Answer: Vinca can be slow to spread but once it does, it grows nice and thick. I think the underlying problem in your situation is that the tree roots and vinca roots are competing for moisture and nutrients. The tree will get the lion's share because it has many more and much larger roots. This doesn't mean that the vinca won't ever root and spread, it will just take a little longer. Be patient and eventually the vince will create a beautiful planting beneath our maple.
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