Commonly known as the cardinal flower, Lobelia cardinalis is a moisture-loving, herbaceous perennial that blooms in bright red flowers during autumn. Lobelia cardinalis plants grow 1 to 3 feet tall and have leaves and stems with a purplish tint. The brilliant red flowers are 1 to 2 inches long and tubular in shape. Cardinal flowers are easy to grow, provided that they're planted in the appropriate location and receive plenty of moisture.
Water your Lobelia cardinalis flowers regularly to keep the soil moist at all times. Cardinal flowers love moisture, so you'll need to water them deeply and thoroughly two or three times per week in the absence of rainfall. If you're growing the flowers in a boggy area or beside a pond, you'll likely need to water them less frequently.
Spread a 1- to 2-inch-thick layer of organic compost on the ground around your Lobelia cardinalis plants to preserve soil moisture and keep weeds at bay. Hand pull any weeds that grow around your cardinal flowers.
Fertilise your Lobelia cardinalis plants once each year in early spring, just as the new growth emerges. Apply 0.454 to 0.907 Kilogram of 8-8-8 NPK formula fertiliser per 100 square feet of flower bed area, following the instructions on the label.
Remove the low-growing rosette leaves on the Lobelia cardinalis in early winter. Doing so helps to ensure that the plant will survive the winter and regrow in spring.
Plant your Lobelia cardinalis in a location that has moist soil. Keep in mind that in the wild, cardinal flowers tend to grow beside streams, in bogs and in moist meadows. Try growing these perennials beside a water garden or pond, or in a flower bed with other moisture-loving plants like ferns or cannas. Although cardinal flowers will grow in full sunlight, they prefer partial shade and can grow even in deep shade.
Don't plant your Lobelia cardinalis flowers in a spot where the soil dries out easily or watering is difficult. Cardinal flowers require moist to wet soil conditions to thrive. If you're growing Lobelia cardinalis with other plants in a flower or garden bed, select companion plants that also enjoy wet conditions.