Asked September 07, 2016, 2:44 PM EDT
Kenton County Kentucky
Do you mean that the "unhealthy" area is under the oak tree?
It's very hard to grow ground covers, grass, etc. underneath a well-established tree. The tree and the other plants compete for the same light, moisture, and nutrients. The tree, having an established root system in that area, will usually "win" the contest. But it's not good for the tree or the other plants to have to struggle this way, either. The healthiest solution for the tree AND the vinca is to put the vinca somewhere else, and place mulch under the tree 2-3" deep in a very wide circle under the tree's shaded area. Here is a publication that explains in more detail how to handle the tree & ground cover situation: http://www2.ca.uky.edu/agcomm/pubs/id/id203/id203.pdf
Please feel tree to call your county's Cooperative Extension office and speak with the Horticulture or Agriculture agent for more information.
I hope this helps!
Please call me at 859-356-3155. I'm the Horticulture Agent, and I'll try to help you figure this out. Our office is located at the corner of Route 16 and Marshall Rd.in Independence.