These clump-forming plants are mint relatives, with the family's typical square stems and paired leaves. Long, tubular flowers flare out into two lips, the upper one narrow and hooded, the lower one broad; in many species, the lower lip is marked in white. The species listed here are easy to grow in light, gravelly, well-drained soil; tolerate drought and alkaline soil.
S. drummondii. DRUMMOND SKULLCAP. Annual. Native to Texas, New Mexico, Oklahoma, Florida, Louisiana, Arizona, and Mexico. Upright, branching plant forms a mound 1 ft. tall and wide. Oval, gray-green leaves to 1 in. long; lavender spring flowers about 12 in. across.
S. ocmulgee. OCMULGEE SKULLCAP. Perennial. Zones MS, LS, TS; USDA 7-10. Native to Georgia and South Carolina. Upright grower to 2 ft. tall and 3 ft. wide, with fuzzy, gray-green leaves to 3 in. long. Midsummer blooms are lavender-blue. Rare in nature and in commerce. Listed as a threatened species in Georgia.
S. resinosa. RESINOUS SKULLCAP, PRAIRIE SKULLCAP. Perennial. Zones MS, LS, TS; USDA 7-10. Native from Kansas and Colorado south to Texas and Arizona. Mounding plant 68 in. tall, 1 ft. wide, with roundish, resinous, grayish green leaves less than 12 in. long. One-sided, elongated clusters of 1-in., blue-violet flowers are produced in late spring. Deadhead to encourage intermittent bloom through autumn. 'Smoky Hills' grows 1012 in. high and 1215 in. wide; flowers are deep purplish blue.
S. suffrutescens. PINK SKULLCAP. Perennial. Zones MS, LS, CS; USDA 7-9. From Mexico. Forms a dense mound to 1 ft. tall, 112 ft. wide, with oval, bright green leaves just 12 in. long. Deep rosy pink flowers to 1 in. long bloom from late spring until frost. 'Texas Rose' is a superior selection.
S. 'Violet Cloud'. Perennial. Zones US, MS, LS; USDA 6-8. Sterile hybrid resulting from a cross between S. resinosa and S. suffrutescens. Forms a low mound about 8 in. high and 12 in. wide, with profuse deep violet blooms from late spring through fall.
S. wrightii. SHRUBBY SKULLCAP. Perennial. Zones LS, CS, TS; USDA 8-11. Native to Texas and Oklahoma. Mounding plant 610 in. high and a bit wider, with small, rounded, green leaves. Deep violet-blue, 12- to 1-in.-long flowers have prominent white anthers that give them a striped appearance. Blooms heavily in June, then off and on until frost. Good for massing and rock gardens. Evergreen in mild-winter areas.