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The summer lilac is a large, fast-growing shrub known for its long cone-shaped flower clusters. A number of cultivated varieties exist, and different flower colors include white, red, and lilac flowers. The shrub is also called the orange-eye butterfly bush because the tiny flowers often have orange eyes. The summer lilac is a popular choice in butterfly gardens but has a tendency to be become invasive. Its scientific name is Buddleja davidii.
Growing to heights of 12 feet (3.7 m) and having spreads of up to 8 feet (2.4 m) in diameter, the summer lilac is a large shrub. The flowers grow in long panicles, or branching flower clusters, that can be as much as 12 inches (30 cm) or more in length. The small blooms are noted for their fragrance. The plant flowers through summer and often into autumn. Its pointed, slender leaves are 2 to 3 inches (5 to 8 cm) long and are usually gray or green in color.
B. davidii is a deciduous shrub, meaning it loses its leaves each year. For this reason, it needs little pruning unless the plant has become overly large during the growing season. After winter, it is recommended that dead growth be removed in early spring. Plants can be shaped in spring if they have not been completely killed, either for aesthetic reasons or to contain their growth, neither of which will affect a plant's summer flowering. Except for the very hottest or coldest zones, the plant is hardy throughout most of the United States.
The summer lilac comes in a number of varieties, most of which have been developed for flower color, but also for a more contained size. Popular varieties include "Black Knight," which bears deep purple flowers. The variety "Harlequin" has white-edged leaves and magenta flowers. Varieties designed to have a smaller spread include "Nanho" and "Summer Beauty." Other varieties like "Royal Red" have longer than average flower clusters that may be as long as 20 inches (50 cm).
Due to its somewhat wild appearance when fully grown, the summer lilac is often seen at garden edges or in less formal spaces. Its frequently large size can overshadow smaller plants in the garden. It is a popular and effective choice in butterfly gardens, but may spread to other parts of the garden or uncultivated areas. Relatively undemanding, the summer lilac requires full sun and prefers light, well-drained soil. Plants may benefit from fertilizer in the spring, as faster growth tends to produce more flowers.