Bigleaf hydrangeas (H. macrophylla) need special attention in the hottest and coldest zones. In the hot climates of the Southeast and Southwest, plant them in a shady spot or where they get only morning sun. And make sure they get consistent moisture, advises the New Mexico State University Extension Service. Extreme heat can make even shade-grown shrubs wilt, but watering revives them.
In cold areas, you must prevent bigleaf hydrangea stems from freezing to the ground, or they may not survive winter. Surround the shrubs with chicken wire, fill the center all around the stems with dried leaves, and cover the cage with burlap. Even with these measures, growing hydrangeas in cold, dry areas such as the Rocky Mountain states is risky, because of the region's drought problems and water restrictions.
H. serrata is much like H. macrophylla, except that it grows into a smaller, more compact shrub. University of Maine Cooperative Extension Specialist Lois Berg Stack notes that in Zones 3a through 5b, H. serrata seems more hardy than H. macrophylla, although neither will survive reliably in climates colder than Zone 5.
The beautiful oakleaf hydrangea (H. quercifolia, Zones 5?9) is native to shady stream banks in eastern North America. Most varieties grow 4 to 6 feet tall and wide, with white flowers, and leaves that are indeed shaped like those of an oak, only larger. The leaves begin the year with a rusty look and continue to have a roughness to them throughout the summer. In fall, the foliage turns varying shades of burgundy.
One of this shrub's finer points is that it suckers (sends up vigorous shoots from its roots) slightly, which makes it easy to divide if you want more plants. You can choose from several varieties, including the astoundingly large 'Alice', which grows to 12 feet tall and wide with 12-inch panicles—highly branched flower structures—to match. On the other end of the spectrum is 'Pee Wee,' which tops out at about 3 feet.
In moist, temperate climates, such as the Pacific Northwest, the oakleaf hydrangea grows beautifully as a dry shade plant, but in hotter climates or if sited in more sun, it needs consistent moisture. It is a robust grower in morning sun, reports Ed Gilman, Ph.D., professor of horticulture at the University of Florida, who has an 8-foot-tall, 12-foot-wide specimen in his own garden. In very cold climates, protect the stems of oakleaf hydrangeas from freezing in the same manner you would bigleaf types.