I am primarily into lawn maintenance and pesticide application. A client wants a price on planting ground cover on a bank. Probably going with periwinkle due to the shade. Area to be covered is about 1000 sq ft. and I've been advised to plant at least every 18 inchs. If my calculations are correct, that's about 500 plants. (13 rows X 33 plants/row) Any advice on how much to add for planting what be much appreciated.