Behind it, she filled in with fertile myrtle also known as vinca minor.
From the toledoblade.com
Vinca minor may be more compact than V. major, but it is the most officious spreader because it sends out lots of short runners and soon forms a tight mesh.
From the telegraph.co.uk
If you get less than that, you might want to think about planting some other ground cover such as vinca minor, ivy, or moss to cover the soil in low-light conditions.
From the toledoblade.com
The white vinca named after her is a wonderful form of lesser periwinkle, Vinca minor, with flowers rounder than normal and a neat habit.