The Right Place
Common periwinkle, the hardier of the two types grows well over much of the United States (zones 4 to 9). It can tolerate cold winters below zero, even colder if protected by snow cover. Large periwinkle does best in warmer climes (zones 7 to 9) where winters temperature rarely dip below zero.
Locate periwinkle in a shady spot in well-drained soil. Plant it under shade trees or on the north sides of buildings in slightly acid to neutral soil (pH 6.0 to 7.0) with some organic matter mixed into it.
Planting Nursery Stock
Before planting, cultivate the soil at least 12 inches deep. Periwinkle is easily transplanted from flats in either spring or fall. Gently separate the sprigs from one another, untangling and spreading their roots. Dig holes 3 to 4 inches deep and wide enough to hold their root systems. If transplants are in pots, make the planting holes large enough to easily accommodate the root ball. Place a plant in each hole making certain that the top of the root ball is at, and not below, ground level. Refill the hole with soil to the level of the surrounding ground and water thoroughly to provide good soil-to-root contact. Spaced 12 inches apart, they will cover the area by the end of the growing season.