Bot. Gaz. 12: 53. 1887.
Stems (11-)40-80(-120) cm; base usually reddish, longitudinally ridged, puberulent. Leaves mostly cauline; basal leaves 0-3 at anthesis; cauline leaves 3-8 at anthesis; petiole 0.5-7 cm, petioles of proximal leaves glabrous to puberulent. Leaf blade round to pentagonal, 1-4 × 2-6 cm, usually puberulent, especially abaxially; ultimate lobes 3-14, width 3-8 mm (basal), 2-5 mm (cauline). Inflorescences (5-)15-30(-100)-flowered, moderately open; pedicel (0.5-)1-2.5(-7.5) cm, puberulent; bracteoles 2-6(-12) mm from flowers, green to blue, margins often white, linear-lanceolate, 3-7(-12) mm, puberulent. Flowers: sepals dark blue to white, puberulent, lateral sepals spreading, 7-13(-16) × 3-10 mm, spurs straight to upcurved, ascending 30-45° above horizontal, 9-18 mm; lower petal blades slightly elevated, ± exposing stamens, 3-8 mm, clefts 1-5 mm; hairs centered, denser on inner lobes near base of cleft, white. Fruits 8-18 mm, 2.2-3(-4.2) times longer than wide, sparse puberulent. Seeds not echinate, appearing ± smooth to naked eye; seed coat cells with margins straight, surfaces smooth or roughened.
Subspecies 3 (3 in the flora).
Delphinium hesperium is thought to be poisonous to cattle (D. E. Moerman 1986, no subspecies specified). It is often confused with D. hansenii. See discussion under that species for distinguishing features.
|1||Lateral sepals 4(–5) mm wide or less; inflorescence dense (individual flowers often touching in pressed specimens).||Delphinium hesperium subsp. cuyamacae|
|1||Lateral sepals more than 4 mm wide; inflorescence open (individual flowers seldom touching in pressed specimens).||> 2|
|2||Sepals dark blue-purple.||Delphinium hesperium subsp. hesperium|
|2||Sepals white to pinkish, rarely light blue.||Delphinium hesperium subsp. pallescens|