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Native to Japan, the Hydrangea macrophylla is a deciduous flowering shrub grown for its copious blossoms. It is commonly known as the bigleaf hydrangea, French hydrangea or florist's hydrangea. This shrub is hardy to 0 degrees Fahrenheit (minus-18 degrees Celsius). There are 23 species of hydrangea, five of which generally are cultivated. The Hydrangea macrophylla is widely cultivated for its generous, rounded growing habit and abundant, colorful flowers.
Hydrangea macrophylla grows quickily, usually reaching 3-6 feet (0.9-1.8 m) in height and width within a few years. The shrub prefers a moist, well-drained soil with rich organic matter. Preferring a cool climate, this plant tolerates full shade but prefers dappled sunshine for prolific blooms.
The flowers of the Hydrangea macrophylla burst into thick, bushy bunches called mopheads between mid-summer and late summer. The lacecap hydrangea produces a small cap of tiny, light-colored flowers rimmed by larger, more colorful flowers. Flowers might be varied shades of blue, pink or white, depending on sun exposure and soil content. Immature flowers of this plant generally are pale green, developing color as they age. Toward the end of the growing season, the flowers take on a dusky rose hue.
Flower color largely depends on soil content and cultivar. Aluminum in soil with a low potenz hydrogen (pH) factor produces bluish flowers or, as in the case of special cultivars such as nikko blue Hydrangea macrophylla, brilliant azure blooms. Alkaline soil, such as is found near house foundations with lime mortar, produce pink blossoms. Gardeners might add aluminum sulfate to the soil to coax blue blooms. Soil that has been amended with lime will encourage pink blooms.
A common complaint among gardeners is that their Hydrangea macrophylla does not bloom. Too little sun exposure, harsh winters, early spring frosts after buds have appeared and insufficient water or over-watering all contribute to a lack of flowers. One of the most common causes for lack of blooms is improper pruning. Bigleaf and oakleaf hydrangeas flower on the previous year's growth, so pruning in autumn, winter or early spring removes the flower buds. Other cultivars, such as panicle and smooth Hydrangea macrophylla, are best pruned during the summer because they produce flowers from the current year's growth.
This plant is susceptible to many disfiguring — but not fatal — diseases and pest problems. The leading plague for bigleaf Hydrangea macrophylla is powdery mildew. The powdery white dust coats the leaves of plants situated in deep shade or during seasons of high humidity. It also is prone to fungal leaf spot and rust, producing very unsightly black or brown splotches. Pests such as Japanese beetles and aphids may feed on young hydrangeas, but their presence rarely reaches epidemic proportions.