Excerpts from Jim Conrad's
from the September 28, 2014 Newsletter issued from the Frio Canyon Nature Education
Center in the valley of the Dry Frio River in northern Uvalde County, southwestern Texas, on the southern border of the Edwards Plateau, USA
Up against the green-painted, corrugated tin walls of an observation building at Cook's Slough Nature Park on Uvalde's south side some healthy-looking grasses were flowering and I couldn't quite place them. They're shown below:
What knee-high grass with , long, wide leaves like these issues pyramid-shaped flowering heads, or inflorescences, with such straight side-branches? The general inflorescence shape reminds me of the barnyard grasses, genus Paspalum, but up close the spikelets don't look at all like Paspalum, as seen below:
With a single grain-producing floret per spikelet, the spikelets look like those of the panic-grasses, genus Panicum. However, the manner by which the spikelets arose on only one side of the flattish inflorescence branch, or rachilla, (they're "secund") is something I don't recall seeing among the panic-grasses. Those long hairs also are good field marks worth remembering.
Deciding that either the plant was closely related to the panic-grasses or else actually a very unusual panic-grass itself, I checked the ligule -- the little wall-like thing that may or may appear where a grass blade meets the stem, and which can take many forms. This grass's ligule is shown below:
The white-hairy ligule stretching across the inside curve of this grass blade could well be that of a panic-grass, but other grass genera can have such ligules, too, so this doesn't prove much. It was just those "secund" spikelets that worried me about calling this a panic-grass.
In the end our grass turned out to be something other than a panic-grass, though very closely related. It's Browntop Signalgrass, UROCHLOA FUSCA, a mostly tropical American grass occurring from Argentina and Paraguay north through the Americas into the southern US from Arizona to Oklahoma and Florida. The "browntop" part of the common name refers to the fact that sometimes spikelets more mature than ours are deep purple-tinged. Even in our picture you can see a little purplishness developing at the spikelets' very tips. Also, in more mature spikelets, typically cross-veins connect the conspicuous green veins running from bottom to top. If you look closely you can see such cross-veins beginning to form in some places.
Browntop Signalgrass's habitat preference is described as moist, often disturbed areas at low elevations, frequently occurring as a weed, and occasionally grown for forage and grain. That fits our grass well, for I didn't see it out in the dry scrub. Moreover, I halfway suspect that someone sowed it around the observation shelter when it was built.
Worldwide, about a hundred species of the genus Urochloa are recognized, and they're all tropical or otherwise preferring warmer climes. Browntop Signalgrass is so closely related to the genus Panicum that in the past it was assigned to that genus. It's also resided in the genus Brachiaria. Really, taxonomists have had a hard time figuring out just where our plant belongs on the Phylogenetic Tree of Life.
One theory about the name signalgrass relates to those "secund" spikelets all on one side of their rachilla. They're like signal flags all pointing one way, signaling the direction the wind is blowing...