Which plant is commonly referred to as the Madagascar Periwinkle in horticulture?

The plant commonly referred to as the Madagascar Periwinkle in horticulture is Catharanthus roseus. This species is known for its attractive flowers and has gained popularity in gardens and ornamental settings. Originating from Madagascar, it thrives in warm climates and is appreciated for its ability to bloom continuously, producing flowers in various shades, including pink and white.

Additionally, Catharanthus roseus is significant in the medical field, as it contains compounds used in the treatment of certain types of cancer, further enhancing its importance and recognition beyond ornamental usage. The name "Periwinkle" is derived from its resemblance to other plants in the Periwinkle family, particularly Vinca spp., but it is distinct from those.

The other plants listed do not share the same common name or characteristics associated with the Madagascar Periwinkle. For instance, Pelargonium x hortorum is generally known as geranium, while Impatiens species are commonly referred to as busy Lizzies. Viola species include pansies and violets, which have a different flower structure and growth habit compared to Catharanthus roseus.