Description of Lobelia siphilitica (Blue Lobelia)

Lobelia siphilitica is a moisture-loving perennial with tall spikes of blue flowers. You will also see it listed as blue lobelia or blue cardinal flower. It is a good pick when you want mid to late summer colour at the pond edge.

Appearance

Lobelia siphilitica grows as an upright clump with leafy stems. Leaves are lance-shaped, mid green, and lightly toothed. Flower spikes rise above the foliage and open from the bottom up. Each flower is a clear blue with a paler throat, and the whole spike keeps going for weeks because new blooms keep opening. In the UK, flowering is often July to September. Height is usually 60 to 90cm, with a spread around 30 to 45cm, depending on soil depth and moisture.

It looks best planted in small groups, as one plant can read as a single vertical line. Try it with irises, rushes, and royal fern for height, and with marsh marigold and primulas for spring interest.

Care Guide

Plant Lobelia siphilitica in deep, fertile soil that stays reliably moist. It suits bog gardens, damp borders, and pond edges where the water table stays close to the surface. Give it sun or part shade. In full sun, keep watering steady so it does not stall.

• Water: Keep the root zone damp through summer. In a lined bog planter, check moisture during hot weeks and top up.
• Soil: Add compost when planting. It likes a richer mix than many pond-edge grasses.
• Feeding: A spring mulch often covers it. If growth is thin, feed every couple of weeks from late spring to midsummer with a balanced liquid feed, then stop.
• Cutting back: Leave stems until late winter for structure, then cut to ground level before new shoots rise.
• Division: Split clumps every 2 to 3 years in spring. Replant divisions straight away in damp soil.

Suitability for Bog Garden

Lobelia siphilitica suits the main body of a bog garden where soil stays wet but not submerged. It also works in a pond basket on a shelf, with the crown kept above water and the compost kept saturated. It brings late-season colour, so it balances spring-heavy plants like primulas and marsh marigold. If your bog garden is in a windy spot, plant it among sturdier stems like rushes so it stays upright. Once settled, it is a steady performer and not fussy, as long as you do not let it dry out.

FAQ

Is blue lobelia hardy in the UK?
Yes. It dies back in winter and comes back in spring in most gardens.

Does it like sun or shade?
Sun or part shade. In a bog garden, part shade can stop the soil baking in summer.

How do I keep it going?
Cut down old stems in late winter and divide clumps every few years.