Lobelia siphilitica - Great Blue Lobelia, Bog Plant

Lobelia siphilitica, known as great blue lobelia or blue cardinal flower, is an upright perennial with toothed, lance-shaped leaves. From late summer to mid-autumn it carries spikes of blue flowers.

Great blue lobelia at a glance

Botanical name	Lobelia siphilitica
Common name	Great blue lobelia, blue cardinal flower
Plant type	Upright, clump-forming perennial; dies back in winter
Height and spread	Up to about 1 m tall, up to 50 cm wide
Flowers	Blue, in upright spikes, August to October
Position	Full sun or part shade
Soil	Moist soil; wet in the wild, but the RHS advises moist but well-drained
Pond depth	None – bog garden or damp soil, not under water
Hardiness	RHS H5 – hardy in a cold winter (-15 to -10°C)
Native to	North America (not UK native)

Is great blue lobelia a pond plant?

No. It is a bog and damp-border plant. In the wild it grows on stream banks and in wet meadows, but no source supports planting it under water. Plant it in a bog garden or moist border beside the pond.

Is it good for wildlife?

In North America the flowers attract mainly bumblebees and other long-tongued bees. We found no UK-specific data.

Is it poisonous?

Yes. The RHS says it is harmful if eaten and can irritate skin, for people and pets. Wear gloves when handling it.

How to plant and care for it

Plant in spring in moist soil. Watch for slugs; it can also get crown rot.

Which size should I buy?

Bare-root plants (7 cm, 9 cm, 1 litre and 3 litre) arrive without a pot or compost and are the most economical way to plant a group. Potted plants (9 cm, 1 litre and 3 litre) arrive growing in our peat-free compost and give a faster display. Packs of 3 come in 7 cm, 9 cm, 1 litre and 3 litre bare-root sizes and cost less per plant. The 2x Pot & Compost option is two plants supplied with pots and compost.