The width of the trench is up to you and how much gravel you want to use; keep in mind that a line of brightly-colored aquarium gravel that's only an inch across will stand out against dark soil.

The aptly named creeping phlox (Phlox stolonifera) is perfect for borders because it's a vibrant, low-growing carpet of color. The dense mat-forming habit can grow up to 6 inches tall and up to 3 ...