Horseweed (Conyza canadensis)
Like most unattractive weeds, this one repays a closer examination. It does not cease to be an unattractive weed when closely examined, but it does have some interesting and even beautiful parts. Its right elbow has a fascination that few can resist. The plants you see here were blooming beside the Birmingham Bridge on the South Side in late July.
Flowers. Heads inconspicuous, in panicles; minute white rays, yellow disk; involucre vase-shaped. The heads quickly turn into tiny dandelion-like seed heads, and an inflorescence usually includes some heads still blooming and others gone to seed.
Leaves. Linear; veins and edges with coarse hairs; upper leaves entire, lower jaggedly toothed or lobed; growing thick on the stem in a spiral.
Stem. Coarsely hairy, thick, strong, straight; branching at the top into a panicled inflorescence. Height quite variable: some plants more than 7 feet high, others blooming at a foot or less.
Formerly this species was placed in the genus Erigeron, with the attractive and inoffensive fleabanes. Thus in Gray; but the less attractive members of the genus have since been given a home of their own in Conyza.
ERIGERON L. FLEABANE. Heads many-flowered, radiate, mostly flat or hemispherical; the narrow rays very numerous, pistillate. Involucral bracts narrow, equal, and little imbricated, never coriaceous, neither foliaceous nor green-tipped. Receptacle flat or convex, naked. Achenes flattened, usually pubescent and 2-nerved; pappus a single row of capillary bristles, with minuter ones intermixed, or with a distinct short outer pappus of little bristles or chaffy scales. — Herbs, with entire or toothed and generally sessile leaves, and solitary or corymbed naked-pedunculate heads. Disk yellow; rays white, pink, or purple. (The ancient name presumably of a Senecio, from er, spring, and geron, an old man, suggested by the hoariness of some vernal species.)
§ 2. CAENÒTUS Nutt. Rays inconspicuous, in several rows, scarcely longer than the simple pappus; annuals.
E. canadensis L. (HORSE-WEED, BUTTER-WEED.) Bristly-hairy; stem erect, wand-like, 0.1-3 m. high; leaves linear, mostly entire, the radical cut-lobed; heads very numerous and small, cylindrical, panicled. (Leptilon Britton.) — Waste places, etc., a common weed, now widely diffused over the world. July-Oct. —Ligule of the ray-flowers much shorter than the tube, white.