Setaria macrostachya

Plains Bristlegrass

Field view of Setaria macrostachya, common name Plains Bristlegrass. The seedheads can be up to a foot long.

The elongated spikes are thick with spikelets. The seeds are enclosed in bracts called the lemma and palea, with two glumes beneath.

The rounded spikelets of Setaria macrostachya completely cover the apex of the stem, the rachis.

Here you can see the seeds and the short bristles. These arise from the base of the spikelets, not from the glumes and lemmas as awns do.

Here also you can see the irregular bristles.

The roots of Plains Bristlegrass.

How to Identify Plains Bristlegrass

  This is a perennial bunchgrass that is common especially in Arizona's lower elevations but occurs in every county. It is spike-like but looks like a spike, with many awn-like bristles showing. All the bristlegrass seedheads look like minute bunches of tiny grapes. Each 1/16-inch "grape" is a complete spikelet with glumes and two florets hidden inside (lemmas, paleas, and the actual seed, the caryopsis). Setaria macrostachya is a common bristlegrass in Arizona, but it is hard to differentiate from the others unless you see a seedhead over 7 inches long. I recommend just calling all of these "bristlegrasses" or "Setaria species" until you gain some expertise (and have access to a microscope).

Similar Species

  In Arizona there are 14 bristlegrasses, all with compact oval spikelets and bristles. All these can be roughly divided into two groups:
  (1) Those with so many bristles that they look like soft puffs, concealing the rachis (stem). Only two of these are common and likely to be found: Green Bristlegrass (Setaria viridis) and Yellow Bristlegrass (Setaria pumila). Both of these have relatively short seedheads, usually 2-4 inches.
  (2) Those with fewer bristles and longer seedheads, often with gaps so that the spikelets are prominently visible and the rachis (stem) shows through. Three of these are common: Plains Bristlegrass (this one is the only one with seedheads sometimes longer than 7 inches), Streambed Bristlegrass (Setaria leucopila), and Grisebach's Bristlegrass (Setaria grisebachii).
  Of course, you may simply call all of these bristlegrasses.