Rhizomes with short, creeping branches, stout, 2–2.5 cm diam., covered with bases of old leaves; roots numerous, fleshy. Stems simple or 1–2-branched, solid, 3–6 dm. Leaves: basal persistent, blade dark green, turning gray or yellow-brown when drying, veins not prominent, linear, 4–7 dm × 0.6–1 cm, usually longer than stem, glaucous or subglaucous, apex acute; cauline 2–3, reduced. Inflorescence units 3–6-flowered; spathes distant, inner sometimes 10 cm distal to outer, green, narrowly linear-lanceolate, 8–15 cm × 20–40 mm, herbaceous, margins and apex scarious. Flowers: perianth pale blue-violet or nearly white, with lilac-purple veins; floral tube funnelform, 0.5–1.3 cm; sepals spreading, then drooping at apex, white, flecked and veined with violet, obovate, 8–10 × 3–5 cm, base attenuate into narrow claw with prominent yellow ridge, signal a yellowish basal patch; petals semierect, oblong, 8–9 × 1.5–2 cm, base gradually attenuate, apex emarginate, bluntly rounded; ovary trigonal with central ridge in each face, 2–5 cm; style narrow basally, widening distally, 3.5–4 × 0.6–1 cm, crests overlapping basally, subquadrate, 1–1.5 cm, margins incised; stigmas obscurely to prominently 2-lobed, margins crenate; pedicels stout, 3–9 cm, unequal at anthesis, becoming ± equal at maturity. Capsules 6-ribbed, oblong-ovoid, tapering to either end, almost round in cross section, 8–9 cm. Seeds dark brown, globular, wrinkled. 2n = 86, 88.
Phenology: Flowering Mar–Apr.
Habitat: Wet, heavy soil along coast
Iris longipetala hybridizes with I. missouriensis and is found from Mendocino County south to Monterey.