The Mexican petunia's vibrant violet-blue blossom appears to fluoresce in bright sunlight - and attracts happy hordes of butterfly in the process.
The Mexican petunia is a tender evergreen perennial that forms colonies of stemmy stalks standing 3 ft (0.9 m) in height and of indeterminate width. The strong semi-woody stalks are distinctly vertical in aspect and hold attractive dark green, leaves oppositely at the nodes. The lance-shaped leaves are to 6-12 in (15.2-30.5 cm) in length and 1/2-3/4 in (1.3-1.9 cm) wide. When grown under hot sunny conditions the foliage assumes a metallic bluish cast that creates the perfect backdrop for the the scores of vibrant blue flowers that appear with the onslaught of hot summer weather. The blossoms are trumpet shaped and about 1.5-2 in (3.8-5.1 cm) in diameter and are borne at the tips of the stems. Varieties with white, pink, and many shades of blue are available, as are dwarf versions that form clumps that are about 8-12 in (20.3-25.4 cm) in height. Mexican petunia is very showy when in full bloom due to the clouds of admiring butterflies that swarm about the plants.
Here's a palette of Mexican petunia varieties: the purple and pink at the top are standard varieties with long stems. The blue and white flowers at the bottom are dwarf varieties that stay below 12 in (30.5 cm) in height.
Mexican petunia is native to Mexico, but it has escaped cultivation and established in disturbed areas in the SE U.S., and can be found invading habitats across Florida (see Warning below).
Prefers fertile soil with moisture, but is very adaptable. After flowering, cut back stems about halfway for a new crop! Mexican petunia can be grown indoors as a houseplant in bright light. It is typically free of pests and disease. Light: Sun to part shade. The quantity of blossoms is related to the amount of light the plant receives. The more direct sunlight the more flowers with fewer flowers appearing in overcast conditions or when grown in shadier conditions. Moisture: Average to moist. Mexican petunia is a water plant that becomes very aggressive with access to abundant moisture. Survives dry spells once established. When growing Mexican petunia indoors water freely when in active growth but water only when dry in winter. Hardiness: USDA Zones 8-10. Marginally hardy in Zone 7 if protected and mulched. Propagation: Cuttings, division, seed. Stems are very easy to root in moist soil. Over time the plant multiplies and the original stem becomes a colony.
Use Mexican petunia towards the back of a flowering border, or as the centerpiece in a container. Plant next to pentas (Pentas lanceolata) for an effective contrast in both foliage and flower color and texture, with a purple verbena (Glandularia puchella)as ground cover below to mirror the larger, purple flowers of R. britonniana (the butterflies like this combination too!). The dwarf varieties also do well (and look great) in containers - especially those situated in hot sunny situations. The dwarf Mexican petunias also make a handy perennial edging plant for flowers beds and as colorful groundcovers.
The white flower dwarf variety is compact and well behaved. It demands little care and rewards with non-stop flowers until frost.
Mexican petunia is an easy to grow plant with strikingly colored flowers that is seldom bothered by disease or pests. It blooms enthusiastically throughout the hottest time of the year. It is a fast grower and is inexpensive and sold at many discount chain garden centers (which is unfortunate in places like Florida).
In the southeastern US the Mexican petunia is becoming a pest. I hope that the dwarf varieties will prove to be better mannered. After six years of observation my blue and white flowered dwarf Mexican petunias remain well behaved little clumps and I've seen no indication that they are reseeding.
Mexican petunia is listed as a Category I invasive species by the Florida Exotic Pest Plant Council. This means that it is "altering native plant communities by displacing native species, changing community structures or ecological functions, or hybridizing with natives". This warning applies to all parts of the state of Florida (and other areas with similar mild climates). Where hardy, the Mexican petunia excels at invading wetlands.