One of the main drawbacks of growing annual flowering plants in Mediterranean and other dry climate regions is their excess water consumption. Annuals in hot, dry summer areas require water based on at least 6mm a day, which is 6 liters per square meter. Over the year, this can amount to more than a cubic meter of irrigation water for every meter square – an unviable and unacceptable rate, unless the area planted with annual flowers is reduced to a minimum. Catharanthus roseus however, can be successfully grown, creating a lovely carpet of color, on a third, perhaps a quarter of that amount, thereby saving you much water and expense.
Catharanthus roseus is also known by its common name, Madagascar Periwinkle, or the very misleading Vinca rosea. As the latter is the botanical name for a separate genus of plants, it's best avoiding in order to prevent confusion. The plants, technically speaking, are perennials, but because they decline as soon as the temperatures drop in the autumn, they are generally grown as summer annual plants, except in tropical regions.
There are many cultivars of Catharanthus now available, the colors ranging from pure white, pinkish-white, and various shades of rose and pink-purple. The erect varieties reach up to 50 cm, (20 in) while the trailing types get to about 15cm (6 in) in height. In addition to the impressive and often continuous blooms through the summer, the dark green, waxy foliage is also attractive. Grown in bold, broad sweeps, it can create a fabulous effect.
As Madagascar Periwinkle needs full sun to flower, the darker, stronger hues are more suited to the harsh light of a Mediterranean summer, than the paler, more pastel shades, which are at their best in soft light or shade. White flowers, which may be excellent for lifting up a shady corner, are far less effective in full, brilliant sun.
Other than full sun, the most important cultural requirement for the plants is a well-drained soil. Poor drain will kill the plants sooner than aridity. This is what makes Catharanthus so suitable to water-preserving gardening, because the plants can be watered deeply but infrequently, together with the trees and shrubs. Such a territory is especially important in heavy, clay soils, where lack of air in the root zone is exacerbated by the constant moisture required by most annual flowers.
It follows there, that consistently adding compost and other organic amendments will create an air / moisture balance in the soil, which is beneficial to the periwinkle. A high organic content also helps to reduce pests and disease by creating balanced population levels between the multitudes of organisms that inhabit the soil, including pathogenic ones.
Catharanthus roseus is hardy to chalky and saline soil, and to both aridity and humidity, but nonetheless vulnerable to soil-borne fungi. Unfortunately, the pathogenic fungi establish themselves best when Catharanthus is planted every year in the same spot. It is safer therefore, to rotate the "crops" from year to year, planting water-preserving perennial flowers, and replacing them with the Madagascar Periwinkle, once every few years.