Mexican Petunia: Wrong Plant, Wrong Place

Several years ago, I bought a plant called Mexican petunia. There are several plants called Mexican petunia; I believe that the plant I have is Ruellia brittoniana. It is a tall, bushy plant that puts up many upright stems with pretty, petunia-like blooms in mid-to-late summer. While this plant has proven to be invasive further south, I have not found it to be a re-seeder. It may be the cultivar I have is sterile or that it blooms so late that the seeds do not have time to mature. 

Ruellia brittoniana

Although this plant has a lot to recommend it (easy to grow and attractive to people and pollinators), it can be a garden thug, spreading around by underground shoots.  I planted the original plant in a bed bordered by a concrete patio and it has not only spread in that bed, but it is also trying to escape to the neighboring bed by exploiting the space between the house and the patio.

So rather than continuing to fight this battle, I decided it was time to remove this plant. The runners it sends out are easily pulled up and re-planted (a definite warning sign), and, unfortunately, I had re-planted these runners to two other beds in this area, so I have a big battle on my hands. I considered three methods for removal: smothering with a layer of mulch, digging out the runners, or the use of a herbicide, such as Round-up. Digging out the roots is the preferred method, but every trace of root must be removed or the plant will re-sprout. 

The area I tackled yesterday was in a place that is hard to get to with a shovel and near a rosebush I don't want to inadvertently damage with herbicide. After unsuccessfully trying to remove the underground runners with a handtool, I decided to cover the area with thick, construction-grade plastic and hope that the combination of heat and lack of light will kill the plant. I'm not sure how long it should take for this to happen, and I'm guessing the plant will outlive my patience for having black plastic in my flower bed. So in a few weeks, you may be reading that I've taken up the plastic to find the Mexican petunia, alive and well. If that happens, applying Round-up with a paintbrush is my next move. Gardening is one big experiment.