A hydrangea bush is a popular shrub with large flowerheads, or clusters of small flowers, on the ends of its stems. There are a number of species in the genus Hydrangea, but the most commonly grown species, H. macrophylla, is known for having flowers that can be blue or pink, depending on the acidity of its soil. It is commonly grown as a border plant in warmer climates. In areas where hard winters or deep freezes are common, it is usually grown as a potted plant or in greenhouses.
Also known as H. opuloides, this hydrangea bush normally reaches heights of 4 to 8 ft (1 to 2.4 m) tall. It can grow as high as 12 feet (3.7 m), however. It has toothed, thick leaves that can be up to eight inches long (20.3 cm). As with most hydrangeas, the plants bloom from early in the spring until late in the fall, and the flowers often last for months.
To obtain a change in flower color with the breeds H. macrophylla and H. serrata, one must change the acidity of the soil. The key pH is six. Having a pH level below this will produce bluish flowers. This can be obtained by adding ammonium sulfate to the soil in the late fall or early spring. To cultivate pink flowers, add lime or superphosphate to obtain a pH level higher than six.
Hydrangea bushes prefer some shade and moist, rich soil. Organic matter, such as peat moss or compost, is best when amending soil for these plants. The shrub will bloom more if grown in the sun, however. If placed in a spot that receives full days of sun, it is important to keep this plant well watered.
When planting hydrangea bushes, the hole should be dug only as deep as the rootball. It should be dug as wide as possible, however. One should fill in the hole with organic matter and loosen the roots to ensure they have adequate contact with the organic matter. It is important that the soil surface be at the same level as was the rootball, and to water thoroughly after planting. Mulching is recommended.
Some hydrangea bushes grow rapidly and require pruning. Pruning greatly affects the size of the flowers. Reducing the number of stems will give the biggest flowers. To get a number of medium-sized flowers, leave a greater number of stems. Optimal pruning timing for a hydrangea bush varies according to the species.
Different species of hydrangea bush vary in whether they produce their flowers on stems produced from new wood, or that accomplished in the current year — or old wood, or growth from the previous year. If the flowers are produced on old wood, and they are pruned in the spring, they will not flower that year. Generally, these species are pruned immediately after they have flowered. Plants that produce flowers on new wood will bloom if pruned in the spring.
Some species only require minimal pruning, including H. macrophylla, which blooms on old wood. This hydrangea bush generally only requires pruning if it grows too tall, or if an older shrub develops dead stems. Along with H. serrata, these species have cultivars with flowerheads known as mopheads or laceheads. Mopheads are striking groups of flowers that resemble a mop, or the pom-poms of a cheerleader. Laceheads have inner and outer rings of flowers.