In New York State there are three nonnative crab grasses, Digitaria ciliaris, D. ischaemum, and D. sanguinalis. These annual grasses begin flowering in mid-summer and continue to flower and fruit until frost knocks them back. The native range of D. ciliaris is unclear. Recent authors (e.g., Wipff 2003, Shouliang and Phillips 2006) considered it to be widespread in tropical and warm-temperate parts of the world. Digitaria ischaemum and D. sanguinalis are native to Eurasia (Wipff 2003). All three are widespread and common throughout New York and occur in a variety of native and nonnative disturbed habitats. They all look superficially similar in that they have long finger-like branches of the inflorescences radiating from the summit or near the summit of the stem.
Inflorescence of Digitaria ciliaris
Each inflorescence branch has numerous spikelets that occur on short pedicels.
Inflorescence branch of Digitaria ischaemum
A closer look reveals that the spikelets are grouped together. In D. ischaemum the spikelets are grouped in threes and in D. ciliaris and D. sanguinalis they are grouped in pairs. The paired spikelets of the latter two species have the spikelets on different length pedicels, one very short and the other longer.
Digitaria ischaemum with spikelets occurring in groups of three. Some spikelets were removed and some were spread apart from each other for ease of observation.
Digitaria ciliaris with spikelets occurring in pairs. Some spikelets were spread apart from each other for ease of observation.
Digitaria ischaemum is also quite different from the latter two species in numerous other ways including:
|
Digitaria ischaemum |
Digitaria ciliaris and D. sanguinalis |
|
|
2nd lemma |
black, dark purple, or dark brown and mostly hidden behind the second glume |
yellow, green-gray, or blue-gray often suffused with purple, the upper portion readily visible |
|
2nd glume |
as long as or slightly shorter than the 2nd lemma |
about 40-75% as long as the 2nd lemma |
|
1st glumes |
absent or vestigial, when present a thin translucent truncate membrane ≤ 0.2 mm long; |
small but apparent, thick, opaque, 0.1-0.8 mm long |
|
spikelets |
1.8-2.3 mm long |
2.3-4.0 mm long |
Spikelet of Digitaria ischaemum
Spikelet of Digitaria ciliaris
Once one becomes familiar with these species the relatively small spikelets of D. ischaemum make it doable to distinguish it from the other two species from a distance. When necessary a quick closer examination of the plants can confirm the determination.
Digitaria ciliaris and D. sanguinalis are very similar species and the two can easily be confused. They are best distinguished by observing the lateral veins of the first lemmas. In D. ciliaris the lateral veins are smooth or sometimes have just a few small spicules while in D. sanguinalis many of the lateral veins are roughened at least in the upper half. The rough texture, created by numerous small spicules, can be difficult to discern without good lighting and magnification.
Spikelets of Digitaria sanguinalis (left) and D. ciliaris (right)
Additionally the upper surface of the uppermost leaf blades of D. ciliaris are sometimes scabrous but otherwise are glabrous or have only a few papillose-based hairs especially towards the base versus the upper surface of the uppermost leaf blade moderately or densely pubescent with papillose-based hairs in D. sanguinalis.
Upper surface of leaf blades of Digitaria sanguinalis (top) and D. ciliaris (bottom)
There are other differences too such as the relative length of the second glume to the spikelet and absolute length of the spikelet although there is some overlap with these characters. And, in New York, I have occasionally encountered intermediates. Intermediate plants have been reported from other areas and these have been hypothesized to be hybrids or represent some level of introgression between the two species (Ebinger 1962, Gould 1963, Webster and Hatch 1981).
The first lemma of Digitaria ciliaris and D. sanguinalis possess interesting hairs. The main hairs are soft, supple, and white (sometimes fading to off-white or pale brown). When young these hairs are densely packed together and appressed against the surface of the lemma. They primarily occur in two places, along the margins and in between the two inner lateral veins. When young they lie parallel to the veins and appear like another vein but when mature they fluff up and project out from the surface of the lemma, their tips often joined together.
