Lobelia siphilitica – Hardy perennial with rich blue blooms

Lobelia siphilitica, also known as blue cardinal lobelia, is a robust, hardy perennial known for its striking, upright inflorescences of intense blue flowers. This species originates from North America and is valued in Europe as a reliable border plant, providing an abundance of color year after year, even in damp or partially shaded locations where other plants sometimes struggle.

With its strong, upright stems, elegant leaf structure and long flowering period, Lobelia siphilitica is one of the most low-maintenance and versatile Lobelia varieties for gardens and landscapes.

Characteristics of Lobelia siphilitica

• Plant type: hardy perennial

• Height: 60–90 cm

• Flower color: deep blue to violet-blue

• Flower shape: tubular, in dense spikes

• Flowering period: July – September

• Location: sun to partial shade

• Soil: moist, humus-rich

• Special features: very winter hardy, excellent for damp garden areas

Intense blue flowers that stand out in the border

The most striking feature of Lobelia siphilitica is its beautiful deep blue to violet flowers, which appear in dense vertical clusters at the top of the stems. The flowers are tubular with light accents in the throat, creating a subtle color gradient.

Thanks to its upright flowering habit, the plant adds structure and height to mixed borders, pond edges, and stream zones. The flowers are abundantly visited by bees, bumblebees, and butterflies, making the plant a valuable contribution to garden biodiversity.

Reliable growth in moist locations

Lobelia siphilitica prefers moist, fertile soil. It thrives alongside ponds, in moist borders, at the edge of bog gardens, or in spots with light, partial shade.

Key benefits:

• tolerates wet or moist soils well

• performs excellently in partial shade

• continues to bloom reliably even in less than ideal summers

However, this Lobelia also feels at home in sunny borders, provided the soil can retain sufficient water.

Hardy and problem-free

Unlike many other Lobelia species, Lobelia siphilitica is very hardy. The plant dies back above ground in winter, but vigorously re-emerges in spring. With minimal care and an annual layer of mulch, the plant will remain healthy and bloom profusely for many years.

Care and cultivation

• Water: keep the soil slightly moist; does not tolerate prolonged drought

• Fertilization: some compost or organic fertilizer annually increases flowering.

• Pruning: remove dead stems after flowering

• Location: sun to partial shade, preferably moist

By dividing the plant annually (every 3 to 4 years), Lobelia siphilitica remains vigorous and the flowering becomes more and more beautiful.

Application in gardens and landscape planting

Lobelia siphilitica is at its best in:

• perennial borders

• natural gardens and wild gardens

• pond zones, stream edges and damp garden areas

• shady planting areas

• prairie and forest edge plantings

The intense blue colour forms a beautiful contrast with yellow or white flowering plants such as Rudbeckia, Helenium, Leucanthemum and Chelone.