If you seek a tough annual flower that can handle dry, nutrient-poor soils and hot, sunny locations, look no further than the herbaceous vinca (Catharanthus roseus). It produces dainty blossoms that resemble those of impatiens or phlox in a wide array of colors except for yellow, orange, blue, or purple. Grow herbaceous vinca as a summer annual or as a perennial in U.S. Department of Agriculture Hardiness Zones 9 and warmer.
Herbaceous vinca is a tropical perennial native only to the island of Madagascar. Where no winter frosts occur, the plants remain evergreen and grow into small mounding sub-shrubs with semi-woody stems.
Confusion often arises at plant nurseries when people discuss vinca. Originally, herbaceous vinca was botanically known as Vinca rosea, but today is widely known as Catharanthus roseus. Common names include periwinkle and Madagascar periwinkle. These names are used to differentiate from the vining, evergreen ground cover plants also known as vinca, or greater and lesser periwinkle, known botanically as either Vinca major or Vinca minor. All of these plants are members of the dogbane family, Apocynaceae.
Herbaceous vinca, or Madagascar periwinkle, is often used as a seasonal annual. Its oval leaves are attractively glossy and deep green. The stem tips bear loose clusters of five-petaled flowers continually across the warm months up until plants are killed by an autumn or winter frost. Flower colors range from white to pink as well as shades of lavender, apricot-peach, red, salmon, magenta and fuchsia. Some varieties bear blossoms that are bi-colored with a contrasting color in the center or "eye" of the blossom. Seeds are produced and often are scattered across the garden and result in seedling plants.
Plant Madagascar periwinkle when there is no danger of frost in the garden. Site it where it will receive abundant sunshine, no less than six hours daily. To prevent root rot and fungal leaf diseases, make sure to plant it in fast-draining soils that never remain soggy or wet for too long after irrigation or rain. Sandy soils that are hot and dry are ideal although heavier clay soils suffice if it is warm, drains well and is in intense sunlight. In general, never over-water this plant and err on the side of under-watering, especially in regions with humid summers.
Substantial breeding of Madagascar periwinkle by seed companies now provides many varieties (cultivars) for gardeners to grow and enjoy. Modern cultivars tend to be more compactly growing, more resistant to fungus, tolerant of cooler summer temperatures or climates with wet and humid summers. Wide arrays of flower colors exist and typically are grouped into plant collections or series that share the same characteristics, such as mature size or disease resistance. The Pacific and Cooler series comprise compact plants that grow no more than 15 to 18 inches tall while the Little series mature less than 10 inches tall. Sprawling plants that are nice for creating ground covers or hanging baskets include the Mediterranean, Carpet or Cora Cascade series. The Tropical series are fast growing and are among the first to flower after sowing, while the Nirvana and Cora series are touted for their tolerance for hotter climates that have rainy or humid weather.