Hydrangeas are flowering shrubs known primarily for their very large, showy blooms. There are about 23 species of hydrangeas, according to the U.S. Department of Agriculture. These shrubs feature flowers of sky blue, pastel pink or creamy white. The exact color depends on the acidity level of the soil and the particular species. The basic care needs of hydrangeas are the same, regardless of the species.
In general, most hydrangeas grow best in U.S. Department of Agriculture plant hardiness zones 4 through 9. The bigleaf hydrangea (Hydrangea macrophylla), which is the most popular hydrangea in America, is only hardy to USDA zone 6. Panicle hydrangeas (H. paniculata), which feature long panicles of flowers rather than round clusters, are the cold-hardiest. They are able to grow in USDA zone 3, according to the University of Illinois.
Hydrangeas grow best in soil that is well-draining, moist and cool, according to the U.S. Department of Agriculture. Choose a location that receives some afternoon shade, as full sun exposure can scorch the broad leaves and bleach the colors of the flowers. In general, the hotter the climate, the more afternoon shade your hydrangea bush should receive. Most hydrangeas grow well on the north side of a house or building. Panicle hydrangeas, as well as being cold-hardy, also do best in hot areas.
It is possible to change the color of the flowers of your hydrangea by changing the acidity level of the soil, according to Clemson University. To cultivate blue flowers, create and maintain a soil pH level of 5.0 to 5.5. Use a soil testing kit to find out if your pH level is above this. If so, add aluminum sulfate or sulfur to the soil just as new growth is emerging on your plant. Apply according to the directions on the label.
To create pink flowers, keep the soil level at 6.0 or higher by adding lime to your soil as new growth emerges in the spring. Follow the directions on the package of lime for the size and age of your plant.
Water and Fertilizer
Keep the soil moist throughout the growing period (spring and summer), especially during times of hot, dry weather. A thorough watering a minimum of one time per week is recommended by Clemson University. In the fall, reduce watering to let the plant go dormant. Fertilize your hydrangea with a balanced (10-10-10) fertilizer. Use 2 cups for every 100 square feet of the planting site. Do this once in early spring, late spring and mid-summer.
Hydrangeas sometimes fail to bloom. This is most commonly caused by cold or freezing temperatures injuring the new buds, according to Clemson University. Poor blooming can also be caused by not enough sun exposure or too much fertilizer in the soil, especially if it is nitrogen-heavy. Make sure your hydrangea is receiving at least some morning sunlight, and do not fertilize more than is recommended. Hydrangeas can also suffer from fungal diseases, including powdery mildew. Avoid wetting the foliage when you water, and provide plenty of room around your hydrangea for air to circulate. Finally, monitor for common insect pests such as aphids or spider mites and treat with an insecticide when necessary.