Annual or short-lived perennial herb. Primary stem up to 1.5 m, usually simple, branched above, erect, terete, ridged, purplish, hairless, and woody towards the base. Leaves alternate thickish, stiffish, hairless, pinnati-nerved, sessile, and variable in outline; lower leaves shrivelled or lacking at flowering; longest stem leaf 5.5—21.5 cm long, 0.4—5.0 cm wide, linear or very narrowly elliptic in outline, apex acute, long attenuate and pseudo-petiolate in the proximal part, at the base somewhat stem-clasping with unequally two-pronged auricles, dark green, midrib impressed with the remaining veins obscure above, below paler, sometimes purplish, midrib prominent with the remaining veins inconspicuous; margins hardly revolute, dentate, serrate, or pinnatifid with segments singly toothed on the proximal edge; uppermost leaves entire or denticulate and without auricles. Capitula few or many, heterogamous, discoid, arranged in broad corymbs at the ends of branches. Involucre cylindrical, calyculate c. 5.5 mm long, c. 2 mm wide, calycle 4-6, very short; involucral bracts 8-12, 4.3-5.2 mm long, green, hairless. Florets 19-21, yellow; central florets 4-6. hermaphrodite with slender tubular corollas shortly 4-5-toothed at the apex; marginal florets 15-16, female with filiform corollas minutely 3-4-toothed at the apex. Filament balusteriform at the base of the anther, endothecium with radial thickenings. Achenes 1.8-2.7 mm long, cigar-shaped, brown, hairy in narrow grooves between broad ribs; hairs 1-2 (-3) seriate as long as or shorter than the width of the ribs. Pappus capillary, very slender, white. Plants of this kind are not catered for in the monograph of Belcher (1956). The fact that all the specimens originate from North Auckland land district in the North Island, and that the earliest gathering was as recent as 1944 suggests Senecio diaschides is adventive in New Zealand.

Herba annua vel perennis brevi summa vitae. Caulis primarius ad 1.5 m, plerumque simplex, superne ramosus, crectus, teres, in longitudinem corrugatus, purpurascens, glaber, basin versus ligneus. Folia alterna, crassiuscula, rigidula, glabra, pinnatinervia sessilia, forma variabilia; folia infima marcescentia vel absentia sub anthesi; longissima folia caulina 5.5—21.5 cm longa, 0.4-5.0 cm lata, linearia vel peranguste elliptica ambitu, apice acuta, inferne longe attenuata pseudopetiolata basi subamplexicaulia auriculis unidentatis vel inaequaliter bifurcatis, supra atroviridia, costa impressa ceteris nervis obsoletis, subtus pallidiora, subinde purpurascentia, costa prominente ceteris nervis inconspicuis; margines vix revoluti, dentati, vel serrati, vel pinnatifidi segmentis 1-dentatis in marginibus proximalibus; folia suprema linearia, integra vel denticulata, exauriculata. Capitula pauca vel multa, heterogama, discoidea, in corymbos latos in apicibus ramorum disposita. Involucrum cylindraceum, calyculatum c. 5.5 mm altum, c. 2 mm crassum; bracteolis calyculi 4-6, brevissimis, bracteis involucralibus 8-12, 4.3-5.2 mm longis, viridibus, glabris. Flores 19-21, lutei; flores centrales 4-6 hermaphroditi, corollis tenuiter tubulosis, apice brevissime 4-5-dentatis; flores marginales 15-16, feminei, corollis filiformibus, apice minute 3-4-dentatis. Filamentum fundo antherae balusteriforme; endothecium antherae crassitudinibus radialibus. Achaenia 1.8-2.7 mm longa, fusiformia, brunnea, pubescentia in angustis sulcis inter costas latas; pili 1-2-(3)-seriati, latitudinem costae aequantes vel ea breviores. Pappus capillaris, tenuissimus, niveus.