Digitaria cognata (Fall Witch Grass)
|Also known as:||Mountain Hairgrass, Carolina Crabgrass|
|Habitat:||part shade, sun; dry, sandy soil; open prairie, open woods|
|Fruiting season:||August - September|
|Plant height:||1 to 2 feet|
|Wetland Indicator Status:||none|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Wispy, branching cluster, initially erect, up to 11 inches tall and 17 inches wide when fully expanded (typically much reduced), with wiry, widely spreading branches. Branches are mostly single with few divergent branches, often reddish, rough textured, with dense clusters of long, straight hairs in the axils, and a spikelet (flower cluster) at the tip of each branchlet.
Spikelets are elliptic, 2.2 to 3.1 mm long, .7 to 1.1 mm wide, with a single sterile (lower) floret, a single fertile (upper) floret, and a pair of bracts (glumes) at the base. The lower glume is reduced to a tiny scale 1mm long or less, the upper glume is nearly as long as the spikelet, 1.8 to 2.8 mm long with 3 or 5 veins, smooth or with bands of fine, white, appressed hairs between the veins giving it a striped appearance. Each floret is surrounded by a pair of bracts (lemma and palea) though the sterile palea is typically absent. The sterile lemma is opposite the upper glume on the spikelet, and similar to upper glume except a little larger and with 7 veins. The fertile lemma and palea are alike, brown turning dark purplish brown, elliptic tapering to a pointed tip, and about as long as the sterile lemma.
Leaves and stems:
Leaves are mostly along the branching stems, mostly ascending, mostly less than 4 inches long and ¼ inch wide. Surfaces are smooth or with fine, spreading hairs, the hairs sometimes with a swollen, pimple-like base (papillose).
The sheath is open, forming a long “V” at the front, surfaces on upper sheaths are smooth or with sparse, spreading hairs; the lowest densely hairy with long, spreading hairs. There is often a lobe (auricle) at the tip on one side. The ligule (membrane where the leaf joins the sheath) is membranous, up to 1.5 mm long, hairless, straight or slightly jagged across the top. Nodes are mostly hairless and green to purplish. Stems are branching at the base, ascending to sprawling and may root at the nodes forming a dense clump. Often several non-flowering branches are at the base of fruiting branches.
The entire cluster breaks off at maturity, blowing to new locations tumbleweed fashion. Grains (seeds) are straw-colored to brown, elliptic, 1.3 to 1.6 mm long.
Fall Witchgrass, formerly Leptoloma cognatum, is a species of dry, open, sandy sites. Minnesota is at the northwest edge of its range and it's only found in our easternmost border counties from Chisago down to Houston County. It is one of three similar purple tumbleweed grasses found in Minnesota, the other two are Purple Lovegrass (Eragrostis spectabilis) and Witchgrass (Panicum capillare). It can be distinguished from E. spectabilis by its single flowered spikelets, multiple branching stems and membranous ligule, where E. spectabilis has multiple florets in a spikelet, mostly just basal leaves, unbranched stems, and a dense cluster of long hairs about the ligule. P. capillare also has branching stems but is an annual and a much larger (taller), stout plant that is more uniformly densely hairy throughout, with larger, broader cornstalk-like leaves and more oval spikelets and fertile florets. Some references list multiple subspecies of D. cognata, but they are not recognized in Minnesota.
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- Fall Witch Grass plant
- a clump of Fall Witch Grass
- stem bases
- densely hairy lower sheath
- sheath is sometimes auricled at the tip
- cluster branches hairy in the axils
- scan of mature flower cluster
- fully expanded cluster is wider than long
Photos by K. Chayka taken at Long Lake and Rice Creek Trail regional parks, Ramsey County. Photos courtesy Peter M. Dziuk taken in Houston County.
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