While both plants belong to the genus Lobelia, their fundamental life cycles and structures are vastly different. Lobelia erinus is a tender perennial almost universally grown as a low-growing, trailing annual. It forms dense mounds or spreading mats, typically reaching only 4 to 9 inches in height. Its stems are thin, herbaceous, and highly branched, making it an ideal plant for the front of borders, containers, and hanging baskets.
In contrast, the Cardinal Flower (Lobelia cardinalis) is a robust, upright herbaceous perennial. It grows in distinct clumps from a basal rosette of leaves, sending up sturdy, often unbranched stalks that can reach impressive heights of 2 to 4 feet. Its growth is focused on creating a strong vertical presence in the garden, often dying back to the ground in winter and re-emerging from its crown in spring.
The light preferences of these two Lobelias highlight their adaptation to different ecological niches. Lobelia erinus thrives in cooler conditions and performs best in full sun to partial shade. In regions with hot summers, providing afternoon shade is crucial to prevent the plant from wilting, becoming stressed, and ceasing to bloom. It is frost-tender and cannot survive freezing temperatures.
The Cardinal Flower, however, has a strong preference for partial shade, especially in hotter climates. While it can tolerate morning sun, it is naturally a plant of moist woodlands and stream banks. It is far more cold-hardy, surviving winters in USDA zones 3 through 9. Its above-ground growth dies back after frost, but the plant itself is dormant and perfectly adapted to survive freezing conditions.
This is arguably the most critical care difference. Lobelia erinus requires consistently moist, well-drained soil. It enjoys regular watering but will quickly succumb to root rot if left in soggy, waterlogged conditions. Good drainage is non-negotiable. It prefers a rich, organic soil but is relatively adaptable as long as moisture levels are maintained without being excessive.
The Cardinal Flower has a completely different relationship with water. It is a true moisture-loving plant, often described as a marginal aquatic. It thrives in consistently wet to soggy soils and is an excellent choice for the edges of ponds, streams, or in rain gardens. It can even tolerate temporary standing water. Denying it constant moisture, especially in full sun, will lead to stunted growth, leaf scorch, and ultimately the death of the plant.
Lobelia erinus produces a profusion of small, delicate flowers in shades of blue, violet, pink, or white. It flowers continuously from spring until the heat of summer causes it to decline. To encourage a second flush of blooms in cooler autumn weather, it can be cut back midsummer. It is almost always propagated by seed, sown indoors many weeks before the last frost.
The Cardinal Flower blooms in late summer, producing a spectacular spike of intense, cardinal-red flowers (occasionally white or rose-pink forms exist). Each flower is a larger, tubular structure specifically adapted for pollination by hummingbirds. After flowering, the main stalk can be deadheaded to encourage a potential second, smaller bloom. Propagation is most commonly done by division of the clump in spring or by collecting and sowing seeds, as it is a short-lived perennial that benefits from regular division to maintain vigor.
