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// SPECIES PROFILE · PERENNIAL · NATIVE · POLLINATOR · WET-SOILS

Great Blue Lobelia

Lobelia siphilitica

Great Blue Lobelia is the cool-blue sister of the famous red cardinal flower — a striking, late-summer-blooming perennial whose dense spikes of rich violet-blue, tubular flowers rise from a basal rosette, lighting up streambanks, seeps, rain gardens, and moist meadow edges from August through October. The flower's pollination mechanism is one of the most elegant in the plant kingdom: a trigger-like column of fused stamens and style deposits pollen on the back of a visiting bumblebee as it probes the flower tube. Lobelia siphilitica is the species to plant where you have consistently moist soil and want a tall, upright accent of true blue in the late-summer and fall garden — and it self-sows into a long-lived colony that expands year after year.

// QUICK FACTS
Family
Campanulaceae (bellflower family)
Life cycle
Herbaceous perennial; short-lived (3–5 years) but robustly self-seeding
Native range
Eastern & central N. America — Maine to Manitoba, south to Georgia and Texas
USDA hardiness
Zones 4–8 (Tulsa = 7a/7b)
Mature size
2–4 ft tall × 12–18 in wide
Bloom period
August – October (NE OK); 6–8 weeks
Flower
Rich violet-blue, tubular, 1 in long; upper lip 2-lobed, lower lip 3-lobed with white markings
Sun
Full sun to part sun; afternoon shade beneficial in Tulsa heat
Soil
Moist to wet; rich loam, clay loam, or muck; pH 5.5–7.5; dislikes drying out
Water
High; needs consistently moist soil; tolerates brief inundation
Wildlife
Bumblebee specialist · hummingbird nectar · occasional swallowtail host · late-season pollinator anchor
Great Blue Lobelia (Lobelia siphilitica) with dense spikes of vibrant blue tubular flowers
Lobelia siphilitica — the rich violet-blue flower spikes are a hallmark of late-summer streambanks and rain gardens. Photo: Rooted Revival.

Identification

[ field key — habit · leaf · flower spike · flower anatomy · vs. cardinal flower ]

Habit & Stem

An erect, mostly unbranched perennial with a basal rosette of leaves and one to several stout flowering stems 2–4 ft tall. Stems are round, hollow, and smooth to sparsely hairy, often tinged reddish near the base. The overall form is upright and clump-forming; older plants produce multiple stems from the crown. Like its close relative cardinal flower (L. cardinalis), individual rosettes are short-lived (3–5 years), but the plant self-sows prolifically and maintains a continuous presence in suitable habitat. In NE Oklahoma, basal rosettes are visible by late March, and flower spikes begin elongating in July.

Leaves

Leaves are alternate, simple, oblong to lanceolate, 2–6 in long and 1–2 in wide, with finely toothed (serrulate) margins and a pointed tip. The leaf base tapers to a short petiole or is sessile on the upper stem. The blade surface is medium to dark green and smooth to sparsely hairy. Leaves are progressively smaller toward the top of the stem. A diagnostic difference from cardinal flower: L. siphilitica leaves are not as narrow and willow-like as those of L. cardinalis, and the leaf bases are not clasping or winged.

Flower Spike & Anatomy

The inflorescence is a dense, terminal raceme (spike) 6–12 in long, packed with 20–50 individual flowers. Each flower is about 1 in long, tubular, and distinctly two-lipped: the upper lip has two narrow, erect lobes; the lower lip has three wider, spreading lobes marked with white or pale-yellow nectar guides at the throat. The five stamens are fused into a curved, tubular column surrounding the style, and the anthers form a ring at the tip. When a bumblebee pushes into the flower tube to reach nectar at the base, the stamen column acts as a trigger mechanism, brushing pollen onto the bee's back. The color is a rich, saturated violet-blue that is rare among native flowers of this stature.

Fruit & vs. Cardinal Flower

After pollination, the flowers develop into small, dry capsules that split open to release numerous tiny, dust-like brown seeds. A single flower spike can produce thousands of seeds, which are wind- dispersed and germinate on bare, moist soil the following spring. Key differences from cardinal flower (L. cardinalis): Great blue lobelia has blue flowers (vs. scarlet red), blooms later (August–October vs. July–September), has smoother, broader leaves, and is generally more cold-hardy and longer-lived as an individual plant.

Habitat & Range in NE Oklahoma

Lobelia siphilitica is native across eastern and central North America, from Maine and southern Ontario west to Manitoba and eastern Colorado, and south to Georgia, Alabama, and eastern Texas. In NE Oklahoma it is a species of moist, open to partly shaded habitats: the edges of streams and rivers, seepy banks, wet meadows, bottomland forest openings, the margins of ponds and lakes, and roadside ditches that hold water into the summer. It is most often found on rich, alluvial soils or organic muck where the water table stays within a foot or two of the surface through the growing season. In the Tulsa region, look for it along tributaries of the Arkansas and Verdigris Rivers, around the margins of farm ponds in Rogers and Wagoner counties, and in the seepy colluvial slopes where the Cross Timbers meets the bottomland hardwood zone.

