘i Island. Pangola grass [HAW - NAT] (see couplet [39b]).
    Digitaria eriantha Steud.
53b

Racemes 2 to 3. Spikelets 3 to 3.5 mm long. Lower glume ovate, 0.5 mm long; upper glume at or a little over 1/2 length of spikelet; upper glume surface pubescent between three veins, margins ciliate. Lower lemma five-veined, pubescent, acute. { Typically clumping small crabgrass. [GUM - IND]

    Digitaria mezii (Retz) Koeler
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
54a (52) Magnifying Lens Panicle with secondary branches from base of lower panicle branches; florets <2 mm long; upper glume >3/4 as long as floret. [HAW - NAT] (see couplet [48a]).
    Digitaria velutina Steud.
54b

Panicle without secondary branches; florets 1.7–2.4 mm long; upper glume 1/3–4/5 as long as floret

[55]
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
55a (54) Magnifying Lens Upper leaf surface glabrous or with a few hairs near base. Upper glume 2/5–4/5 as long as floret; lower glume usually absent on most florets. { [HAW - NAT]
    Digitaria nuda Schumach.
55b Upper leaf surface usually hairy. Upper glume between 1/3 and 1/2 length of spikelet; lower glume present on most spikelets. { Uncommon on O‘ahu only; Racemes held vertically together (see Fig 7E, below). [HAW - NAT]
    Digitaria horizontalis Willd.
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
56a (36) Magnifying Lens Plant upright, clumping, inflorescence branches 2 to 9. Upper lemma dark brown to violet in color .. . [57]
56b Plant decumbant with stolons. Inflorescence branches 2 or 3(5) Upper lemma light in color .. . [58]
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
57a (56) Magnifying Lens Apex of pedicel without any hairs. Spikelets elliptic, 1.5 - 1.7 mm long. First glume absent; second glume 3/4 as long as spikelet. florets with pilose hairs or appearing nearly glabrous. { Smooth or violet crabgrass [HAW - NAT]
    Digitaria violascens Link
57b Apex of pedicel (base of spikelet) with a crown of hairs. Spikelets elliptic, 1.2 - 1.4 mm long. First glume absent; second glume 1/4 to 1/2 as long as spikelet (or rarely absent). Sterile lemma with conspicuous club-shaped hairs. { Widely distributed on O‘ahu. [HAW - NAT]
    Digitaria stricta Roth
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
58a (56) Magnifying Lens Leaf sheaths and blades conspicuously hairy (pilose). Racemes 2 or 3, digitate. Spikelets 2.1–2.3 mm long, hairy { small, creeping annual or perennial; rare on Hawai‘i Island. [HAW - NAT].
    Digitaria mollicoma (Kunth) Henrad
58b Leaf sheaths largely glabrous. Racemes usually two or 3(4), digitate (attached close together). Spikelets 1.2–1.6 mm long, elliptic or slightly obovate, acute; fruit inside pale or light brown; lower glume absent; upper glume about as long as spikelet .. . [59]
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~
59a (58) Spikelets with hairs. { Small, spreading by long, branching stolons. Rare on O‘ahu or possibly not naturalized or is D. fuscescens. [HAW - NAT]
    Digitaria longiflora (Retz.) Pers.
59b

Spikelets entirely without hairs (glabrous). { Small, creeping annual forming mats in disturbed (mesic) locations. Creeping kūkaepua‘a [HAW - NAT]

    Digitaria fuscescens (J. Presl.) Henrard
~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~ Spikelets single on rachis
60a (36) Magnifying Lens Leaf blades under 1 inch (1-2 cm) long. Racemes paired. Glumes absent. Sterile lemma with 5 smooth veins. { Small, prostrate hairy, creeping annual with slender stems; originally reported from Cabras Island. [GUM - IND]
    Digitaria mariannensis Merr.
60b

Leaf blades 6-12 in (15-25 cm) long; Racemes 7 to 16 in number. Upper glume against rachis about 1/6 length of spikelet. Sterile lemma with 5 scabrid veins. { Moderate-sized, erect, hairless grass; originally reported from Rota and Cabras on Guam. [GUM - END]

    Digitaria gaudichaudii (Kunth) Henrad

Species of Digitaria reported to be in Hawai‘i or Guam 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)
    D. timorensis (Kunth) Balansa (=D. radicosa)
    Tricachne insularis (L.) Nees (=D. insularis)

CRABGRASS PHOTOS
[CLICK ON THUMBNAIL TO OPEN AN ENLARGED IMAGE]

Thumbnail Thumbnail Thumbnail

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

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

Fig. 7F. Scanned image of Digitaria sp. (indet.) Note racemes held vertical together at anthesis (1.1 MB).
 
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