Centranthus is a genus of flowering herbaceous perennial plants native to southern Europe, northern Africa and Asia Minor. There are between eight and 12 species of centranthus within the genus. The most common is Centranthus ruber, also known as red valerian, of which there are several varieties. All the members of the Centranthus genus are members of the larger family Valerianaceae, commonly known as the valerian family.
The plants bloom from late spring into the summer in varying shades of red, crimson and white, depending of the species. Most varieties of red valerian grow 18 inches to 3 feet (about 45 to 90 cm) tall. The small flower clusters form bunches at the top of woody stalks. Each plant is between 1 and 2 feet (about 30 to 60 cm) wide. In harsh, inhospitable conditions, centranthus grows smaller and more compact to conserve limited resources.
Centranthus is a hardy plant that will grow in rocky and nutritionally poor soil if there is adequate drainage around the root system. For optimum growth, slightly alkaline, sandy, well-draining soil and full sun are best. Varieties of this plant genus are used in gardens as borders along pathways and flowerbeds and to add color to rock walls and rock gardens. It also is planted to control erosion on hillsides and in disturbed areas. The hardy root systems create a sturdy web to hold the soil in place.
Cultivation of centranthus in a garden setting requires moderate effort. The seeds are sown directly into the ground after the last frost has passed. The seeds germinate in three to four weeks, as long as the temperature remains consistently above 64 degrees Fahrenheit (18 degrees Celsius). A single application of a balanced flower food fertilizer in the early spring will provide adequate nutrients for the duration of the growing season. Centranthus is a popular and hardy garden plant, which has led to its cultivation and naturalization in many areas of the world.
It often is found along roadways and between highways in areas where it has become naturalized. Self-propagation methods rely on prolific seed distribution in the fall. In areas outside of its native range, centranthus can spread to the point of being a noxious weed. To control red valerian from spreading in the garden or in a wild setting, the heads of the flowers are cut from the plant before the seeds have a chance to form and disperse.