Flower:

A single, densely packed, cylindric, spike-like panicle at the tip of the stem, 1 to 6 inches long, mostly straight. Clusters of 1 to 3 spikelets (flower clusters) are densely crowded on short branches along the length of the panicle, the branches tightly crowded along the stem or with short gaps (interruptions) between at least some branches especially near the base. Spikelets are green to purple, flattened on one side, oval-elliptic in outline with a blunt tip, 1.5 to 2.5 mm long, with one fertile and one sterile floret. At the base of a spikelet stalk are 1 to 4 (usually 1) straight bristles, 3 to 8 mm long, about 2 to 4 times as long as the spikelet. Bristles are green to straw-colored or purple, flexible but with minute, downward pointing (retrorse) barbs that latch onto anything that passes by, including neighboring panicles.

At the base of each spikelet is a pair of bracts (glumes) that are thin with translucent edging, elliptic to egg-shaped with a pointed tip, the lower glume 1 to 3-veined and ¼ to 1/3 as long as the spikelet, the upper glume 5 to 7-veined and as long as the spikelet. Florets are surrounded by a pair of bracts (lemma and palea). The sterile lemma is like the upper glume and as long as or slightly longer than the spikelet; the sterile palea is shorter and may be visible through the translucent lemma. The fertile lemma and palea are both much thicker than the glumes, their surfaces covered in tiny pits or fine wrinkles and with a conspicuous swelling at the base. The fertile lemma is oval to egg-shaped, blunt at the tip, 1 to 2 mm long, and the edges curl around the palea edges; the fertile palea is flat, as long as the lemma and with a similar texture but smooth and shiny on the edges.

Leaves and stems:

Leaves are alternate, 2 to 10 inches long, 4 to 16 mm (to ~2/3 inch) wide, lance-linear, flat, hairless or with a few scattered hairs on the upper surface especially near the base, rough on both surfaces. The sheath is open, green, hairless except for a fringe of hairs along the edge especially near the tip. The ligule (membrane where the leaf joins the sheath) is a fringe of hairs about 1 mm long. Nodes are hairless and green to purplish.

Stems are smooth, erect to ascending, the lower stem often prostrate then rising at the lower node (geniculate), but not rooting at the node. Stems are multiple and branching from the base, forming loose to dense clumps.

Fruit:

The glumes and sterile lemma turn tan and papery as they mature, the fertile lemma and palea hardening and turning darker brown. The entire spikelet drops off when mature, leaving the bristles behind on the stem.

Notes:

While an agronomy professor once remarked to me that Bristly Foxtail was one of the most common and widespread weeds worldwide, it is one of the least common and widespread weedy Foxtail species in North America. While criss-crossing every corner of the state over the past decade, every time we approached a suspected Bristly Foxtail plant, per suggestions from multiple references we'd run our hands along the panicle to determine whether it felt sufficiently “bristly” to us. We were never really certain at the time, but it invariably turned out to be Green Foxtail (Setaria viridis). We only encountered Bristly Foxtail for the first time in 2017 in extreme western MN in a highly human disturbance impacted site. It was instantly recognizable so once you experience it, you'll never have doubts again! What was most interesting to me was experiencing its ability to attach Velcro-like to anything in reach—something I instantly recalled from childhood experiences on my aunt and uncle's farm in Pope County.

Not all weeds succeed. In spite of its obvious ability to attach and travel, it has not nearly been as successful as most other non-native Foxtails. All but two of Minnesota's small handful of herbarium collections are over 50 years old. All plants have their preferred habitats, and the unique combination of disturbance and competition levels required for this species to flourish apparently does not occur across the broader landscape.