In Oklahoma, L. siphilitica occurs primarily in the eastern third of the state, extending along riparian corridors into central counties. It reaches its southwestern range limit in the state, becoming progressively less common west of Tulsa. High-quality populations are associated with intact riparian zones that have not been channelized or heavily grazed.

Ecology & Wildlife Value

[ bumblebee pollination · hummingbirds · lepidoptera · seed ]

Bumblebee Pollination

The flower structure of Lobelia siphilitica is adapted primarily for bumblebee pollination. The flowers are too deep- tubed for most small bees, and the lever mechanism of the fused stamen column requires an insect large enough to push past the reproductive structure. In NE Oklahoma, the primary pollinators are common eastern bumblebees (Bombus impatiens), American bumblebees (B. pensylvanicus), and brown-belted bumblebees (B. griseocollis). Carpenter bees (Xylocopa virginica) also visit and are effective pollinators. The bees collect nectar by entering the tube, and pollen is deposited on their backs and thorax as they trigger the staminal lever.

Hummingbirds

While the red cardinal flower (L. cardinalis) is the more famous hummingbird plant, ruby-throated hummingbirds do visit L. siphilitica, especially in late summer and during fall migration when the blue flower spikes are at their peak. The tubular shape and abundant nectar are compatible with hummingbird foraging, though the blue color is less attractive to hummingbirds than the red of its congener.

Lepidoptera

The foliage of Lobelia species hosts larvae of the pink-washed looper moth (Enigmogramma basigera) and a few other specialist lepidoptera. The showy flowers are occasionally visited by eastern tiger swallowtails and spicebush swallowtails for nectar, though butterflies are less effective pollinators than bees because the flower tube is deep and the pollination mechanism requires significant force to trip.

Herbivory & Toxicity

All parts of Lobelia siphilitica contain piperidine alkaloids (especially lobeline), which are toxic to mammals in quantity. As a result, the plant is strongly deer-resistant and rabbit-resistant — an important attribute for Tulsa-area gardens subject to deer browse. The alkaloids also deter most generalist insect herbivores, though specialist insects that have co-evolved with Lobelia are unaffected.

Historical name note: The specific epithet siphilitica and the common name "Great Blue Lobelia" belie a peculiar piece of botanical history: the plant was once believed (erroneously) to be a cure for syphilis, a belief that Linnaeus enshrined in the scientific name. It has no such medicinal property and is toxic if ingested.

Horticulture & Care

[ moisture management · self-seeding · propagation · companion planting ]

Site selection & moisture

The single non-negotiable requirement for Great Blue Lobelia is consistent soil moisture. It will not survive in soil that dries out for more than a day or two during the growing season. Plant it in a rain garden basin, at the base of a downspout, along a frequently irrigated stream or pond edge, in a seep, or in any low-lying area where the soil stays damp through July and August. In full sun, the plant is compact and blooms heavily but requires correspondingly more water. In part sun (especially with afternoon shade), it is taller and a bit lankier but needs less supplemental water. In the Tulsa summer heat, afternoon shade is strongly recommended to reduce water stress.

Self-seeding & propagation

Individual plants are short-lived (3–5 years), but L. siphilitica is one of the most reliable self-seeders among native perennials. The tiny, dust-like seeds germinate on bare, moist soil the spring after they are shed, and a colony will perpetuate itself indefinitely. To encourage self-seeding, leave at least a few flower spikes standing through October and scatter the seeds onto bare, moist soil nearby. Divide basal rosettes in early spring or fall to expand the colony manually. Seeds require light for germination — surface-sow and do not cover.

Pests & diseases

Companion plants in a rain garden or wet meadow

Great Blue Lobelia is spectacular when massed with its red cousin, cardinal flower (Lobelia cardinalis), which blooms slightly earlier. Other excellent companions in a rain garden or streambank planting include: fox sedge (Carex vulpinoidea) for grassy texture, buttonbush (Cephalanthus occidentalis) for shrub structure, joe-pye weed (Eutrochium purpureum) for pink late-summer plumes, pickerelweed (Pontederia cordata) for deeper water edges, false indigo bush (Amorpha fruticosa) for nitrogen-fixing shrub backdrop, switchgrass for vertical grass accent, and golden alexanders for early-spring yellow before the lobelia comes on.

Sources & Further Reading

Hero photo: Rooted Revival.