The Big Leaf Hydrangea (Hydrangea macrophylla) is a deciduous shrub prized for its large, showy flower clusters that bring vibrant color to the summer garden. Native to Asia, this species features two main flower forms: the globe-shaped mophead and the flattened lacecap. Its popularity stems from its lush foliage and the unique ability of its blossoms to display different color tones. Success relies on understanding its specific needs, from initial planting conditions to seasonal maintenance.
Site Selection and Soil Foundation
Selecting the correct planting location is necessary for a thriving Big Leaf Hydrangea. This shrub performs best with bright morning sun followed by dappled or full shade during the afternoon heat. Too much direct sun can cause leaf scorch and wilting, while deep shade often results in fewer blooms.
The root system requires rich, well-drained soil that remains consistently moist but never waterlogged. Incorporating organic material, such as compost or aged leaf mold, before planting helps improve fertility and drainage. Since this species has shallow roots, the soil must retain moisture while allowing excess water to pass through.
Soil composition determines flower color, which is controlled by the availability of aluminum ions. Soil testing should be performed early to establish the baseline pH level necessary for color manipulation.
Watering and Routine Feeding
Hydrangea macrophylla is a water-demanding shrub, especially during establishment and hot summer weather. Watering should be deep and infrequent to encourage a robust root system. Check soil moisture by inserting a finger two inches deep; if dry, apply water slowly at the base until saturated.
A balanced, slow-release granular fertilizer (e.g., 10-10-10 or 8-8-8) supports steady growth and flower production. Apply this fertilizer in light, measured doses throughout the early and mid-growing season, typically starting in spring and repeating in May and July. Applying small amounts multiple times is better than a single heavy dose, which can burn the shallow roots.
Stop routine feeding by late July or early August to prevent the shrub from producing tender new growth. Late-season growth is susceptible to damage from early frosts, compromising the plant’s ability to set buds for the following year.
Pruning for Structure and Bloom
Understanding the flowering habit is necessary for successful bloom production. Most H. macrophylla cultivars bloom on “old wood,” meaning flower buds are set on the previous year’s stems. Improperly timed pruning removes these overwintering buds and is the most common reason for a lack of flowers.
Pruning should be done immediately after the flowers fade in summer, typically before August. This timing allows the shrub time to develop new growth and set flower buds before cold weather. Focus on selective pruning, removing old, non-productive canes down to the base to encourage fresh growth.
Remove dead, weak, or spindly stems to improve air circulation and maintain shape. For re-blooming varieties, which flower on both old and new wood, pruning is less restricted, but summer pruning maximizes potential blooms. Deadheading spent flowers can encourage a second flush of blooms on re-bloomers.
Controlling Flower Color
The ability of Hydrangea macrophylla to change flower color stems from the presence or absence of aluminum compounds in the flower tissues. Soil pH directly controls the availability of aluminum to the plant roots, making color manipulation achievable through soil amendment.
Achieving Blue Flowers
To achieve blue flowers, the soil must be acidic, with a pH maintained between 5.0 and 5.5. This range allows aluminum to be readily absorbed by the roots. Amendments like aluminum sulfate or elemental sulfur can be added to decrease the pH level. Using aluminum sulfate is the most direct method, as blue flowers require both low pH and the presence of aluminum.
Achieving Pink Flowers
For pink flowers, the goal is to raise the soil pH to 6.0 or higher, which binds the aluminum and makes it unavailable. Agricultural lime or dolomitic lime is used to increase the soil’s alkalinity. White-flowering cultivars lack the necessary pigment and will not change color regardless of soil pH.
The process of changing flower color is not immediate and often requires several applications over a full growing season. Always perform a soil test before applying amendments to determine the starting pH. High phosphorus fertilizers can also inhibit aluminum uptake, indirectly promoting pink coloration.
Winter Protection and Dormancy
Gardeners in colder regions, particularly USDA Hardiness Zones 5 and 6, must protect the dormant flower buds of H. macrophylla through the winter. Since the buds are formed on old wood in late summer, they are susceptible to damage from extreme cold or fluctuating temperatures. Protecting these buds is necessary for a reliable bloom display the following season.
The most basic protection involves applying a thick layer of organic mulch (straw, shredded leaves, or wood chips) around the base of the plant. This layer should be 6 to 8 inches deep and applied after the ground has frozen to insulate the crown and roots from severe temperature drops. Do not apply mulch too early, as this can trap moisture and encourage disease.
For exposed locations or the coldest zones, the entire shrub can be protected by constructing a cage around the canes using chicken wire or stakes and burlap. Fill this structure loosely with insulating material like straw or dry leaves to shield the stems from winter wind and desiccation. Container-grown hydrangeas should be moved to an unheated garage or sheltered area where temperatures remain just above freezing.