Great blue lobelia (Lobelia siphilitica) is a native perennial for the spots most flowers hate: damp, partly shaded ground along streams, rain gardens, low corners and the shady side of a pond. It grows 2 to 3 feet tall, opens spikes of true-blue tubular flowers from August into October, and is one of the best late-season plants for bumblebees. It is hardy in USDA Zones 4 to 9, wants soil that never fully dries out, and keeps itself going by self-sowing. Give it consistent moisture, morning sun or bright shade, and bare soil nearby for seedlings, and it will be one of the least fussy plants in your garden.
In this guide
Key Takeaways
- Native to wet meadows, stream banks and bottomland woods across the eastern and central US; hardy in Zones 4 to 9.
- Plant in part shade to full sun with constantly moist, humus-rich soil; it tolerates wet feet but not drought.
- Blooms late summer to fall on 2 to 3 foot spikes; bumblebees are the main pollinators.
- Individual plants are short-lived (often 2 to 4 years), so let it self-sow or divide rosettes to keep a colony.
- All parts are toxic if eaten, so site it away from grazing pets and children’s play areas.
Great Blue Lobelia at a Glance
| Botanical name | Lobelia siphilitica |
| Plant type | Herbaceous perennial, native to eastern and central North America |
| Hardiness | USDA Zones 4 to 9 |
| Size | 2 to 3 feet tall, 12 to 18 inches wide |
| Bloom | Blue (rarely white), August to October |
| Light | Part shade to full sun (sun only with steady moisture) |
| Soil | Moist to wet, rich in organic matter, pH roughly 5.5 to 7 |
| Wildlife | Bumblebees, occasional hummingbirds; somewhat deer resistant |
Where Great Blue Lobelia Grows Best
In the wild you find it in wet prairies, floodplain woods, ditches and along creeks from New England to the Great Plains and south to the Gulf states. Copy those conditions and the plant needs almost no help. The most important factor is moisture. Great blue lobelia will sit happily in soil that stays soggy for days after rain, but it sulks and browns at the leaf edges if the root zone dries out in July.
Light is the second dial. In the North it can take full sun if the soil is reliably wet, such as at a pond edge or in the low basin of a rain garden. In the South and Midwest summer heat, morning sun with afternoon shade or bright dappled shade under high trees is better. Deep shade produces leggy stems and fewer flowers. A spot with four to six hours of sun and moist ground is ideal. If you are planning a whole border around it, our flowers and perennials hub has care guides for most of its natural partners.
Good garden spots
- The lower zone of a rain garden, where water collects for a day or two after storms.
- Pond and stream margins, in soil just above the waterline rather than submerged.
- Low spots where downspouts drain or lawns stay soggy.
- Woodland edges with leaf-mold soil and afternoon shade.
- Large containers with a saucer, kept moist, for patios with no wet ground.
How to Plant Great Blue Lobelia
Spring planting is easiest because plants have a full season to root before they bloom in late summer. Early fall works in Zones 6 to 9 if you get plants in at least six weeks before the ground freezes. Most native nurseries sell it in quart or 1-gallon pots, and plug trays are common for rain garden projects.
- Prepare the soil. Work 2 to 3 inches of compost or leaf mold into the top 6 to 8 inches. Clay is fine as long as it holds moisture; very sandy soil needs extra organic matter.
- Space plants 12 to 18 inches apart. Closer spacing gives a fuller drift faster, which also shades the soil and keeps it cool and damp.
- Set the crown at soil level. Planting too deep invites crown rot, especially in wet sites.
- Water deeply and keep the soil consistently moist for the first month.
- Mulch lightly with shredded leaves or fine bark, about 1 to 2 inches, but keep mulch off the basal rosette.
Growing From Seed
Great blue lobelia is easy and economical from seed, which matters for larger rain gardens. The seed is dust-fine and needs light to germinate, plus a period of cold, moist conditions to break dormancy.
- Winter sowing (easiest): In December through February, surface sow on moist seed-starting mix in a covered container with drainage and vent holes, and leave it outdoors. Spring temperature swings break dormancy naturally.
- Indoor stratification: Mix seed with a little damp sand in a bag and refrigerate for about 60 days, then surface sow under lights at 65 to 75 degrees F. Do not cover the seed. Germination usually follows within a few weeks.
- Direct sow in fall: Scatter seed on bare, moist soil in late fall. Mark the spot, because seedlings are tiny and easy to weed out by mistake.
Seedlings grow slowly at first and usually form a rosette the first year, then bloom in their second summer. Transplant when they have several true leaves and harden them off for a week before going into the ground.
Ongoing Care
Watering
Outside naturally wet sites, plan on about 1 to 1.5 inches of water a week in summer, delivered in one or two deep soakings. Soaker hose under mulch works well. Wilting in afternoon heat that recovers by evening is normal; leaves that stay limp or crisp at the edges mean it is too dry.
