Use a rototiller to dig up the area where the periwinkles will be planted down to about 8 inches. Work in compost until the bed drains well. Dig out areas for spot-planting with a shovel and mix as much compost as possible into the hole; areas with lots of rocky outcrops may be challenging places to amend soil so use a hand trowel if necessary.
Inspect the periwinkles for visible problems, such as mold and rot. Weigh the bare root in your hand -- it should feel heavy for its size. Trim any broken or seriously damaged roots from the plant before proceeding. Submerge the periwinkles in a bucket of water for 10 to 20 minutes immediately prior to planting.
Space periwinkles 6 inches apart for quick coverage or up to 18 inches for gradual fill-in. Dig a hole that is large enough to sit the bare root inside without having to crush or bend its root system. Spread the roots out in the hole. Gently pack dirt around the bare root and be careful to keep each plant's crown flush with the ground. Water the periwinkles thoroughly.