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The plant genus Coronilla belongs to the Fabaceae family, but formerly people listed it as a member of the Leguminosae, or pea, family. This older classification is still valid; therefore, gardeners may need to search for it under both names. Growers cultivate the plants for their attractive foliage and pea-like flowers. Generally, Coronilla plants are shrubs that are native to many areas from Europe to western Asia, including northern Africa and the Mediterranean region.
The flowers of the Coronilla plants are pea-like, meaning they resemble the pea plant flower. It has an upright central petal backed by two large lateral petals and fronted by two lower petals that may be keeled and/or fused at the base or along the petals. The lower petals house the stamens and pistil. The flowers usually are borne on axillary umbel, which looks like a flattened crown at the end of the flower stalk.
Gardeners raise most Coronilla plants as perennials in temperate climates where frost is not a threat. One of the species that is frost tolerant is C. varia, or crown vetch, which may survive temperatures of about -30°F to -20°F (about -35°C to -30°C). Crown vetch is a creeping herbaceous plant, meaning that it is a non-woody plant that loses its leaves at the end of the growing season. People often use crown vetch for erosion control, but in many areas it is an invasive plant.
Most of the approximately 20 species in the genus are desirable garden plants. One of these is the scorpion senna, or C. emerus, that is sometimes labeled as Hippocrepis emerus. Scorpion senna is a bushy shrub that may grow to be 6 feet (2 m) tall and wide, giving it a rounded appearance. It is deciduous or evergreen, depending on the climate, and has pinnate leaves up to 2.5 inches (about 6 cm) long that have up to nine obovate, or triangular, shaped leaflets. The small flowers are about 0.75 inches (about 2 cm) long and usually appear in late summer through the autumn.
The C. glauca is a smaller shrub, measuring about 32 inches (about 80 cm) tall and wide. The leaves are glaucous, meaning that they have a fine, waxy coating that rubs off. Gardeners like this shrub because it produces fragrant, yellow blooms in late winter and early spring when very few flowers are blooming. Often it may produce blooms again in late summer.
Another shrub that gardeners use for ground cover is C. cappadocica. The prostrate shrub often is barely 18 inches (about 46 cm) tall. It forms a dense mat of pinnate leaves, composted of approximately ten small obovate leaflets. The bright yellow 0.5-inch (about 1.3-cm) flowers compliment the glaucous leaves. Gardeners typically plant it as a border plant or in rock gardens.
There are many cultivars of various Coronilla plants for landscaping. Some of the popular ones include C. varia aurea, which features golden foliage, and C. varia penngift. Penngift, which was cultivated at the Pennsylvania State University, bears pink and white flowers throughout the summer and autumn. C. glauca variegata has yellow flowers and attractive variegated leaves, green edged with cream or white.