Feeding
Skip synthetic fertilizer. A yearly top-up of compost or shredded leaves in spring is enough. Rich feeding produces floppy stems.
Deadheading and staking
Plants rarely need staking unless they are in too much shade. Deadheading the main spike right after it fades can push a few side spikes. If you want seedlings, leave at least some spikes to ripen and drop seed in fall.
Winter care
Great blue lobelia overwinters as a flat, evergreen-ish rosette of leaves at ground level. That rosette is the plant, and it rots if smothered. Cut old stems down in late fall or early spring, but do not bury the rosette under thick mulch or matted leaves. A light layer of loose leaves around it is fine.
Keeping a Colony Going
Like its close relative, red cardinal flower, great blue lobelia is a short-lived perennial. Individual crowns often fade after two to four years. Gardeners who lose it usually lost it to either drought or a lack of new seedlings. There are two ways to keep it:
- Let it self-sow. Seed needs bare, moist soil and light to sprout. Pull mulch back in a small area around plants in fall, and avoid heavy mulching where you want seedlings. Thin to 12 inches in spring and move extras.
- Divide rosettes. In early spring or right after flowering, look for small offset rosettes at the base. Lift them with a trowel and replant at once. This keeps the colony vigorous and is the only way to keep a named form true.
Pollinators and Wildlife
The flower shape is built for bumblebees. A bee lands on the lower lip, pushes inside, and gets dusted with pollen from the overhanging column. Because it blooms after many summer perennials fade, it is an important late nectar source for queen bumblebees fattening up before winter. Hummingbirds occasionally visit, though they prefer red cardinal flower. The leaves are bitter, and deer and rabbits usually leave it alone, though no plant is deer-proof in a hungry winter.
Companion Plants
Pair great blue lobelia with other moisture-loving natives so the planting reads as a designed drift rather than a lone spike. Our native plant garden guide covers the design side, and these are proven partners:
- Cardinal flower (Lobelia cardinalis): red spikes at the same time, same conditions. See our guide on how to grow cardinal flower.
- Turtlehead (Chelone glabra and C. lyonii): pink or white blooms in late summer for wet shade.
- Swamp milkweed (Asclepias incarnata): a monarch host plant for sunny wet spots.
- Joe-pye weed (Eutrochium): tall back-of-border height for big rain gardens.
- Blue flag iris, sedges and ferns: structure and foliage before and after bloom.
A simple rain-garden combination: a center of joe-pye weed, a ring of swamp milkweed and great blue lobelia at 15 inches apart, cardinal flower tucked between the lobelias for red and blue together, and tussock sedge along the edge. For more ideas on choosing plants by light, see sun vs shade perennials.
Varieties and Hybrids
- Straight species: the best choice for pollinators and naturalizing; seed-grown plants vary slightly in shade of blue.
- ‘Alba’: a white-flowered form, nice in shade where white reads brighter.
- Lobelia x speciosa hybrids: crosses between great blue lobelia and cardinal flower in purple, magenta and rose shades. They are showy but are often shorter-lived and do not come true from seed.
Problems and Fixes
- Plant disappeared after winter: usually crown rot from mulch or matted leaves sitting on the rosette, or simply age. Keep rosettes exposed and let seedlings replace old crowns.
- Brown, crispy leaf edges: drought stress. Water deeply and add mulch around, not on, the crown.
- Holes in leaves: slugs and snails in damp shade. Hand-pick at night or use iron phosphate bait.
- Floppy stems, few flowers: too much shade or too much fertilizer.
- Leaf spots in humid weather: improve air flow by thinning crowded seedlings; remove badly spotted leaves.
A safety note: Lobelia species contain alkaloids, including lobeline, and are toxic if eaten. The “siphilitica” name comes from a historical, unproven folk use. Do not use it medicinally, and plant it where pets that chew plants and small children are not likely to graze.
Frequently Asked Questions
Does great blue lobelia spread?
It spreads mainly by seed, not aggressive roots. In moist soil with some bare ground it self-sows modestly and forms a loose colony over a few years. Seedlings are easy to pull or move.
Can great blue lobelia grow in full sun?
Yes, if the soil stays consistently moist, such as at a pond edge or in a rain garden basin. In hot summer regions or average garden soil, part shade with afternoon protection works better.
When does great blue lobelia bloom?
Usually from August into October, depending on region. Seed-grown plants typically bloom in their second year after forming a rosette in the first.
Is great blue lobelia the same as annual edging lobelia?
No. Edging lobelia (Lobelia erinus) is a small trailing annual for pots and baskets. Great blue lobelia is an upright native perennial for moist beds and rain gardens.
Why did my great blue lobelia die after two years?
It is naturally short-lived. Crowns often fade after two to four years, and winter crown rot speeds that up. Leave bare soil for seedlings and divide offset rosettes in spring to keep plants coming.