Culture
Winter hardy to USDA Zone 8 (marginally hardy in Zone 7 with protection and mulch) where it is best grown in medium to wet soils in full sun to part shade. Best flowering is in full sun. This plant thrives in moist, fertile, humusy but well-drained soils. It is a versatile plant that tolerates an extremely wide range of growing conditions. It thrives as a marginal water plant and in boggy soils. It also does well in average garden soils with even moisture. Established plants have respectable drought tolerance. Plants also tolerate high heat and humidity. Cut back stems after flowering to encourage new flowers. Plants will spread by rhizomes and self-seeding in the garden, and have escaped gardens and aggressively naturalized in parts of the southeastern U.S. Notwithstanding its value as an excellent flowering plant, this species is currently listed as a Category One invasive species by the Florida Exotic Pest Plant Council (FEPPC) because it has been found to invade natural areas and displace native flora in the State of Florida. Plants are most invasive in moist areas. Invasiveness is clearly not a problem in St. Louis, however, where these plants are grown as annuals, with stem cuttings overwintered indoors if desired. Easy to propagate by cuttings, division and seed.
Noteworthy Characteristics
Ruellia simplex, commonly called Mexican petunia or Texas petunia, is a vigorous, shrubby, woody-based, rhizomatous perennial that is grown as an annual north of USDA Zone 8. It is native to Mexico, but has escaped gardens and naturalized somewhat aggressively in parts of the southeastern U. S. from South Carolina to Texas plus Hawaii, Puerto Rico and the U. S. Virgin Islands. It typically grows to 3-4’ tall in the wild, but to 2-3’ tall in gardens. Plants branch from the ground into several woody-based stems clothed with elongated, linear, willow-like, dark green leaves (to 6-12” long and ¾” wide) that are often tinged with purple. Tubular, trumpet-shaped, 5-lobed, petunia-like, lavender to violet flowers (to 1.75” long) bloom from the upper leaf axils in loose purple-stemmed clusters (long-stalked cymes). Each flower blooms for only one day. Best flowering occurs in the deep South near the temperatures of its Mexican origin where flowers may appear from May to November, but sometimes year round. Flowering is very respectable but less frequent when plants are grown as annuals in northern gardens, but will typically bloom from May to September. Flowers are followed by bean-like pods (to 1” long) which explosively dehisce mature seed in all directions.
Considerable confusion has existed over the years as to the correct specific epithet for this plant. It is been given a number of different names, including R. brittoniana, R. coerulea, R. malacosperma and R. tweediana. At this time, R. simplex is the preferred name because it has been determined that this was the name first given to this plant in 1870 when it was described in Cuba, and accordingly that name has priority.
Straight species plants are rarely sold, but several established cultivars are regularly sold (pink, purple or white flowers in tall and dwarf forms).
Genus name honors Jean de la Ruelle (1474-1537), French herbalist and physician to Francois I (1494-1547) who was king of France from 1515 until his death in 1547.
Specific epithet of simplex is in reference to the leaves being simple.
‘Katie’ is a vigorous, dwarf, herbaceous plant which typically grows to 6-10” tall spreading to 12” wide. It features linear, sword-shaped, green leaves to 6” long and ½” wide and short rhizomes. It is a chance seedling discovered by Katie Ferguson growing in her Conroe, Texas nursery. Bluish-purple, tubular, trumpet-shaped, petunia-like flowers (1.5") cover this plant throughout the summer to frost.
Problems
No serious insect or disease problems. Once established in the wild, plants are very difficult to eradicate. Plants may spread invasively by self-seeding and rhizomes. Seeds persist in the soil. Seeds explosively dehisce. Both seeds and bits of rhizome will float in water.
‘Katie’ is reportedly less invasive than species plants.
Uses
Excellent annual flowering plant for areas where it is not winter hardy. For USDA Zones 8-10, it may be effectively grown in bog gardens, as a pond marginal, and in beds and borders. It can be significantly invasive in areas such as the Florida where it is has been declared to be a Category 1 invasive species. Excellent free-blooming plant in large containers. May be grown indoors as a houseplant.
‘Katie’ is a dwarf form that spreads to form an attractive ground cover.