Answer: There are two different plants with the common name periwinkle. Vinca minor is one of the plants commonly called periwinkle and it has either white or blue flowers. It is a vine and grows best in a woodland setting where there is afternoon shade and rich, loose, moist soil. The second plant is Madagascar periwinkle (Catharanthus). It is an annual plant and has either white or pink flowers. This plant is commonly found in local nurseries so I think this is what you are growing. Madagascar periwinkle grows best in the cooler weather (any time except May, June, July and August). When grown in average soil it needs water every 4-5 days and appreciates morning sun and afternoon shade. Since these are annual plants they are short-lived so expect to replace them every year. Try planting in September and enjoy them until January or until it gets cold. Then plant new plants in March and enjoy them until mid-summer when they will die out.
Best wishes with your periwinkle plants!
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