The Mexican petunia (Ruellia simplex) is a popular perennial plant cherished for its trumpet-shaped, purple flowers and remarkable tolerance to harsh conditions. While its adaptability makes it a favorite for many gardeners, the answer to whether it spreads is a definitive yes, often with aggressive tendencies. In warm climates, particularly in the southeastern United States, the species is known to be highly invasive and can rapidly colonize garden beds and natural areas. Understanding the plant’s dual-action spreading mechanism is the first step in managing its reputation as a difficult garden resident.
Dual Mechanisms of Spread
The aggressive colonization by the Mexican petunia is primarily due to two distinct reproductive strategies working in tandem. The first method involves extensive underground runners called rhizomes, which are modified stems that grow horizontally beneath the soil surface. These rhizomes continuously send up new shoots, allowing the plant to form dense, ever-expanding colonies far from the original parent plant. This vegetative spread is the main source of lateral expansion and can quickly turn a single plant into a large, crowded patch.
The second method of propagation is through prolific seed dispersal from dehiscent capsules. After the short-lived flowers fade, they produce cylindrical seed pods that mature and then explosively fling the seeds several feet away when they dry. This mechanism allows the Mexican petunia to rapidly colonize new, distant areas of the garden or landscape. The seeds are also known to form a sticky, gel-like substance when wet, which helps them adhere to surfaces and persist in the soil for long periods, leading to persistent re-infestation.
Managing Aggressive Growth
For gardeners who already have the aggressive variety of Mexican petunia established, effective control requires a multi-pronged approach targeting both the roots and the seeds. Physical control is often the most demanding, requiring the complete removal of all underground mass. Simply cutting the plant back will not stop the spread, so deep digging with a fork or shovel is necessary to extract the entire root system and the extensive rhizomes.
One way to prevent the lateral spread of the rhizomes is to install a physical root barrier around the plant’s designated area. These barriers, similar to those used to contain bamboo, must be buried deep enough to block the underground runners from escaping the bed. Consistent maintenance is also necessary, which includes regularly pulling any new shoots that emerge outside of the barrier.
Preventing seed production is a critical maintenance step accomplished through consistent pruning and deadheading. Removing spent blooms before they can form and mature into explosive seed capsules will eliminate the plant’s ability to self-sow. For severe infestations where manual removal is impractical, systemic herbicides containing glyphosate may be required to eliminate the stubborn root mass. This chemical application must be targeted carefully to the plant’s foliage or cut stems, as glyphosate is non-selective and will damage surrounding vegetation.
Choosing Non-Invasive Varieties
Gardeners who desire the distinctive look of the Mexican petunia without the commitment to aggressive management have excellent alternatives available in sterile or semi-sterile cultivars. Plant breeders have developed these varieties specifically to significantly reduce or eliminate the plant’s capacity to spread. These cultivars are highly recommended for responsible planting, especially in areas where the native species is considered invasive. The popular ‘Purple Showers’ cultivar does not produce viable seeds, although it can still spread slowly via rhizomes in moist conditions. Other well-known, less aggressive varieties include the dwarf ‘Katie’ series and the sterile ‘Mayan Purple’ and ‘Mayan White’ series. These options provide the same drought-tolerance and vibrant flowers as the wild type, relying on division or cuttings for propagation, making them safe additions to a controlled garden environment.