Flower:

A single, densely packed, nodding, cylindric, spike-like panicle at the tip of the stem, 2 to 7 inches long and 3/8 to 1 inch thick. Clusters of spikelets (flower clusters) are densely crowded on short branches along the length of the panicle, sometimes with a short gap between the lowest branches (interrupted). Spikelets are flattened on one side, oval to egg-shaped in outline, usually pointed at the tip, 2.5 to 3 mm long, 1.5 to 1.7 mm wide, with one fertile and one sterile floret. At the base of a spikelet stalk are 1 to 6 (commonly 3) straight bristles each 10 to 20 mm long, averaging 5 or 6 times as long as the spikelet. Bristles are mostly green to yellowish, rarely purplish, and soft with minute, upward pointing (antrorse) barbs.

At the base of each spikelet is a pair of bracts (glumes) that are thin with translucent edging. egg-shaped with a blunt to pointed tip, the lower glume 3-veined and 1/3 to half as long as the spikelet, the upper glume 5 to 7-veined and 2/3 to ¾ as long as the spikelet. Florets are surrounded by a pair of bracts (lemma and palea). The sterile lemma is like the upper glume except as long as the spikelet; the sterile palea is shorter and inconspicuous. 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 egg-shaped, blunt to pointed at the tip, 2.5 to 3 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.

Leaves and stems:

Leaves are alternate, 4 to 16 inches long, 7 to 20 mm (~¼ to ¾ inch) wide, lance-linear, flat, hairless to softly hairy on the upper surface or sparse hairs especially near the base. 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 1 to 2 mm long. Nodes are hairless and green to purplish. Stems are smooth, erect or more often ascending, the lower stem often prostrate then rising at the lower node (geniculate), and rooting at the node. Stems are single but branched from the base and lower nodes, forming clumps and often dense colonies.

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:

Giant Foxtail is a relative newcomer compared to other non-native weedy Setaria species found in Minnesota. It was first documented in the state in the late 1950s, and in only a short time spread across much of our central and southern farmland, becoming a nuisance in corn and soybean fields. The national distribution map might suggest it much prefers the rich mesic soils of the eastern Great Plains where other Foxtails have readily invaded the arid west, though we have observed it in rock outcrop habitat. Time will tell how far it will continue to spread but we're likely to see more of it in coming years.

While it might seem easy to ID, it bears a striking resemblance to the more robust forms of Green Foxtail (Setaria viridis var. major), which can also have thick, nodding heads. The two are often confused but can be distinguished by a combination of traits: Giant Foxtail panicles mostly nod from the lower half, where Green Foxtail mostly nods from the upper half or barely nods at all. Giant Foxtail spikelets are usually 2.5 to 3 mm long and more pointed at the tip where Green Foxtail spikelets are usually 2 to 2.5 mm and more rounded at the tip. Giant Foxtail upper glumes are up to about ¾ as long as the spikelet (consistently exposing the fertile lemma) where Green Foxtail upper glumes are mostly as long as the spikelet (more consistently hiding the fertile lemma). Giant Foxtail leaves are often softly hairy on the upper surface where Green Foxtail leaves are hairless, though may be slightly rough. Giant Foxtail roots from the lower nodes where Green Foxtail does not.