Though not native to the types of climates found in Arizona, hydrangeas (Hydrangea spp.) can be successfully grown in most parts of the state. You just need to choose the right species for your area, and take a few steps to provide healthy growing conditions.
Hydrangeas are hardy in U.S. Department of Agriculture plant hardiness zones 3 through 9, depending on species. Arizona covers USDA zones 5 through 10, so certain species of hydrangea can be grown through most of the state. Parts of southwestern Arizona are too hot for all hydrangea species, and only the more heat-tolerant varieties can grow in the Phoenix area. Provided their other growing needs are met, smooth hydrangea (Hydrangea arborescens, USDA zones 3 through 9), bigleaf hydrangea (Hydrangea macrophylla, USDA zones 6 through 9) and oakleaf hydrangea (Hydrangea quercifolia, USDA zones 5 through 9) are hardy throughout most of Arizona.
Selecting a Site
Hydrangeas' sunlight needs vary by species. Smooth hydrangea is native to woodland locations, and prefers part shade. Oakleaf hydrangea also prefers part shade. Bigleaf hydrangea grows well in sunny locations with afternoon shade to prevent wilting. Panicle hydrangea (Hydrangea paniculata, USDA zones 3 through 8) can take the most sun of any hydrangea species, and is also the most tolerant of drought. In addition to some afternoon shade in hot climates, hydrangeas also need protection from drying winds. Planting on the leeward side of a building, near a fence or by a wind-break of trees, should provide enough shelter.
One challenge facing hydrangea growers in Arizona is soil conditions. Arizona climates include cold intermountain regions, intermediate areas with warm summers and chilly winters, high desert regions and areas with a subtropical desert climate. The soil conditions found in these regions are often well-draining, which hydrangeas need, but most do not contain the high levels of organic matter that hydrangeas require. Before planting hydrangeas, spread a 1- to 2-inch layer of organic matter like compost across the soil surface, then work it in to the top 12 inches of soil. Incorporating organic matter also improves drainage in compacted or clay soils.
Fall is the best time to plant shrubs, including hydrangeas. They can also be planted in spring and summer, or while dormant in winter, but planting in the fall allows more time for the plants to establish a good root system before hot weather arrives. After amending soil, dig a planting hole the same depth as the hydrangea's root ball is high and two to five times wider than the root ball. Space plants about 6 feet apart. Set the hydrangea in the center of the hole, and fill in around the root ball with the soil dug out of the hole. After filling in the hole, mound-up a ring of soil about 3 inches high that circles the root ball. This will help keep water from flowing away from the plant roots. Apply a layer of mulch 2 to 3 inches deep around the base of the plant, but keep the mulch from touching the crown. Water thoroughly, and continue to keep the soil moist but not saturated for the first few months after planting.
An established hydrangea plant needs about 1 inch of water per week. In very hot weather, supply up to 2 inches of water to keep the plants from wilting. Try to avoid getting water on the leaves, which can cause leaf spot disease, or water in the morning so there is plenty of time for the water to evaporate during the day. On an ongoing basis, hydrangeas have moderate fertilizer needs. Apply a balanced, slow-release dry fertilizer with an N-P-K ratio of 10-10-10 at a rate of 2 cups per 100 square feet in the spring after plants start to green-up and again after flowering. There is no need to fertilize at planting time.