The many names for the Mexican petunia (Ruellia brittoniana) include “common ruellia” or “wild petunia.” This herbaceous shrub grows as a perennial in U.S. Department of Agriculture plant hardiness zones 8 through 11 and an annual in cooler climate zones. The plant produces petunia-shaped flowers in vibrant blue or purple, with deep green foliage. One advantage of growing a Mexican petunia is its extreme vigor and hardiness, including disease resistance.
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Disease-Resistance and Conditions
The Mexican petunia is known for its resistance to common pests and diseases that plague other garden plants. The generally hardy plant will grow in a variety of cultural conditions, even poor ones. Mexican petunias thrive in full sun to partial shade, but the plant blooms more prolifically in full sunshine. Flowering begins in the late spring and continues through the summer, into the early fall.
Mexican petunias come in regular and dwarf varieties; both are resistant to diseases, as well as resisting insect pests, including snails and caterpillars. Mexican petunias are aggressive growers and can become invasive, spreading via seeds and underground rhizomes, so be aware of this when you plant them. There are some less aggressive cultivars, such as the dwarf “Katie,” also called “Nolan’s Dwarf.” This disease-resistant shrub stands approximately 6 inches high at maturity. The regular-sized “Purple Showers” cultivar is also disease resistant and spreads less aggressively because its seeds are sterile. This variety grows to approximately 3 feet in height at maturity. Both varieties are hardy in USDA zones 8 to 10.
Most garden and landscape plants require the best cultural care to keep them vigorous and disease-free. This is not the case with the Mexican petunia, which can thrive in many different conditions, even with poor cultural care or neglect and multiple soil types and moisture levels. To give it the best care, provide the Mexican petunia full sun to partial shade; fertile, well-draining soil with steady moisture, especially when the plant is young; and a warm, Mediterranean climate. However, because of the aggressive growing nature of the Mexican petunia, a less than favorable environment provides a form of control.
While Mexican petunias are generally disease-free, in 2010, a nursery on the Council of Agriculture in Taiwan reported loss of Mexican petunias from a fungal pathogen called Sclerotium rolfsii, also known as Southern blight. This root rot disease caused wilting and yellowing leaves, as well as mycelium growth in both the roots and stems of the Mexican petunias in the nursery, killing infected plants within a period of 10 days. The result was heavy commercial loss in Mexican petunia production. While this was a limited incident, Southern blight diseases are also present in the Southern U.S. The best way to avoid fungal rots and diseases of the root system is to ensure that your plants have well-draining soil and the soil does not remain overly saturated for any length of time.