Lobelia oahuensis, a member of the Campanulaceae (bellflower) family, is a stout, erect, unbranched shrub 1 to 3 m (3 to 10 ft) tall. The elliptic leaves, 40 to 60 cm (16 to 24 in) long and 4 to 6 cm (1.6. to 2.4 in) wide, are typically stalkless and form a very dense rosette at the end of the stem. The upper surface of the leaves is hairless and the lower surface is covered with rather coarse grayish or greenish hairs. The inflorescence is branched 3 to 5 times from its base with each erect spike 0.1 to 1.5 m (3 to 5 ft) tall and comprised of 50 to 200 flowers. Each flower measures 42 to 45 mm (1.7 to 1.8 in) long and about 5 mm (0.2 in) wide with a 3 cm (1.2 in) long bract just below it. The linear calyx lobes are about 16 mm (0.6 in) long and 3 mm (0.1 in) wide. The fruits are hairy, oval capsules 10 to 17 mm (0.4 to 0.7 in) long and about 9 mm (0.4 in) wide that contain numerous brownish seeds.