Periwinkle plants spread quickly, and for some that is not what fits best in their garden. Cut your plant back with pruning shears in late spring or early fall when temperatures range between 70 and 80 degrees. Once you have made the cut where you want your plant to stop growing, spray the cut stems with a ready-to-use, 5-percent glyphosate weed killer. Spraying it with this will stunt the growth and your plant should stay the perfect size for your liking.