Multiple branching stems 12–40 inches tall grow from a woody root crown and are covered with thick arrays of yellow flower heads. Note the flower heads have 8 narrow, yellow rays, the foliage is hairless, and the leaves are divided into thread-like segments. The plant is toxic to cattle and usually removed from rangeland.

FLOWER: April–October. Tight clusters with 5–20 or more flower heads bloom on the stem tips; each flower head has ±8 narrow, yellow, petal-like ray flowers 5/16–3/8 inch long (8–10 mm); ±13 phyllaries, all even-sized and with green tips, are below the rays; beneath the phyllaries there are no bractlets or 3–8 minute ones less that 1/3 the length of the phyllaries.

LEAVES: Alternate, same size, evenly distributed along stem. Blades green, hairless, 1 2/2–3 1/2-inches long (4–9 cm) with linear, filament-like lobes to 3/16-inch wide (5 mm).

HABITAT: Dry to periodically moist sandy, gravelly soils; shrublands, grasslands, floodplains, dry washes, ditch banks, sandy hills, roadsides, disturbed areas; desert grasslands and scrub, prairies, pinyon-juniper, ponderosa woodlands.

ELEVATION: 3,500–8,700 feet.

RANGE: AZ, CO, KS, NE, NM, OK, SD, TX, WY.

SIMILAR SPECIES: About 22 species of Senecio in NM, 2 others have hairless, thread-like leaves. Threadleaf Grounsel, S. falccidus var. monoensis, is hairless, has ±21 phyllaries, with bractlets 1/2 the length of the phyllaries or longer. Broom Groundsel, S. sparticoides, has ±5 rays and 0–3 tiny bractlets beneath the phyllaries. See key below.

NM COUNTIES: Nearly statewide in low- to mid-elevation, dry to periodically moist habitats: Bernalillo, Carton, Chaves, Colfax, Curry, Dona Ana, Eddy, Grant, Hidalgo, Lea, Lincoln, Los Alamos, McKinley, Moro, Otero, Quay, Rio Arriba, Roosevelt, San Juan, San Miguel, Sandoval, Santa Fe, Sierra, Socorro, Torrance, Union, Valencia.

KEY to 3 species with thread-like leaves

1. Woolly-hairy leaves, bractlets beneath phyllaries minute or absent ... S. flaccidus var. flaccidus

+  Leaves hairless or nearly so ... (2)

2. Conspicuous bractlets 1/4 to 1/2 or more the length of the phyllaries, phyllaries 13 or 21, tips green or black … S. falccidus var. monoensis

+ tiny or no bractlets under phyllaries ... (3)

3. 3.Tiny bractlets, less that 1/3 length of phyllaries, phyllaries mostly 13, tips green, rays 8 … S. riddellii
   

+ Bractlets absent or inconspicuous, phyllaries mostly 8, tips green … S. sparticoides