Great Blue Lobelia

Lobelia siphilitica

Great Blue Lobelia (Lobelia siphilitica) is a perennial native wildflower. Hardy in USDA zones 4 to 9. Prefers full sun.

Sun

Full sun to partial shade

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Zones

4–9

USDA hardiness

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Height

1-4 feet

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Planting Timeline

Zone:

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Transplant

Direct Sow

Transplant

Direct Sow

Showing dates for Great Blue Lobelia in USDA Zone 7

All Zone 7 native-wildflower →

Zone Map

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Great Blue Lobelia · Zones 4–9

What grows well in Zone 7? →

Growing Details

Difficulty

Easy

Spacing12-18 inches

SoilMoist, well-drained soil; tolerates slightly acidic to neutral pH

WaterHigh

SeasonPerennial

ColorBlue to purple-blue

Zone-by-Zone Planting Calendar

Zone	Indoor Start	Transplant	Direct Sow	Harvest
Zone 4	—	June – July	April – June	—
Zone 5	—	May – July	April – June	—
Zone 6	—	May – July	April – June	—
Zone 7	—	May – June	March – May	—
Zone 8	—	April – June	March – May	—
Zone 9	—	March – May	February – April	—

Complete Growing Guide

Light: Dappled Sunlight (Shade through upper canopy all day), Full sun (6 or more hours of direct sunlight a day), Partial Shade (Direct sunlight only part of the day, 2-6 hours). Soil: High Organic Matter. Drainage: Moist, Occasionally Wet. Height: 1 ft. 6 in. - 4 ft. 0 in.. Spread: 1 ft. 0 in. - 1 ft. 6 in.. Maintenance: Low. Propagation: Division, Seed. Regions: Mountains, Piedmont.

Harvesting

Displays from August to November

Harvest time: Fall

Storage & Preservation

Great Blue Lobelia is an ornamental wildflower, not typically harvested or stored for consumption. If cutting flowers for fresh arrangements, place stems in cool water immediately and store in a cool location (50-55°F) away from ethylene-producing fruits. Keep humidity moderate to prevent wilting. Cut flowers typically last 7-10 days. For preservation, press flowers between absorbent paper for dried arrangements, or dry entire stems upside-down in a well-ventilated area for 2-3 weeks. Alternatively, preserve seeds by collecting mature seedpods in late summer and storing in cool, dry conditions for next season's planting.

History & Origin

Lobelia siphilitica, commonly called Great Blue Lobelia, is a species native to eastern North America rather than a deliberately bred cultivar with a documented origin story. The plant occurs naturally from Quebec and Ontario south to the Carolinas and Missouri, where it inhabits wetlands, stream banks, and moist meadows. Its scientific name derives from the Latinized form of Philippe de Sistèle, a 16th-century Flemish botanist, and the Greek word siphon, referring to its tubular flowers. As a wild species, Great Blue Lobelia entered horticulture through plant collectors and native plant enthusiasts rather than formal breeding programs, making it a heritage selection preserved through seed saving and propagation of wild populations.

Origin: Eastern North America

Advantages

+Attracts: Butterflies, Hummingbirds, Pollinators, Predatory Insects
+Low maintenance

Considerations

-Toxic (Flowers, Fruits, Leaves, Roots, Sap/Juice, Seeds, Stems): Low severity

Companion Plants

Cardinal Flower (Lobelia cardinalis) is the most natural pairing — same genus, overlapping mid-to-late summer bloom, and both draw hummingbirds and long-tongued bumblebees without competing at the root level. Wild Bergamot and New England Aster extend that pollinator activity into fall, so you end up with a continuous forage stretch rather than a single spike of bloom. Joe Pye Weed and Swamp Milkweed belong in the same wet-meadow or rain-garden planting for practical reasons: they share the same moisture preference, so none of them are fighting each other for water. Wild Ginger and ferns make useful low groundcovers in shadier spots where Great Blue Lobelia is running on 4–5 hours of sun.

Black Walnut (Juglans nigra) is the one exclusion that really matters — it releases juglone from its roots, and Lobelia siphilitica is sensitive enough that even planting on the outer edge of a walnut's root zone can stunt or kill it. Autumn Olive and Purple Loosestrife are both invasive enough that growing anything near them is asking for trouble; you'll spend more time pulling their seedlings than tending the lobelia.

