The sun requirements and growing conditions for lobelia plants vary depending on the type of lobelia. One of the most commonly cultivated varieties, Lobelia inflata, is used as a herbal medicinal supplement. In the landscape, Lobelia cardinalis makes a bold statement with bright red flowers. For a more subtle flowering perennial ground cover, select the blue-flowering Lobelia erinus. Lobelia siphilitica has delicate blue flowers but provides more height than the low-growing Lobelia erinus.
Lobelia inflata is an annual that grows well in clay soil, making it ideal for areas with heavy soil. The plant is versatile, thriving in both full sun and partial or dappled shade. It grows 1 foot tall and blooms in mid to late summer. As a medicinal herb, Lobelia inflata is cultivated for home use and grown commercially. Though technically an annual, this lobelia sometimes grows as a biennial, according to Plants for a Future.
Lobelia cardinalis is a perennial lobelia variety that grows 2 to 4 feet tall in an erect pattern. The bright red flowers bloom from mid to late summer, creating a splash of vibrant colour in the perennial flower bed. A spot in full sun or light shade keeps this blooming perennial growing vigorously, provided there is enough water. Lobelia cardinalis requires consistently damp soil to thrive. The Missouri Botanical Garden gave this plant one of its 'Plant of Merit' awards as a landscape perennial.
Lobelia erinus grows just 1/2 to 3/4 foot tall, making it an ideal ground cover in a perennial flower bed or shrub bed. Like other lobelia varieties, this plant enjoys full sun or partial shade. It is not suitable as a ground cover for deep shade areas. The flowers are commonly blue, though some different cultivars display a variety of flower colours, including purple, violet, white and red, according to the Missouri Botanical Garden. Flowers start blooming in spring and continue until the heat of the summer sets in.
Lobelia siphilitica creates height in the garden, growing 2 to 3 feet tall. A spot in the sun suits this lobelia well, though a dappled shade area is also tolerated. The blue flowers bloom between July and September. Plant Lobelia siphilitica in damp or boggy soil where it will remain consistently damp. This flowering perennial is ideal for water gardens and the shallow edges around ponds and other water features.