Great Blue Lobelia (Lobelia siphilitica): Cool Blues for Late Summer

You likely know red cardinal flower (Lobelia cardinalis), but great blue lobelia (Lobelia siphilitica), or blue cardinal flower, is easier to care for and longer-lived. It flowers late summer to early fall—calming violets and blues when summer is hottest. Diverse native bees visit, even occasional moths or butterflies. Planted in drifts and masses, it's a stunner for front or middle of border. Seed in fall or winter for flowers next year.

Botanical name: Lobelia siphilitica
Common names: Great blue lobelia, blue cardinal flower
Origin: Native from Minnesota southwest to Wyoming and Colorado, south to Oklahoma and Louisiana, all points east
Hardiness: -40°F (USDA zones 3–9; find your zone)
Water requirement: Medium to wet soil
Light requirement: Full sun if wet soil; partial shade if medium soil
Mature size: 2–3 feet tall, up to 18 inches wide
Benefits and tolerances: Likes wet soil; tolerates some shade; unique color and form
Seasonal interest: Blooms midsummer to early fall; some winter interest with mature multi-spired plants
When to plant: Potted/bare-root spring to fall; seed fall or winter.

Photo: rockerBOO

Distinguishing Traits

Turns on when many gardens are in late-summer transition. Blooms sometimes purple, sometimes bluer. Hosts long-tongued pollinators (bees, butterflies, moths). Rabbits and deer tend to leave it alone.

Photo: Aaron Carlson

How to Use It

Prefers slightly loamy, rich soil. Looks great massed with other late-summer bloomers. Perfect at water's edge (not mucky), providing vertical interest as a midsize plant.

Planting Notes

In southern range, appreciates consistent moisture or partial shade. Self-sows only in optimal conditions. Seeds small, need light to germinate—thickly planted garden keeps unwanted seedlings at bay.

Photo: rockerBOO

More:
See more Houzz guides to beautiful blue flowers
Browse plants native to other regions of the U.S.