Lobelia siphilitica is a member of the Campanula, or bellflower, family. It is also called Great Blue Lobelia or Blue Cardinal Flower. This lovely perennial blooms in the summer from tall, slender stalks. The flowers, unlike the red of the cardinal flower, are blue. Lobelia siphilitica is hardy in zones 4 to 8 and prefers light to part shade and moist, conditions. They self-sow, but are not an invasive plant.
Lobelia siphilitica is a tall perennial growing up to 3 feet. It's slender stalks are topped with tubular blue flowers from late summer to fall. Lobelia siphilitica 'Alba' and 'Albiflora' are white-flowered cultivars. Lobelia siphilitica 'Blue Peter' has pale blue flowers. Although short-lived, lobelia will self-sow, growing new plants for the following year.
Where to Plant
Choose a site with moist, rich soil in light to partial shade. Areas that have a warmer climate should provide afternoon shade. Along a stream, by a pond or in a bog garden are ideal sites for these plants. If planting in a cottage garden or border, place them in the rear, as they will get tall. They are perfect companion plants for ferns, astilbes and hostas.
How to Plant
Plant container-grown plants in the spring. If planting in groups, space them at least 1 foot apart. Dig a hole large enough to accommodate your plant. Place it in the hole, backfill with soil and compact the earth. Water generously.
Lobelias need to be kept moist, but not water logged, otherwise they can suffer from root rot. Deadhead any spent flowers to encourage new ones. To increase your plant's bushy appearance, pinch back the stalks to the top of the plant.
In the fall, cutting back is not needed. A thin layer of mulch is recommended over the winter months. Plants can be divided every few years. New clumps that form from the mother plant can be dug up and transplanted to another site.
Lobelia siphilitica may be prone to rust and leaf spots, which are the result of getting the leaves wet when watering, and poor air circulation. Water the plant at its base and divide clumps if overcrowding occurs to provide more air circulation. Areas with too much moisture may attract slugs and snails as well.