Blue Treasures for Rock Gardens

While blue flowers are at a premium in much of the gardening world, there is (almost) a joyful embarrassment of riches when it comes to selecting rock garden specimens. Some are well known, such as the spring favorites crocus, muscari, and scilla. Many others—not all spring-blooming—are available as well.

Codonopsis, also called Asia bell, is of the Campanulaceae family. These thirty species of bellflowers are herbaceous perennials. The plants sprawl but reach a height of two to three feet. Although they are hardy, they do not do well in exposed positions. They flourish in full sun or partial shade with a well-drained soil. The stems, leaves and flowers all have a strong and (some say) unpleasant odor. It is helpful to mark the location of the plants near the end of the season since their location is invisible when the plants go dormant in winter.

Plants are usually reproduced by seed sown in cold frames, young plants being set out in June; stem cuttings can also be taken in autumn; established plants can be divided in spring.

Only one species, C. clematidea (Asia bell), a native of central Asia, is commonly available. It is characterized by pale blue flowers with yellow or black markings on the interiors; they bloom in late summer. Plants may reach three feet and require support; they are not reliably winter-hardy in the north.

C convolvulacea, from the mountains of western China, produces violet-blue flowers on two- to three-foot plants in summer.

Corydalis, a member of the Fumariaceae family, includes more than 300 species of hardy herbaceous annuals or perennials. They are found in temperate regions of the northern hemisphere. Typically they grow from underground rhizomes or tubers, and are less than two feet tall. Foliage can be fern-like and resembles that of bleeding heart (Dicentra), to which it is closely related. The flowers have a distinctive long corolla tube that terminates in a spur. Most bloom in spring. These plants do well in light shade in a moist, well-drained soil.

Corydalis is multiplied by seed or division of dormant plants. Annuals can be sown in situ in April. Division is the most common propagation method for perennial types forming rhizomes or small bulbs; this can be done in March.

C. cashmeriana, a Himalayan native, reaches only six inches in height and produces blue flowers in spring.

C. flexulosa is a native of China; it is popular in Europe and is also now available in the U.S. Characterized by electric-blue flowers and fern-like foliage, it grows to a height of one foot.

Sources

Geneve, Robert. 2000. A Book of Blue Flowers. Timber Press. Portland, Oregon.

Hunt, Peter, Ed. 1970. The Marshall Cavendish Illustrated Encyclopedia of Gardening. Marshall Cavendish, New York.

Tebbitt, Mark et al. 2008. Bleeding Hearts, Corydalis, and their Relatives. Brooklyn Botanic Garden and Timber Press, Portland, Oregon.