Smooth Crabgrass

Digitaria ischaemum is very close to D. violascens, but has a slightly stouter habit with broader leaves, a few short, widely spreading racemes, and larger, plumper spikelets. It also has a more temperate distribution.

Digitaria ischaemum, D. fauriei, D. longiflora, and D. violascens belong to a group of Digitaria known as the Verrucipilae because of their warty spikelet hairs. A compound microscope is required to see this, but the hairs have a beadlike appearance under moderate magnification. These spikelet hairs also have curled tips in D. ischaemum (and sometimes in D. violascens), which led to the hairs being described as capitate or clavate in the past.

Digitaria ischaemum is much more closely related to Digitaria violascens than has hitherto been supposed. Henrard placed it in his section Clavipilae, along with Digitaria stricta, but close examination of the hairs has shown that they are not in fact clavate Although they appear, under moderate magnification, to have a clavate tip, this is not due to a swelling, but to a curious crozier-like curling of the tip of the hair itself. Furthermore, when viewed under higher magnification (preferably through a compound microscope), the hairs can be seen to be verrucose. Apart from geographical distribution, the main distinctions between Digitaria ischaemum and Digitaria violascens lie in the larger spikelets of Digitaria ischaemum and its longer, matted, pseudo-clavate hairs.

Annuals; culms tufted, erect or ascending, 30-60 cm high. Sheaths loose, keeled, usually shorter than internode; ligule membranaceous, ca. 2 mm long; blade 2-10 cm long, 2-5 mm wide, leaf-base rounded. Racemes 2-5, 1-20 cm long, digitately arranged; rachis 0.8-1.2 mm wide, indurate, margins scabrous, green; midrib white, narrow, each node with 2-3 spikelets. Spikelets elliptic, 1.5-2.3 mm long, greyish green or purplish; lower glume minute or absent, translucently membranaceous, veinless; upper glume equaling spikelet or slightly shorter, narrow, 3-veined, interveins and margins coverd with soft clavate hairs; lower lemma 5-veined, interveins and margins also clavate hairs; upper floret coriaceous, blackish brown when mature, equaling spikelet. Caryopsis ca. 1.2 mm long.

Annual, whole plant often purple-tinged. Culms loosely tufted, erect or ascending, 15–40 cm tall. Leaf sheaths loose, usually keeled, glabrous or pilose; leaf blades linear-lanceolate, 5–12 × 0.4–0.8 cm, ± pilose, base subrounded, apex acuminate; ligule ca. 0.6 mm. Inflorescence digitate or on a short axis; racemes 2–4, divaricate, 2–9 cm; spikelets ternate; rachis ribbonlike, winged, 0.8–1.1 mm broad, midrib white, rounded or triquetrous, narrower than the green or purple wings, margins serrulate; pedicels angular, scabrous, tips discoid. Spikelets elliptic, 2–2.2 mm, pilose with verrucose hairs, some with curled tips; lower glume absent or a tiny hyaline rim; upper glume as long as spikelet or slightly shorter, 3–5-veined, intervein spaces and margins appressed-pilose; lower lemma as long as spikelet, 5–7-veined, interveins and margins appressed-pilose; upper lemma purplish brown to blackish at maturity. Fl. and fr. Jun–Nov. 2n = 36.

Annual; culms (10-)1540 cm high, erect or geniculately ascending. Leaf-blades 2.5-11 cm long, 3.5-8 mm wide. Inflorescence composed of 2-4(-6) racemes; racemes 2.5-8(11) cm long, subdigitate or arranged along an axis 1.5-2(4) cm long, the spikelets ternate on a ribbon-like winged rhachis with rounded or triquetrous midrib; pedicels terete to flattened and winged, scabrid, with discoid or cupuliform tip. Spikelets elliptic or lanceolate, 1.9-2.4 mm long; lower glume an obscure hyaline rim; upper glume almost as long as the spikelet, 5-nerved, with matted verrucose hairs between the nerves; lower lemma as long as the spikelet, 7-nerved, pubescent with matted verrucose hairs; fruit ellipsoid, dark brown to almost black.

The temperate regions of the world. Taiwan, in open waste places of the northeastern part.

Distribution: Pakistan (N.W.F.P., Gilgit & Kashmir); North America; Europe extending eastwards into northern Asia.

Other, unlocalised, records are Kashmir, J.L. Stewart 850b(K) and Gilgit, R.R. Stewart 26652(K).

Fl. & Fr. Per.: July-August.

Open grassy places. Anhui, Fujian, Gansu, Hebei, Heilongjiang, Henan, Jiangsu, Jilin, Liaoning, Nei Mongol, Ningxia, Shaanxi, Shandong, Shanxi, Sichuan, Taiwan, Xinjiang, Xizang [Japan, Pakistan, Russia; Europe, North America].

Panicum ischaemum Schreb., Spec. Fl. Erlang. 16. 1804.

Panicum ischaemum Schreber in Schweigger, Spec. Fl. Erlang. 16. 1804; Digitaria asiatica (Ohwi) Tzvelev; D. humi-fusa Persoon; D. ischaemum subsp. asiatica (Ohwi) Tzvelev; D. ischaemum var. asiatica Ohwi; Panicum humifusum (Persoon) Kunth; Paspalum humifusum (Persoon) Poiret; Syntheris-ma humifusa (Persoon) Rydberg.