Plant Together

Wild Bergamot

Shares similar moisture requirements and attracts beneficial pollinators like bees and butterflies

Cardinal Flower

Both prefer moist soils and create stunning red-blue color combinations while attracting hummingbirds

New England Aster

Extends blooming season into fall and provides habitat for beneficial insects

Joe Pye Weed

Thrives in similar wet conditions and attracts butterflies while providing structural support

Swamp Milkweed

Shares preference for moist soils and supports monarch butterflies and other pollinators

Wild Ginger

Provides ground cover and retains soil moisture that Great Blue Lobelia requires

Ferns

Create ideal understory conditions with consistent moisture and partial shade

Elderberry

Provides natural windbreak and attracts beneficial insects while tolerating wet conditions

Keep Apart

Black Walnut

Releases juglone toxin that inhibits growth and can kill sensitive plants like lobelia

Autumn Olive

Aggressive invasive shrub that outcompetes native plants for nutrients and space

Purple Loosestrife

Invasive wetland plant that aggressively displaces native species in moist habitats

Pests & Disease Resistance

Common Pests

Spider mites, aphids, slugs

Diseases

Powdery mildew, root rot in poorly drained soil

Troubleshooting Great Blue Lobelia

What you'll see, why it happens, and what to do about it.

White powdery coating on leaves and stems, usually appearing mid-summer when nights cool down

Likely Causes

Powdery mildew (Erysiphe or Golovinomyces spp.) — thrives when humidity is high but leaves stay dry, common in crowded plantings
Poor airflow from dense neighboring plants or spacing tighter than 12 inches between crowns

What to Do

1.Thin the planting to at least 12–18 inches between crowns to open up airflow
2.Spray affected foliage with a diluted solution of 1 tablespoon baking soda per gallon of water — catches it early if you stay on it
3.Cut back badly infected stems to the base; this plant rebounds well from a hard cut

Stems collapsing at soil level, roots dark and mushy when you pull the plant

Likely Causes

Root rot from Phytophthora or Pythium spp. — both thrive in waterlogged, low-oxygen soil
Planting in a site that holds standing water between rain events, even though this species wants consistent moisture

What to Do

1.Don't plant in standing-water spots — Great Blue Lobelia wants moist soil, not saturated soil; there's a real difference
2.Amend heavy clay beds with 2–3 inches of compost worked in before planting to improve drainage
3.Remove and discard collapsed plants; don't replant Lobelia siphilitica in that exact spot for at least two seasons

Ragged holes in leaves or seedlings disappearing overnight, especially in spring

Likely Causes

Slugs (Deroceras reticulatum and related species) — active at night and in wet weather, nearly invisible during the day
Young transplants under 6 inches tall are most vulnerable in the first few weeks after setting out

What to Do

1.Set out iron phosphate bait (Sluggo or equivalent) around new transplants — it's wildlife-safe and works
2.Check under mulch and debris near the crowns in the early morning with a flashlight and handpick
3.Pull mulch back a few inches from the base of each plant to remove slug daytime cover until plants are established

Frequently Asked Questions

When should I plant Great Blue Lobelia?▼

Plant in spring after the last frost date, or in fall (4-6 weeks before first frost) for established plants by next season. This perennial performs best when given time to establish strong roots before winter. In warmer zones (8-9), fall planting is often ideal as it takes advantage of cooler growing conditions.

Can you grow Great Blue Lobelia in containers?▼

Yes, Great Blue Lobelia grows well in containers filled with well-drained potting soil. Choose a pot at least 12 inches deep to accommodate its root system. Container plants may dry out faster, so monitor moisture regularly. This is an excellent option for areas with poor garden soil or for creating portable flower displays on patios.

How long does it take Great Blue Lobelia to flower from seed?▼

Seeds typically germinate in 10-15 days under appropriate conditions. Plants grown from seed usually reach flowering size in their first year when started indoors 6-8 weeks before transplanting, blooming from mid-summer through fall with stunning blue-purple spikes.

Is Great Blue Lobelia good for beginners?▼

Absolutely. With an 'Easy' difficulty rating, this native wildflower is forgiving and low-maintenance. Once established, it's drought-tolerant and self-seeds readily. It attracts hummingbirds and pollinators naturally, making it perfect for beginner gardeners wanting to support wildlife while enjoying stunning blooms.

What does Great Blue Lobelia look like?▼

Great Blue Lobelia features tall, leafy stems topped with showy spikes of bright blue to purple-blue flowers. Plants grow 24-36 inches tall with a narrow, upright form. The flowers bloom sequentially along the spike from bottom to top, creating a long season of color from July through September.