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Blue Lobelia

Flower

Lobelia siphilitica

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Blue Lobelia is a tall native wildflower of moist meadows and streambanks producing vivid royal-blue flower spikes from late summer through autumn. It is one of the most important late-season native bee plants in eastern North America and is especially attractive to bumblebees, which are primary pollinators. It pairs beautifully with Cardinal Flower in rain gardens and streamside plantings.

Native Range

Origin
Native to eastern and central North America.
Native Habitat
Moist meadows, streambanks, wet woods, floodplain edges, and seeps.
Current Distribution
Widespread across eastern North America within native range; cultivated in wet and rain gardens across temperate regions.
Blue Lobelia

Growing Conditions

Sunlight

Full Sun

Water Needs

Moderate

Soil

Rich, consistently moist to wet soil; pH 5.5 - 7.0; tolerates clay and wet conditions

Spacing

12 - 18 inches

Days to Maturity

Blooms August - October in year two onward; self-sows reliably

Growing Zones

1
2
3
4
5
6
7
8
9
10
11
12
13

Thrives in USDA Zones 4 - 9

Companion Planting

Good Companions

Keep Away From

No known antagonists

When to Plant

  • Direct Sow

    Surface sow in autumn for natural stratification, or start indoors 10 weeks before last frost

  • Harvest

    Collect seed capsules as they begin to brown in autumn; seed is very tiny

Phenology (Natural Timing Cues)

Start Indoors

Blue Lobelia is easily started from seed indoors 10 weeks before the last frost date. Surface sow on moist potting mix without covering - seed needs light to germinate. Keep at 65-70°F and germination occurs in 2-3 weeks. Transplant to the garden in moist conditions. It also self-sows reliably once established; a single plant will produce a colony over 3-5 years. The plant typically flowers in its second year from seed.

  • Start indoors 10 weeks before the last frost date.
  • Surface sow - do not cover seed; light is required.
  • Transplant after last frost to consistently moist garden site.
  • For autumn sowing outdoors: sow after last hard frost, before ground freezes.

Start Dates (Your Location)

Based on your saved growing zone and this plant's timing notes.

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Typical Last Frost

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Typical Harvest Window

August to October

Organic Growing Tips

  • Allow seed heads to mature and disperse naturally; self-seeding maintains the colony.

  • Mulch around plants to maintain consistent soil moisture - the single most important cultural factor.

  • In drier gardens, plant in a rain garden, swale, or near a downspout where extra moisture accumulates.

  • Divide established clumps in spring to propagate; each rosette can be separated and replanted.

Common Pests

All pest management in Garden uses safe, organic, non-toxic methods only. No synthetic pesticides, ever.

Taxonomy

Kingdom
Plantae
Family
Bellflower family (Campanulaceae)
Genus
Lobelia
Species
siphilitica

Natural History

Lobelia siphilitica is native to moist meadows, streambanks, wet woods, and floodplain edges across eastern and central North America from New England west to Nebraska and south to Alabama. The species epithet siphilitica reflects a 17th-century account - reported by the explorer Sir William Johnson, who received the information from Iroquois peoples - that the root was used to treat syphilis. This claim attracted considerable European medical interest, and specimens were sent to European herbalists and physicians who investigated but could not confirm the claimed properties. Despite the misleading epithet, the plant has an extensive and genuine documentation of use for other conditions in Indigenous traditions. Blue Lobelia is ecologically paired with Cardinal Flower (Lobelia cardinalis) - their flowering times, habitats, and pollinators overlap but differ: Blue Lobelia serves bumblebees (with tongues long enough to access the deep blue flowers) while Cardinal Flower serves hummingbirds. Together they represent an elegant partitioning of pollinator resources in late-summer wet habitats.

Morphology (Plant Structure & Identification)

  • Root System

    Fibrous, shallow roots with a basal rosette that persists through winter; plants spread slowly by offsets.

  • Stem

    Single upright stem 2 - 4 feet tall; hairy; leafy throughout; bearing a long terminal flower spike.

  • Leaves

    Alternate, lance-shaped to oblong; slightly toothed; pale green; progressively smaller up the stem.

  • Flowers

    Irregular, two-lipped blue to blue-violet flowers 1 inch long crowded into a long terminal spike; bloom August through October; deeply attractive to long-tongued bumblebees.

  • Fruit

    Small round capsules containing many tiny seeds; capsules split to release seed when mature in autumn.

Known Varieties

Common cultivars worth knowing

  • Straight Species

    Seed-grown Blue Lobelia with the classic royal-blue flower color; some natural variation in flower intensity.

    Best for: Rain gardens, streamside plantings, late-season pollinator support

  • Alba

    White-flowered selection; otherwise identical in habit and ecology.

    Best for: White gardens, contrast with Cardinal Flower

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