Stems (40-)60-120(-180) cm; base usually reddish, glabrous to puberulent. Leaves mostly cauline at anthesis; basal leaves 0(-3) at anthesis; cauline leaves 5-12 at anthesis; petiole 0.5-17 cm. Leaf blade ± pentagonal, 4-8 × 7-16 cm, margins ± incised, nearly glabrous; ultimate lobes 0-9, width 15-30 mm (basal), 5-20 mm (cauline), widest at middle or in proximal 1/2. Inflorescences (5-)14-40(-75)-flowered, ± open, at least 2 times longer than wide; pedicel 1-4(-9) cm, puberulent to glabrous; bracteoles (2-)6-12 mm from flowers, green, linear, 5-9(-14) mm, puberulent. Flowers: sepals dark blue, glabrous, lateral sepals spreading, (8-)14-21 × 5-9 mm, spurs straight or downcurved at apex, within 20° of horizontal, (10-)16-23 mm; lower petal blades covering stamens, 5-10 mm, clefts 1.5-3 mm; hairs sparse, mostly near junction of blade and claw, centered or on inner lobes, well dispersed, yellow. Fruits (15-)23-34 mm, 3.8-5.5 times longer than wide, glabrous. Seeds unwinged; seed coats smooth. 2 n = 16.

Flowering spring. Oak woods, coastal chaparral, wet woodlands; 30-1100 m; Calif., Oreg.

Delphinium trolliifolium occurs in the northern Coast Range of California, the Columbia River Valley to just east of Mt. Hood, and the Willamette Valley of Oregon upstream to Lane County. California plants differ somewhat from Oregon plants in pubescence patterns and habitat preferences. Further study may show that two entities are involved here.

Hybrids between Delphinium trolliifolium and D . decorum , D . menziesii subsp. pallidum ( D . × pavonaceum Ewan, Peacock larkspur), D . nudicaule , D . nuttallianum , and D . nuttallii are known. Delphinium trolliifolium is likely to be confused only with D . bakeri . Refer to discussion under that species for differences.