Lobelia siphilitica aka Great Blue Lobelia

Lobelia siphilitica is a perennial in the bellflower family. Gardeners call it great blue lobelia or blue cardinal flower. Its native range spans south-central and southeastern Canada into the United States. It grows in wet woods, marshes, and low meadows. It also grows beside streams, springs, and ponds. These damp sites help explain its need for steady moisture.

The plant forms upright clumps with stiff, leafy stems. These stems bear no branches and have no scent. Most garden plants stand 2–3 feet [0.61–0.91 metres] tall. Some sources report heights of 4 feet [1.22 metres]. Clumps spread about 1–1.5 feet [0.30–0.46 metres] wide. The light green leaves grow in an alternate pattern. They lack leaf stalks and have a narrow, lance-shaped outline. Fine teeth line their edges, and the leaves lack hairs. The plant loses its leaves at the season's end.

Blue flowers crowd the tops of the stems in racemes. Each tubular flower has two lips and a striped tube. Three lobes form the lower lip; two form the upper. The blue shades range from pale to deep. Bloom begins in July and runs through September or October. Fruit appears from August to November. Bumblebees and other long-tongued bees seek the nectar and pollen. The flowers also draw butterflies and hummingbirds.

Grow it in rich soil that stays moist to wet. It suits partial shade, though it can take full sun. Sun works best in cool climates or with constant moisture. The RHS lists clay, loam, sand, and chalk as suitable. It also accepts acid, neutral, and alkaline soils. Sources give USDA zones 4–9 and UK hardiness H5. Divide clumps in spring, and remove spent flower stems in autumn. Plants may seed themselves and form groups where conditions suit them. Use them in borders, rain gardens, or native plant beds. They also fit woodland gardens and planting beside water.

Care is light when the soil stays damp. Snails and slugs can chew the leaves. The RHS also notes crown rot and virus risk. Sources disagree about how well plants resist deer damage. No edible use is known, and ingestion can cause poisoning. The RHS warns of harm to people and pets. It also warns that handling may irritate skin. Past medicinal uses do not establish that the plant is safe. Reports of its use against syphilis later proved unfounded.