Does it need full sun?

It prefers partial shade but will tolerate full sun if the soil stays consistently moist.

Can I grow it in a dry spot?

No, it requires moist to wet soil to thrive.

Plant guide

Great blue lobelia.

Lobelia siphilitica

Pronounced loh-BEE-lee-uh sif-ih-LIT-ih-kuh

Also known as Blue Cardinal-flower, Blue Lobelia, Great Blue Lobelia, Great Lobelia, blue cardinal flower

The great blue lobelia is a lovely perennial that brings a splash of vibrant blue to damp, shaded areas in your garden. It thrives in moist soil and is a fantastic choice for attracting pollinators like bees and butterflies. You'll find it's a hardy plant that adds a nice vertical element to your landscape.

At a glance

Care level: Easy
Plant type: Perennial
Light: Partial shade to full sun
Indoor light: Bright indirect
Water: Every 2 to 4 days
Mature size: 2 to 3 feet
Growth rate: Medium
Hardiness: USDA 4 to 9
Bloom time: Mid summer to early autumn

Care

Keep the soil consistently moist because this plant really dislikes drying out. Watch for wilting leaves as a sign that it needs more water. It does best when protected from the harsh afternoon sun.

Watering: Keep the soil constantly damp to the touch.
Feeding: Apply a balanced liquid fertilizer once a month during the growing season.
Pruning: Cut the plant back to the ground after the first frost in autumn.
Repotting: Repot only if the plant becomes root bound or needs a larger container.
Soil: Moist, rich, well draining soil
Soil pH: 6.0 to 7.5
Temperature: -30 to 30°C
Humidity: 60 to 90%
Drought tolerance: Low
Lifespan: Short lived perennial, typically 3 to 5 years.
Deer and rabbits: High

Signs it needs repotting

Roots growing out of the drainage holes
Water running through the pot too quickly
Plant growth slowing significantly

Safety

People: All parts of the plant contain lobeline, which can cause nausea, vomiting, and respiratory distress if ingested.
Pets: This plant is considered toxic to cats and dogs if ingested.
Children: Keep this plant away from children as ingestion of any part can lead to serious illness.
Contact allergens: The sap may cause mild skin irritation in sensitive individuals.
Airborne allergens: There are no significant reports of airborne allergens.
Edibility: This plant is considered toxic and should not be consumed.

How to identify it

Spikes of tubular blue flowers
Lance shaped toothed green leaves
Sturdy unbranched upright stems
Flowers bloom in late summer

Height: 0.6 to 1m
Spread: 0.3 to 0.45m
Leaves: green, deciduous
Flowers: blue, white, 2 to 3cm

Problems to watch for

Slugs · These mollusks often feed on the tender young foliage in spring.

By symptom

Plant problems by symptom

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Growing it

Propagation: seed, division
When to plant: spring, autumn
Overwintering: Cut back dead foliage in late autumn or early spring. Ensure the crown is not sitting in ice, though it is quite cold hardy.
In containers: It grows well in containers if kept in a self watering pot or a saucer filled with water to maintain constant moisture.

Division

1. Dig up the clump in early spring.
2. Gently pull the root mass apart into smaller sections.
3. Replant immediately in moist soil.

Seed

1. Collect seeds in late autumn.
2. Sow on the surface of moist soil.
3. Provide light, as seeds need it to germinate.

Group this plant with other moisture loving species to ensure the soil stays damp and the microclimate remains humid.

Grows well with

Cardinal Flower · They share similar moisture requirements and bloom at the same time.
Joe Pye Weed · This tall perennial provides a nice structural contrast in wet garden beds.
Blue Flag Iris · It thrives in the same boggy conditions as the lobelia.

Similar species

Cardinal Flower · It is a close relative with identical growth habits but bright red flowers.

Science

Family: Campanulaceae
Genus: Lobelia
Order: Asterales
Class: Magnoliopsida
Native range: Eastern and central North America.
Conservation: It is secure across most of its range and not of conservation concern.

It naturally grows in damp meadows, stream banks, and swampy thickets where it relies on consistent moisture.

Value to wildlife: nectar source
Pollinators: bees, butterflies, hummingbirds
Uses: rain gardens, pollinator gardens, wetland restoration

Through the year

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Flowering

Fruiting

Foliage

Ecological indicators

Salinity: 1/9
Soil reaction: 6/9
Light: 5/9
Temperature: 5/9
Moisture: 8/9
Nutrients: 5/9

Taxonomy resolved against GBIF.

Questions people ask

Does it need full sun?: It prefers partial shade but will tolerate full sun if the soil stays consistently moist.
Can I grow it in a dry spot?: No, it requires moist to wet soil to thrive.

Where the name comes from

The specific epithet siphilitica refers to its historical, though erroneous, use as a treatment for syphilis.

Good to know

The flowers are perfectly adapted for bumblebees to crawl inside.
It thrives in damp soil where other plants might struggle.
The blue color is quite intense and stands out in late summer gardens.

Also in

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This page is a general guide. Save your plant in Pocket Botanist and you get care reminders timed for this plant, plus a care history that shows what worked.

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This guide was generated with AI on October 19, 2026 and checked against GBIF taxonomy. It is general information, not horticultural or veterinary advice. Read about how we build these guides.