The primary issue with planting hydrangeas under trees is the intense competition for essential resources. Trees, especially large, mature ones with extensive root systems, are highly efficient at absorbing water and nutrients from the soil. The area directly beneath a tree's canopy, known as the "drip line," is often the most densely packed with feeder roots. Hydrangeas are notoriously thirsty plants that require consistent moisture, particularly during the warmer months, to support their large leaves and prolific blooms. When planted in this zone, hydrangeas must compete directly with the tree's roots for every drop of water and every available nutrient. Even with regular watering, the tree's roots will often absorb the moisture before the hydrangea's shallower root system can access it, leading to chronic water stress for the shrub.
Light is another non-negotiable factor for hydrangea health and flowering. Most hydrangea species, including the popular bigleaf (Hydrangea macrophylla) and panicle (Hydrangea paniculata) hydrangeas, require a significant amount of light to produce their signature blooms. While they often thrive in partial shade or dappled sunlight, the deep, dense shade cast by many trees is typically insufficient. The hydrangea may survive in deep shade, but it will likely become leggy as it stretches for light and will produce few, if any, flowers. The plant's energy will be directed toward survival rather than reproduction, resulting in a disappointing display. The light conditions change with the seasons, so a spot that receives dappled light in spring before the tree fully leafs out may become deeply shaded by summer, precisely when the hydrangea needs light most for blooming.
Planting under a tree also presents significant challenges related to soil quality and physical space. To install a hydrangea, you must dig a hole, which inevitably involves cutting through the tree's surface roots. This can damage the tree and create an entry point for disease. Furthermore, the soil in these areas is often depleted of organic matter because the tree has been drawing from it for years. It may also be compacted and lack good drainage or aeration. Amending the soil with compost is beneficial, but it can be difficult to adequately improve a large enough area without further disturbing the tree's root system. The hydrangea will be confined to a small pocket of improved soil, while its roots will quickly encounter the native, competitive environment.
Despite these challenges, it is not always impossible to grow hydrangeas under trees if specific conditions are met and careful strategies are employed. The first step is to choose the right tree. A tree with a deep taproot (like a walnut or pine) is preferable to one with a shallow, mat-like root system (like a maple or willow). The canopy should be high and open, allowing for dappled sunlight rather than deep shadow. Selecting a more adaptable hydrangea variety is crucial. Smooth hydrangeas (Hydrangea arborescens), such as 'Annabelle', are generally more tolerant of these difficult conditions than other types. The planting process must be meticulous. Instead of digging a deep hole, create a wide, shallow basin for the hydrangea on top of the existing soil, and fill it with a generous amount of rich, well-draining compost and topsoil. This minimizes root disturbance and gives the hydrangea a better start. Finally, a strict and generous watering regimen is non-negotiable, ensuring water reaches the hydrangea's roots directly.
