All are grown for their tubular, lipped flowers, which resemble those of honeysuckle (Lonicera) or salvia. Annual kinds are low plants for edgings or hanging baskets; perennial sorts are larger, vertical-growing plants with flowering stalks that rise above the foliage clumps.
L. cardinalis. CARDINAL FLOWER. Perennial. Zones US, MS, LS, CS; USDA 6-9. Native to eastern U.S. and to a few sites in mountains of the Southwest. Erect, single-stemmed plant to 24 ft. high, 1 ft. wide. Sawtooth-edged leaves are set directly on the stems. Spikes of flame-red, inch-long flowers in summer. A bog plant in nature, it needs rich soil and ample moisture throughout the growing season. 'Queen Victoria' grows 45 ft. tall, with purple-red foliage and scarlet flowers. 'St. Elmo's Fire' ('Elmfeuer') grows 3 ft. tall, with bronze foliage, scarlet flower spikes. 'Summit Snow' is a white-flowering form.
L. erinus. EDGING LOBELIA. Annual. Zones US, MS, LS, CS, TS; USDA 6-11. From South Africa. Popular and dependable edging plant to 36 in. high. Compact forms reach 59 in. wide; trailing types spread to 1 ft. Leafy, branching stems with green or bronzy green foliage. Blooms from spring to frost, bearing -in.-wide flowers in light blue to violet (sometimes pink, reddish purple, or white) with white or yellowish throats. Lives over winter in mild climates. In the Lower, Coastal and Tropical South, grow it as a winter-to-spring annual. If started from seed sown in pots, takes about 2 months to reach planting-out size. Give rich soil, regular water. Self-sows where adapted.
Compact types include 'Cambridge Blue', with green leaves and flowers in a soft pure blue; and 'Crystal Palace', with rich deep blue blossoms and bronzy green leaves. Newer strains include the heat-tolerant Magadi series, in mounding (12 by 10 in.) and trailing (10 by 12 in.) forms, in several shades of purple, blue, and white. Also heat tolerant is the Laguna series in various shades of blue, purple, lavender, and white. Trailing, green-foliaged kinds include the Cascade series, with carmine-red, violet-blue, blue, pink, or white blooms, and 'Sapphire', with bright blue flowers. Trailers are well suited to hanging baskets, wall plantings. They also make a graceful underplanting in containers, where the stems, loaded with flowers, can spill over the edges.
L. siphilitica. BLUE CARDINAL FLOWER. Perennial. Zones US, MS, LS; USDA 6-8. Native to eastern U.S. Leafy plant to 23 ft. tall, 1 ft. wide. Blue summer flowers. Give ample water. 'Alba' has white blossoms.
L.x speciosa (L. xgerardii). Perennial. Zones US, MS, LS, CS; USDA 6-9. Group of hybrids of uncertain ancestry. Can reach 5 ft. tall, 1 ft. wide; many combine red leaves and red flowers. Provide rich soil. Compliment series, to 2 ft. high, has dark green leaves; blossoms may be scarlet, deep red, or blue-purple. Fan series grows 2 ft. high, 16 in. wide; comes in blue, burgundy, red, rose, salmon, and scarlet. 'Grape Knee-Hi', 2 ft. high, is a sturdy plant with rich purple blooms. 'Cotton Candy', named for its bright pink blossoms, grows ft. tall. 'Monet Moment', to 3ft. tall, bears big clusters of rich rose-pink blossoms. 'Ruby Queen' reaches 3 ft. high and has ruby-red blooms; 'Ruby Slippers' grows to 34 ft. tall, with spikes of velvety, ruby-red flowers and foliage that may take on purple tones.