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From The United States Arborium:

While there are approximately 23 species of Hydrangea, only five are widely cultivated in the U.S. The most popular species is Hydrangea macrophylla, which is commonly known as bigleaf, French, garden or florist’s hydrangea.

Plants, particularly those of the cultivar 'Grandiflora' ('Pee Gee'), are sometime pruned into a tree form and grown as a specimen plant. Panicle hydrangea is also suitable for use in a mixed border or as a deciduous hedge.

Smooth hydrangea (H. arborescens) is the other U.S. native. It is found in the eastern U.S. from New York to Florida and west to Iowa and Louisiana. In cultivation, plants usually reach about 5 feet in height, with a similar or greater spread. The species is rated as hardy from USDA Cold Hardiness Zones 4 to 9. Flowering occurs in early to mid-summer. The most common cultivar, 'Annabelle', produces rounded inflorescences that may reach up to a foot in diameter.

Hydrangeas grow best in moist, well-drained soil. Most hydrangeas benefit from some shade, especially in hot climates.  Growing hydrangeas in deep shade is not necessary and can greatly reduce flowering.

The amount of sun that hydrangeas can tolerate depends on species, climate and availability of water. Panicle hydrangea tolerates more sun than do other species. Plants grown in hot climates require more shade than do those grown in the colder limits of adaptation. In southern climates, providing frequent and adequate watering will allow hydrangeas to tolerant more sun than if they were subjected to moisture stress.

There are three possibilities for lack of flowering among the hydrangea species. The first two – too much shade and improper pruning – apply to all hydrangeas, while the other – weather-related damage to flower buds – applies primarily to the bigleaf hydrangea.   While most Hydrangea species benefit from some shade, too much shade can reduce flowering.  Improper pruning can also reduce flowering in Hydrangea. Since bigleaf and oakleaf hydrangeas flower on previous year’s growth, potential flowers buds would be removed if the plants were pruned in fall, winter or spring. Panicle and smooth hydrangea flower on this year’s growth, so pruning them in early summer would reduce or eliminate flowering for that year.

The most common reason for lack of flowering in the bigleaf hydrangea is unfavorable weather. Most H. macrophylla cultivars flower primarily on previous year’s growth. Weather conditions that damage aboveground parts of the plant can reduce flowering. Damaging weather conditions include early fall freezes that occur before the plant is completely dormant, extremely low winter temperatures, and late spring freezes that occur after the plant has broken dormancy. In USDA Cold Hardiness zone 6 and warmer, which is the recommended growing area for H. macrophylla, the most common of these unfavorable weather events is late spring freezes that damage tender new growth. This is particularly true in the southeastern U.S., where "see-saw" temperatures are very common in the spring.

Other cultivars, such as 'Nikko Blue' are a bit more flexible, and will flower from the buds that develop from the base of the stem. However, if another cycle of warm weather followed by freezing temperatures damages shoots developing from these buds, these cultivars may flower only lightly, if at all.

Flower color in H. macrophylla is dependent on cultivar and aluminum availability. Aluminum is necessary to produce the blue pigment for which bigleaf hydrangea is noted. Most garden soils have adequate aluminum, but the aluminum will not be available to the plant if the soil pH is high. For most bigleaf hydrangea cultivars, blue flowers will be produced in acidic soil (pH 5.5 and lower), whereas neutral to alkaline soils (pH 6.5 and higher) will usually produce pink flowers. Between pH 5.5 and pH 6.5, the flowers will be purple (see image at left) or a mixture of blue and pink flowers will be found on the same plant.

Established bigleaf, panicle, oakleaf and smooth hydrangea plants can often benefit from regular pruning. Removing about one-third of the oldest stems each year will result in a fuller, healthier plant. This type of pruning is easiest to do in winter, since the absence of leaves makes it easier to see and reach inside plants.

Gardeners may also want to prune to control height or to remove old flower heads. The best time for this type of pruning differs between species. Bigleaf and oakleaf hydrangea, which flower on previous year's growth, should be pruned shortly after flowering is complete. Panicle and smooth hydrangea flower on current year's growth and can be pruned anytime from late summer until early spring. If pruning these two species in the spring, try to prune before leaves appear. Plants of H. arborescens 'Annabelle' have been known to produce a second flush of flowers if pruned lightly after the first flowering.

While hydrangeas in landscape settings are relatively pest free, under certain growing conditions some diseases and insects can become problems. For the bigleaf hydrangea, the major disease problem is powdery mildew (see image at right). It is most common on plants growing in shade and under high humidity conditions. Powdery mildew infested leaves are covered with a light gray powdery-looking substance. Purple splotches may also appear. Powdery mildew rarely kills plants, but is unattractive. Powdery mildew may occur on other hydrangea species, but is most severe on bigleaf hydrangea.

One of the easiest ways to preserve these flowers is to allow them to almost completely dry on the plant. Do not collect them until the flowers have developed a papery feel. On a dry day with low humidity, cut the stems the length you need for making floral arrangements. Strip off all leaves and then find a dry place indoors where the flowers can finish drying. Some people recommend using a warm, dark location, such as an attic. Others prefer a cool, dry location. Flowers can be hung upside down while being dried, or can be placed in a vase with or without water. Whichever method you choose, be sure to keep individual inflorescences separated as they dry so that none of the flowers get squashed. The flower heads of some cultivars dry better than others.