I have a thick stand of Mexican petunia (Ruellia brittoniana or Ruellia simplex) next to a couple of the greenhouses at school. The plants have been established for close to 20 years. This past week we replaced the plastic covering on one of the houses, and I didn’t realize the job it would be.
I’ve replaced plastic dozens of times, but never had I been forced to deal with this invasive perennial to get the job done. Roots and rhizomes had really built up over the years. There was such an overgrowth of plant material that the easiest fix was to put new baseboards above the existing ones and install new locking rails.
Mexican petunia is not really a petunia at all. It’s an upright perennial plant highly adaptable to the southeastern US. It gets covered with purple, pink or white petunia-like flowers in summer, and they hang on well into the fall.
Each flower lasts only a day, but you would never know it. When plants are in bloom they’re loaded. A thunderstorm can strip plants of every flower and the next day these guys are a sea of color again.
Mexican petunia tolerates wet soils. It will even grow in ponds. Plants can be maintained at less than waist height but left unpruned they can grow six feet tall in rich wet soil. For best flowering, plants should be in full sun. They make a great screen and they’re easy to grow.
Propagation is easy. Stem cuttings root well without the need for rooting hormone. Division is another productive method. They also naturally spread from seed. Butterflies and hummingbirds are attracted to the tubular flowers.
The problem is that Mexican petunias don’t play nice. If used in the landscape, they must be planted alone and a place where they can’t spread.
Purple types are the most aggressive. White and pink flowered types are usually somewhat shorter and don’t spread as quickly.
Recently, plant breeders have developed shorter less aggressive cultivars in all three colors. The drawback is that most of them are also less winter hardy. Last winter killed all my dwarfs but none of the full-size types.
This exotic perennial has few insect or disease problems. Mine get covered with mealybugs, but they don’t seem to be bothered by them. Deer generally save their foliage for last.
Chemical control is usually successful. Most broadleaf herbicide mixtures will control Mexican petunia as will glyphosate. The problem is that they will also kill or injure most other ornamental plants in your perennial garden.
Fortunately, we are near the edge of the hardiness zone for Mexican petunia. When planted in open areas, winter will spank them every few years. Those planted in sheltered places are rarely killed or even injured.
We don’t propagate very much Mexican petunia for our school plant sales anymore. We used to sell a lot and some people still ask for it. Its bloom can be spectacular. However, its invasiveness is a turnoff, and I can’t bring myself to promote it.
Ted Manzer teaches agriculture at Northeastern High School.