Mexican-petunia (also known as Mexican bluebell or Britton's petunia) is described as a "hardy perennial edging plant for flower beds and as colorful groundcovers." Scientific names include Ruellia brittoniana, R. coerulea and R. tweediana, but taxonomists now use the name Ruellia simplex, which was the first name used to describe this species. The wild form has purple flowers, and is native to Mexico, Western South America and the Antilles. It was introduced to Florida in the 1940s. Since then it has naturalized in most counties in Florida, plus in six other southern states, Hawaii, Puerto Rico and the Virgin Islands.
Mexican-petunia is a perennial in zones 8 to 11 that stands up to 3 feet in height. Stems are green or purple and leaves are dark green, oppositely arranged and lance-shaped, roughly 6 to 12 inches long and ½ to ¾ inches wide. Veins are prominent on the underside of the leaf. Leaf margins are can be smooth or wavy. Flowers are trumpet shaped (1-1/2 to 3 inches in diameter), solitary or borne in clusters at the tips of the stems, and are attractive to butterflies, bees and other pollinators. In fertile forms, cylindrical fruit or capsules containing 4 to 28 seeds are produced. Capsules have explosive dehiscence and seeds are spread long distances. Seeds produce a gel-like substance when wet that enables them to stick to surfaces when they dry. Seeds generally have high germination rates, and can germinate in both light and dark conditions. Stands of Mexican-petunia can also spread via underground stems or rhizomes.
Mexican-petunia is able to tolerate a wide range of environmental conditions including variations in light, temperature, and moisture. Other characteristics that make wild Mexican-petunia a successful invasive are its rapid growth rate, affinity for disturbed locations, prolific production of seed, and lack of germination requirements such as scarification or stratification. Established plants can further spread by rhizome production. Mexican-petunia can also resprout from crowns or rootstocks when cut back or killed back by frost.
Control in invaded natural areas as well as control at the propagule source (home-gardens and landscapes) is needed for long-term management of this invasive species. Reinvasion and/or occurrence of new invaders can be a problem in home-gardens; therefore we recommend that herbicide treatments be followed by replanting with native or appropriate non-aggressive ornamental plants.
It is important to install appropriate plant material into bare areas in the home-landscape. Bare ground can quickly become invaded by weeds or different invasive species common to urban areas. Installing appropriate non-invasive or native ornamental species into newly bare ground right away provides sufficient plant competition to hold the space. Use sterile Ruellia cultivars such as "Mayan Purple," "Purple Showers," "Mayan Pink" and "Mayan White." Native alternatives to Mexican-petunia for use in home landscaping include wild petunia (Ruellia caroliniensis), blue curls (Trichostema dichotomum), butterflyweed (Asclepias tuberosa), or swamp milkweed (Asclepias perennis).
Wild Mexican-petunia must be removed completely, both above and belowground, or it will continue to resprout. Mowing, tillage or weed-eating the tops off Mexican-petunia will not remove the plants entirely. Instead, plants can be dug up with a shovel, with the aim to remove the entire root mass.
There are no known biological control agents for Mexican-petunia.
If the area of established wild Mexican-petunia is large, then an herbicide application might be needed. Many ready-mixed and readily available herbicides can successfully reduce Mexican-petunia cover, including glyphosate, which can be purchased by home-owners at most retail garden stores under the trade name Roundup. A single spray of a 2% foliar application of glyphosate will control small areas of Mexican-petunia; however, if the area of established plants is large or especially dense, then a second herbicide application may be needed after 2 to 3 months. Glyphosate can be applied at any time of year for Mexican-petunia control in the home-landscape. If you are planning to apply an herbicide, be aware of and follow all instructions and safety precautions outlined on the package.