Lobelia siphilitica, Blue Cardinal Flower
Blue cardinal flower, also known as great blue lobelia, is a tall native perennial that blooms for long weeks at the end of summer. Like its striking cousin, the red cardinal flower, this lobelia relishes plenty of water and is happy growing in full-sun rain gardens, bogs, and damp meadows. But it will also grow successfully without supplemental water in semi-shaded flower borders if its roots remain cool and mulched. Blue cardinal flower brings vertical interest to the garden and is a vividly blue counterpoint to the colors of late summer and early autumn.
Above: The spires of blue cardinal flower can grow up to four feet tall.
Above: An illustration of blue cardinal flower from William Barton’s Vegetable Materia Medica of the United States (circa 1825).
Lobelia siphilitica’s species name is derived from one of its many Native American uses as a treatment for syphilis (in conjunction with other toxic-slash-medicinal ingredients like cherry and May apple—don’t try this at home).
Above: Blue cardinal flower with a late summer tangle of obedient plant, ageratum, and rudbeckia.
Above: In the middle of a border, the tall spikes of blue cardinal flower hold their own with grasses and asters.
Above: Blue cardinal flowers support a wide range of native bees and butterflies.
Cheat Sheet
- Blue cardinal flower is native to eastern North America.
- It is a member of the bell flower (Campanulaceae) family.
- The species name of siphilitica is derived from an historical use of the plant in the treatment of venereal diseases.
- Bees, butterflies, and hummingbirds feed on the flowers’ nectar.
- Lobelias produce a toxic compound called lobeline, which makes them less palatable to deer.
Above: Blue cardinal flower appreciates afternoon shade where summers are hot.
Keep It Alive
- Blue cardinal flower is hardy from USDA growing zones 4 to 9.
- Plant its seeds or seedlings in rich soil with plenty of organic matter.
- It thrives in damp soil and wet edges of ponds, rain gardens, or streams.
- Plant in full sun only in cooler climates.
- Where summers are hotter, plant blue cardinal flowers where it will grow in afternoon shade.
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