How much periwinkle do I need to plant to cover an area 40' x 3'? Allowing for it to grow in some, but looking quite lush to start.
This will depend greatly on the size nursery plants you use.
Space the holes 4 to 5 feet apart for traditional ground cover planting, or as close as 6 yo 8 inches apart for faster coverage.
Make sure you plant the plants at the same level as the nursery pots.
Put a layer of mulch in between the plantings but keep it an inch away from the base of the plants to allow excess water to evaporate.
Here is a link with proper growing information.