Spikelets of Digitaria ciliaris immature (left) and mature (right)
In addition to the supple white hairs, the first lemmas of Digitaria ciliaris and D. sanguinalis sometimes also have stiff glassy yellow hairs, although I have not seen specimens of D. sanguinalis from New York with such hairs. When present, the stiff glassy yellow hairs usually occur with the supple white hairs between the two inner lateral veins and generally project well beyond the supple white hairs.
Spikelet of Digitaria ciliaris
There is a lack of consensus within the botanical community as to whether plants of D. ciliaris with stiff glassy yellow hairs are worthy of taxonomic recognition. Henrard (1950), in his worldwide monograph of the genus Digitaria, recognized these plants at the specific rank as D. chrysoblephara. Wipff (2003), in his treatment of Digitaria in the Flora of North America North of Mexico, recognized these plants as D. ciliaris var. chrysoblephara but noted that further study was needed. Shouliang and Phillips (2006), in the Flora of China noted that this “form” was sometimes distinguished as a variety. Wilhalm (2009), in his review of D. ciliaris in Europe, considered the plants with stiff glassy yellow hairs to represent only a form. Veldkamp (1973), in his revision of Digitaria of Malesia, considered D. ciliaris to never possess yellow glassy hairs and considered such plants to be part of the variability of the closely related D. bicornis. Webster (1987), in his revision of species closely related to D. ciliaris from North America, recognized that D. ciliaris can be variable for the presence or absence of glassy yellow hairs but did not give taxonomic recognition to plants of D. ciliaris with such hairs. In terms of application of names, Webster considered the name D. ciliaris var. chrysoblephara to be synonymous with D. bicornis while Wipff (2003) and Shouliang and Phillips (2006) placed it with D. ciliaris.
In specimens of D. ciliaris from New York that I have observed, when present, there are usually not a lot of these glassy yellow hairs per lemma and sometimes many of the lemmas appear to lack these hairs while others possess just one or two. It can be difficult to accurately assess if a specimen possesses such hairs because these hairs develop the yellow color as they mature and when immature are mixed in with the white supple hairs and are appressed with them against the lemma body, making them hard to discern. In 2020, I collected ten specimens of D. ciliaris from western, central, northwestern, and northeastern regions of New York. Upon a thorough examination of these specimens all turned out to possess at least some stiff glassy yellow hairs. This appears to be in contrast with Wipff’s (2003) statement that plants of D. ciliaris without stiff glassy yellow hairs are more common in North America north of Mexico than plants with these hairs.
Digitaria ischaemum and D. sanguinalis have long been known to be common in New York (e.g. House [1924]). On the other hand, Smith (1965), in his Checklist of The Grasses of New York, may have been the first to report D. ciliaris (as D. ascendens) from New York. He noted that it was only found as a waif in the southeastern part of the state. The NYS Museum Cards, a series of large manila cards that NYS Museum botanists used to record and map species distribution in the state, reflect Smith’s assessment of the species at the time; the NYS Museum Card map for D. ciliaris shows three sites for the species all in the southeastern part of the state.
NYS Museum Card map of Digitaria ciliaris showing three populations (black dots), which are restricted to the SE portion of the state.
But Digitaria ciliaris has since become widespread and common throughout much if not all of the state; between 2003 and 2020 I collected this species from all regions of the state and it was never too hard to find.
Map of NY showing where I collected herbarium vouchers for Digitaria ciliaris. Note that I have observed this species elsewhere in the state and there are herbarium specimens documenting other populations too.
It appears that this species has spread rapidly in the state in the recent past although a thorough review of herbarium specimens is needed to confirm this hypothesis. Yatskievych (1999) reported a similar recent (since 1963) spread of this species in Missouri. Digitaria ciliaris likely has spread beyond the borders of New York into some regions that have yet to report this species such as Ontario (Oldham 2017) and Vermont (Gilman 2015).
In New York, similar to D. ischaemum and D. sanguinalis, D. ciliaris grows in naturally disturbed habitats such as gravel and sand bars in rivers and draw-down zones of ponds, lakes, and rivers as well as in human disturbed sites such as agricultural fields, roadsides, and cracks in sidewalks in urban areas.
