Ask Mr. Smarty Plants

The focus and expertise of the Lady Bird Johnson Wildflower Center is limited to plant species native to North America. Vinca minor(common periwinkle) is a native of Europe and out of our area of expertise. Stenotaphrum secondatum(St. Augustine grass), is of uncertain enough provenance that the Lady Bird Johnson Wildflower Center does not include it in its Native Plant Database. It wouldn't thrive in settings where it faces long dry periods, especially in full sun, without lots of supplemental water.

As to which plant would dominate, we tend to think that it depends on the setting and timing. If the site has good conditions for Stenotaphrum secondatum(St. Augustine grass), and the grass is well-established and dense, it might resist invasion by Vinca minor(common periwinkle) pretty well. If the Vinca minor(common periwinkle) is the established plant, or if the site stresses the grass, the  Vinca minor(common periwinkle) is likely to dominate.

Perhaps you are looking for a new solution to a problem spot? Try looking at the  Native Plant Database for some ideas.... It's in the Explore Plants section. When you get there you can use a couple of methods to look for plants suitable for your site. By narrowing the search to the type(s) of plants you are looking for – grasses, shrubs, etc. – and choosing specific site conditions – sun, soil moisture – you can generate a list of native plants suited to your needs. You can further narrow by using the Recommended Native Species  function to select plants that thrive in your part of Texas.