Blue cardinal flower (Lobelia siphilitica)
Vibrant Lobelia siphilitica, the Blue cardinal flower, offers striking blue spikes in late summer gardens. Essential for wet, sunny spots.
Complete Plant Information
Overview
The stunning Blue cardinal flower (Lobelia siphilitica) is a highly valuable native perennial celebrated for its brilliant, deep blue floral displays late in the growing season. Its upright spikes provide vital color when many other summer bloomers begin to fade, making it a spectacular addition to perennial borders and naturalized settings throughout Eastern North America. This plant thrives in consistently moist to wet conditions, making it perfectly suited for rain gardens and waterside plantings.
Lobelia siphilitica forms attractive clumps of foliage topped by dense racemes of tubular, two-lipped flowers. These flowers are particularly attractive to pollinators, including hummingbirds, drawn to the rich blue hue that persists from July through September. Despite its dramatic appearance, maintenance is generally low once established in an appropriate, damp location.
Gardeners seeking resilient, moisture-loving native selections should prioritize the Blue cardinal flower. It offers significant aesthetic impact while reliably meeting ecological needs in partial shade to full sun environments, provided the soil never dries out. Successful cultivation centers around managing its high water requirements.
Fast Facts
- Plant Family: Campanulaceae
- Plant Type: Herbaceous perennial
- Native Range: Eastern North America
- Hardiness Zones: USDA Zones 4-9
- Size at Maturity:
- Height: 2-3 ft
- Spread: 1-1.5 ft
- Bloom Time: July to September
- Bloom Description: Blue
- Sun Exposure: Full sun to part shade
- Water Needs: Medium to wet
- Maintenance Level: Low
How to Grow
Planting Lobelia siphilitica is best done in spring or early fall, ensuring the selected spot offers rich, humusy soil that retains consistent moisture. While this plant appreciates full sun in cooler, northern climates, most gardeners should aim for a location that receives part shade, especially during hot summer afternoons, which helps prevent wilting due to its significant water demands. The plant needs constant moisture to perform optimally.
Once established, maintenance for the Blue cardinal flower is notably low. Routine fertilization is usually unnecessary if the native soil is rich. The primary care involves monitoring moisture levels, ensuring the area remains damp or wet throughout the blooming cycle. If clumps become overly dense or start to crowd out neighbors, dividing them in the spring is straightforward.
This species has a tendency to self-seed gently in areas where conditions—moisture, soil quality, and light—are perfectly matched to its needs. This natural reseeding habit is excellent for establishing attractive, naturalistic colonies along streams or in meadows. Be aware that snails and slugs may target the foliage, so preventative measures may be required in vulnerable areas.
Landscape Uses
The suggested uses for Lobelia siphilitica highlight its strength in wetter parts of the landscape. It is an ideal choice for naturalizing areas, allowing it to spread gently and form drifts of late-season blue color. Furthermore, it is highly effective in rain gardens or along the edges of ponds and streams, where its requirement for medium to wet soil is easily met.
As a vertical accent, the towering blue spikes draw the eye toward the back of perennial borders or within native plant gardens. Pair Lobelia siphilitica with moisture-loving companions like ferns, Joe Pye weed (Eutrochium species), or cardinal flower (Lobelia cardinalis) for complementary textures and colors. Its height (2-3 feet) works well against shorter groundcovers in woodland edges or wild garden settings.
Standout Features
Flower Qualities
- Showy
Noteworthy Characteristics
Lobelia siphilitica, commonly called great lobelia or blue cardinal flower, is a Missouri native perennial which typically grows in moist to wet locations along streams, sloughs, springs, swamps, meadows and in low wooded areas. A clump-forming perennial which features light to dark blue, tubular, 2-lipped flowers with the three lobes of the lower lip appearing more prominent than the two lobes of the upper lip. Flowers arise from the upper leaf axils forming a dense terminal raceme atop stiff, unbranched, leafy stalks typically rising 2-3’ tall. The species epithet siphilitica arose from a prior medicinal use of the plant in the treatment of venereal disease.
Tolerances
- Deer
- Heavy Shade
- Wet Soil
Potential Problems
This plant suffers from few serious pest or disease issues, which contributes to its low maintenance rating. The most common culprits impacting foliage health are common garden gastropods, specifically snails and slugs, which are attracted to the succulent leaves, especially new growth. Management should focus on proactive, organic controls like slug bait, beer traps, or manual removal, particularly during damp periods when these pests are most active.
Frequently Asked Questions
What hardiness zones is Blue cardinal flower suitable for?
Lobelia siphilitica is reliably hardy across USDA Zones 4 through 9, offering a wide range of adaptability in eastern US gardens.
How large does Blue cardinal flower grow?
This perennial typically reaches a height between 2 and 3 feet, spreading moderately between 1 and 1.5 feet wide, forming an upright clump.
When does Blue cardinal flower bloom?
The peak bloom time for Lobelia siphilitica extends from July well into September, showcasing striking blue flowers.
What sun exposure does Blue cardinal flower need?
It performs best when situated in full sun to part shade, although consistent moisture is more critical than precise sun exposure thresholds.
Conclusion
The vibrant, late-season color provided by the Blue cardinal flower makes Lobelia siphilitica an irreplaceable element in any garden aiming for extended visual interest and native planting support. Its adaptability to wet conditions makes it a superb choice for marginal areas where many other perennials fail. Before planting, assess your site’s moisture levels and ensure you can consistently meet the water needs of this beautiful native species.
Companion Planting
To create cohesive, moisture-loving habitats, plant Lobelia siphilitica alongside other waterside enthusiasts. Good companions include sedges (Carex species), marsh marigold (Caltha palustris), or irises that tolerate wet feet. Mixing it with taller natives like swamp milkweed (Asclepias incarnata) creates layered interest that capitalizes on the Blue cardinal flower’s strong vertical form during late summer.
Wildlife Benefits
The tubular structure of the Blue cardinal flower is perfectly adapted for attracting specialized pollinators. Hummingbirds are frequent visitors, drawn to the bright blue spikes when they begin flowering in mid to late summer. Furthermore, the flowers provide essential nectar resources to late-season bees and butterflies, offering a crucial food source before the onset of colder weather.