Habitat for Digitaria ciliaris in New York. Draw-down on edge of Lake Champlain, Clinton Co., NY
Habitat for Digitaria ciliaris in New York. Draw-down on edge of the Cayuga Inlet, Tompkins Co., NY
Habitat for Digitaria ciliaris in New York. Village of Friendship, Allegany Co., NY
Habitat for Digitaria ciliaris in New York. Agricultural field in Tompkins Co., NY
One final note. In preparing this blog post I realized that some of my recent collections of D. ciliaris appear to have at least some characteristics of the closely related as well as controversial species D. bicornis. Webster and Hatch (1981) provided some evidence that these two species are distinct and many authors continue to recognize them as such although with differing circumscriptions (Veldkamp 1973, Wipff 2003, Shouliang and Phillips 2006, Weakley 2015, Boonsuk et al. 2016). Others are a bit more skeptical (Wilhalm 2009). Digitaria bicornis is reported to be widespread in the tropics and subtropics of the world (Webster 1987) as well as to be common on the coastal plain of the southeastern United States, perhaps growing as far north as Virginia or Maryland (Wipff 2003). Clearly a modern revision of the D. ciliaris complex is needed. So stay tuned for more crab grass stories!
Literature cited:
Boonsuk, B., P. Chantaranothai, and T. R. Hodkinson. 2016. A taxonomic revision of the genus Digitaria (Panicoideae: Poaceae) in mainland Southeast Asia. Phytotaxa 246:248–280.
Ebinger, J. E. 1962. Validity of the grass species Digitaria adscendens. Brittonia 14:248–253.
Gilman, A. V. 2015. New flora of Vermont. Memoirs of the New York Botanical Garden 110:1–615.
Gould, F. W. 1963. Cytotaxonomy of Digitaria sanguinalis and D. adscendens. Brittonia 15:241–244.
Henrard, J. T. 1950. Monograph of the genus Digitaria. Universitaire Pers Leiden, Leiden, Netherlands.
House, H. D. 1924. Annotated list of the ferns and flowering plants of New York State. New York State Museum Bulletin 254. The University of the State of New York, Albany, NY, USA.
Oldham, M. J. 2017. List of the vascular plants of Ontario’s Carolinian zone (Ecoregion 7E). Carolinian Canada and Ontario Ministry of Natural Resources and Forestry, Peterborough, ON, Canada.
Shouliang, C., and S. M. Phillips. 2006. Digitaria Haller. Pages 539–547 in Z. Y. Wu, P. H. Raven, and D. Y. Hong, editors. Flora of China. Vol. 22 (Poaceae). Missouri Botanical Garden Press, St. Louis, MO, USA.
Smith, S. J. 1965. Checklist of the grasses of New York State. New York State Museum Bulletin 403. The University of the State of New York. The State Education Department, Albany, NY, USA.
Veldkamp, J. F. 1973. A revision of Digitaria Haller (Gramineae) in Malesia. Notes on Malesian grasses VI. Blumea 21:1–80.
Weakley, A. S. 2015. Flora of the southern and mid-Atlantic states. Working draft of 21 May 2015. University of North Carolina Herbarium, North Carolina Botanical Garden, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA.
Webster, R. D. 1987. Taxonomy of Digitaria section Digitaria in North America (Poaceae: Paniceae). Sida 12:209–222.
Webster, R. D., and S. L. Hatch. 1981. Taxonomic relationships of Texas specimens of Digitaria ciliaris and Digitaria bicornis (Poaceae). Sida 8:34–42.
Wilhalm, T. 2009. Digitaria ciliaris in Europe. Willdenowia 39:247–259.
Wipff, J. K. 2003. Digitaria Haller. Pages 358–383 in Flora of North America Editorial Committee, M. E. Barkworth, K. M. Capels, S. Long, and M. B. Piep, editors. Flora of North America north of Mexico, volume 25, Magnoliophyta: Commelinidae (in part): Poaceae, part 2. Oxford University Press, New York, NY, USA.
Yatskievych, G. 1999. Steyermark’s flora of Missouri. Volume 1. Revised edition. Missouri Department of Conservation, Jefferson City, MO, USA.