Four species of hydrangea shrubs are commonly grown in the United States. Bigleaf hydrangea (Hydrangea macrophylla), the most popular variety, is hardy in U.S. Department of Agriculture plant hardiness zones 6 through 9. Panicle hydrangea (Hydrangea paniculata) grows in USDA zones 4 to 7, while smooth hydrangea (Hydrangea arborescens) thrives in zones 4 to 9. Oakleaf hydrangea (Hydrangea quercifolia) is hardy in zones 5 and 9. Each species has different care requirements.
Hydrangeas prefer partial shade and moist, well-drained soil. Make sure plants receive about an inch of water each week. Smooth and bigleaf hydrangeas are heavy drinkers, so provide them with slightly more water than oakleaf or panicle shrubs. Protect the roots and preserve moisture levels in the soil by spreading 2 to 3 inches of mulch under each shrub.
The Mississippi State University Extension Service recommends fertilizing hydrangeas with a general purpose fertilizer such as 10-10-10. Give larger plants 1 to 2 cups of fertilizer, but only feed small plants about 1/4 cup. Sprinkle the fertilizer around the plant about 1 foot past the edge of the foliage. Fertilize smooth hydrangeas once a year late in winter. Panicle and oakleaf hydrangeas should be fertilized in April and June, while bigleaf hydrangeas do well with several light applications over the course of the growing season, or one application in early spring.
Pink and blue bigleaf hydrangeas change color depending on the soil pH level. You can apply a liquid soil drench in March, April and May to alter the plant’s color. If you have blue flowers but want pink, mix a tablespoon of hydrated lime into a gallon of water. If you want blue flowers, use aluminum sulfate instead. Drench the soil around the plant. White flowers and cultivars do not change color.
Newly planted hydrangea shrubs require extra winter protection. Mature hydrangeas are cold-hardy to 0 degrees Fahrenheit, but all varieties need protection when temperatures drop below -40 degrees F. Protect your plants by covering them to the ground with materials such as burlap, sheets, tarps or blankets. Secure the covering with bricks or rocks so that it stays in place.
Most hydrangeas bloom during the summer. Typically, panicle and smooth hydrangeas bloom on new wood, which means that their flowers grow from new stems. Oakleaf and most bigleaf hydrangea shrubs bloom on old wood; they produce flowers on stems that emerged from the previous year’s buds. Prune smooth and panicle hydrangeas late in winter or early in spring. Cut smooth hydrangeas back to about half their height. Remove about half of a panicle hydrangea’s older stems and their lower suckers. Prune bigleaf and oakleaf hydrangeas once a year, soon after they flower. Cut between one-fourth and one-half of the older stems to the ground and cut back all dead wood. To prevent passing diseases from infected to healthy plants, disinfect your pruning shears every two hours or 10 plants by soaking them for about five minutes in a mixture of 3 parts water to 1 part bleach.
Common Hydrangea Diseases
Powdery mildew is a common hydrangea problem. Infected leaves become coated with a grayish-white powdery fungus. Botrytis blight, another fungal infection, attacks new blossoms when the weather is chilly and humid. The blooms develop water-soaked spots before deteriorating, and gray fungus covers the decaying flowers. Avoid problems by spacing your shrubs far enough apart to allow adequate air circulation. Treat recurring powdery mildew and botrytis blight infections by adding a tablespoon of thiophanate-methyl to a gallon of water. Spray plants as soon as the first symptoms appear, For powdery mildew, reapply every 10 days to two weeks as needed. Treat botrytis blight every seven to 14 days or as long as the damp, cool weather lasts. Hydrangeas occasionally develop leaf spot and rust infections, but these are more unattractive than dangerous.
Mites occasionally feed on hydrangea shrubs, damaging new growth. Spider mites also create unattractive webs between the leaves. These pests are the most active during dry, hot weather, so make sure your plant is well watered. Aphids feed on new foliage. You can dislodge them by washing the shrub with a steady stream of water. Japanese beetles occasionally feed on oakleaf hydrangea leaves, but they are rarely a serious problem.