Great Blue Lobelia (Lobelia siphilitica)

Great Blue Lobelia botanical diagram

About Great Blue Lobelia

Great Blue Lobelia (Lobelia siphilitica) is grown for garden performance matched to its site needs. Expect full sun or partial sun, typically 0.61–0.91 m tall, high water once established.

Sun Full, Partial
Water High
Height 2–3 ft (60–90 cm)
Spread 1–2 ft (30–60 cm)
Bloom Aug–Sep [1][3]
Habit Herbaceous Perennial
Origin North America, Alabama, Arkansas, Colorado, Connecticut, Delaware, Georgia, Illinois, United States [2]
Season Summer, Fall
Hardiness Zones 4–9

Can Great Blue Lobelia take your winter? Type your ZIP below and compare it with Zones 4–9.

2023 USDA ZIP listings (PRISM / USDA-ARS). Find Your USDA Hardiness Zone · Official USDA map.

Is Great Blue Lobelia Right for Your Garden?

Best for

  • Mixed borders and general garden use
  • Reliably moist soil or pondside positions
  • Open, sunny positions with good air movement
  • Wildlife-friendly gardens — known to attract bees

Not recommended if

  • Free-draining or sandy soils prone to drying out
  • Dense shade or north-facing walls

Trade-off to know

Great Blue Lobelia thrives in moisture but will not tolerate stagnant waterlogging. The distinction between 'moist' and 'wet' matters — good drainage is still required.

Growing Conditions for Great Blue Lobelia

Site checklist

  • Sun: full sun or partial sun
  • Soil drainage: Moist but Well Drained
  • Water: High
  • Hardiness zones: 4, 5, 6, 7, 8, 9

Adjustments

  • Wrong light level is the most common cause of weak performance
  • Fix drainage before treating leaf symptoms

Shared growing steps for this plant’s group: How to Grow Lobelia. Notes below stay specific to this plant.

How to Care for Great Blue Lobelia

Establishment

  1. Plant Great Blue Lobelia in a site matching its sun and drainage needs
  2. Water regularly until roots settle, then match watering to the plant’s need

Ongoing care

  • Mulch to buffer moisture swings
  • Remove dead or damaged growth in season
  • Avoid constant wet foliage where disease pressure is high

Diagnosing Problems with Great Blue Lobelia

SymptomLikely causeConfirming signAction
Leaves yellowing, soft, or dropping in wet conditions Waterlogging or root rot Soil soggy; roots brown and soft when uncovered Improve drainage before replanting Great Blue Lobelia. Do not water until the top 5 cm of soil is dry.
Sudden wilting on one branch while rest of plant looks healthy Root rot from poor drainage in one spot Affected branch has dark, mushy roots when gently uncovered Cut back affected branch to healthy wood. Improve drainage in that area before the next growing season.

Leaves yellowing, soft, or dropping in wet conditions

Likely cause
Waterlogging or root rot
Confirming sign
Soil soggy; roots brown and soft when uncovered
Action
Improve drainage before replanting Great Blue Lobelia. Do not water until the top 5 cm of soil is dry.

Sudden wilting on one branch while rest of plant looks healthy

Likely cause
Root rot from poor drainage in one spot
Confirming sign
Affected branch has dark, mushy roots when gently uncovered
Action
Cut back affected branch to healthy wood. Improve drainage in that area before the next growing season.

Great Blue Lobelia FAQ

  1. Is Great Blue Lobelia easy to grow?

    Great Blue Lobelia needs reliable moisture and is not suitable for dry or neglected gardens. The most common mistakes are poor drainage and incorrect sun placement.

  2. How big does Great Blue Lobelia get?

    Great Blue Lobelia typically reaches 0.61–0.91 m. Final size depends on soil quality and available root space. Allow adequate spacing at planting — most plants reach their full spread within 5–10 years.

  3. How much sun does Great Blue Lobelia need?

    Great Blue Lobelia grows best in full sun or partial sun. Planting in the wrong light level is the single most common cause of poor performance. Check the aspect of your site before choosing a position.

  4. Does Great Blue Lobelia attract wildlife?

    Great Blue Lobelia is known to attract bees, birds. For maximum benefit, avoid deadheading all flowers — seed heads and spent blooms are often as valuable to wildlife as the flowers themselves.

What Great Blue Lobelia Symbolises

  • generous
  • abundant
  • connecting
  • lively

References

  1. [1] GBIF occurrence phenology — Bloom months inferred from StillImage observation seasonality (temperate Northern Hemisphere filter). Not an official cultivar calendar. - https://www.gbif.org/species/5409120
  2. [2] GBIF species distributions — Native range summarised from GBIF distribution records; locality noise filtered to country and macro-region terms. - https://www.gbif.org/species/5409120
  3. [3] NC State Extension - https://plants.ces.ncsu.edu/plants/lobelia-siphilitica/

Taxonomic identifiers

  • https://www.gbif.org/species/5409120 (GBIF 5409120)
  • https://www.wikidata.org/wiki/Q2720384 (Wikidata Q2720384)
  • https://powo.science.kew.org/taxon/kew-352898 (POWO kew-352898)
  • https://www.ipni.org/n/143920-1 (IPNI 143920-1)

Plant details on Bosquelle are curated from published references and garden practice. Height, bloom time, hardiness, and look still shift with climate, soil, and care — use this profile as a reliable starting point, then check what local growers see in your area.