Seedling Description. The seed leaf (coleoptile) of yellow foxtail is pale green, less than 1/8 inch (3 mm) wide, and up to two inches (5 cm) long. It usually has a few cobwebby hairs on the upper surface near the base of the leaf blade, but these hairs are more conspicuous on true leaves. True leaves of yellow foxtail are rolled in the bud and taper evenly to a point. ln seedlings it is difficult to see the ligule—a fringe of hairs about 1/32 inch (0.5 mm) long. Ligule hairs are fused together at the base.
Leaf sheaths are smooth and distinctly flattened, usually green, and often purple-tinged, especially near the soil surface. Sheaths are split and the smooth transparent margins overlap in front. The back edge of the leaf sheath is sharply keeled, or folded along its length, forming a flattened, creased stem.
Biology. Yellow foxtail is a shallow-rooted summer annual grass that reproduces by seeds. Stems grow 1 to 2 feet (30 to 60 cm) tall and branch at the base. The grass tends to grow in clumps because the stems often tiller, or root, at the lower joints.
Stems are smooth, and leaf sheaths are smooth and flattened; the back edge forms a sharp crease, especially at the base of the plant.
The ligule of the mature leaf blade is a fringe of hairs about 1/16 inch (1 to 2 mm) long. The hairs of the ligule are fused together near the base. Auricles are absent.
A major identifying characteristic of yellow foxtail is the straggly white hairs clustered near the base of the upper leaf surface, as described under the Similar Species section. Except for these long hairs near the collar, the leaves are smooth above and below.
Leaf blades are 4 to 12 inches (10 to 30 cm) long and about ¼ inch (6 mm) wide at the base. They broaden slightly up to the midpoint, then taper evenly to a pointed tip.
The seedhead (panicle) is a compact, slightly tapered cylinder covered with soft yellow bristles (awns) about ¼ inch (6 mm) long. The erect panicle grows 1 to 5 inches (2 to 12 cm) long and ½ to ¾ inch (1 to 1.5 cm) wide, including the awns.
The small seeds are densely packed along the main axis of the panicle. Since they do not require a dormant period, they can germinate as soon as they mature. Seedlings can grow to maturity and produce seed in forty days or less. Seeds germinate at temperatures between 68° and 95°F (20° to 35°C) and at depths of about ¾ to 1¼ inch (19 to 30 mm).