Morphology

Growing up to 4 feet (1.2 m) tall, Lobelia siphilitica has a single ridged, unbranched stem, which is smooth or sparsely hairy. Leaves are hairless or scarcely hairy, that vary in shape from elliptical to lance-like, slightly narrowing at the base. They are about 2 inches (51 mm) wide and 2 to 6 in (51 to 152 mm) in length. Leaves are alternately attached to the stem with no leaf stalk.^[6] The root system is a central taproot, that occasionally produces basal offshoots.^[6]

The plant produces a spike of zygomorphic flowers in the late summer. The flowers are more closely clustered towards the top of the raceme and become more infrequent further down with the lower flowers begin blooming first.^[6] Great lobelia flowers are notably larger than many other lobelia species. Flowers are typically blue or violet, but can be lighter, even white and are usually about 1 in (25 mm) long.^[8] The tubed portion of the flower is often striped. Great lobelia flowers have five petals; the three lower petals are fused, and the two upper petals are usually curled back. A curved style pokes between the upper two petals. The lower center lobe is wider and has two small bumps near the throat that are typically lighter in coloration.^[6] Seed pods are two-chambered with an auriculate base and have many small seeds. Seeds are ovular, translucent, and golden brown and are presumed to be distributed by wind or water.^[9]