Hydrangeas are popular garden shrubs known for their large, colorful summer blooms. Because some varieties spread rapidly, many gardeners wonder if these plants are classified as invasive species. Understanding the difference between an aggressive garden plant and a true invasive species is key to assessing the risk hydrangeas pose. Members of the Hydrangea genus are generally safe additions to the garden when their specific growth habits are understood and managed.
Defining Aggressive vs. Invasive
An invasive species is non-native to the local ecosystem and causes environmental or economic harm. This harm occurs when the plant spreads rapidly beyond cultivation into wild areas, outcompeting native flora and disrupting the ecosystem’s balance. In contrast, an aggressive plant, or vigorous spreader, grows quickly and spreads readily within a managed garden space. While this behavior can crowd out nearby plantings, it does not threaten the greater natural environment. Hydrangeas are generally not listed as invasive species in North America; their reputation stems from the local aggression exhibited by some varieties.
Spread and Growth Habits by Hydrangea Species
The growth habits of hydrangeas vary significantly by species, determining the level of management required. Hydrangea macrophylla (bigleaf) and Hydrangea paniculata (panicle) are clump-forming shrubs that grow from a central root crown. They expand upward and outward, presenting a low risk for unwanted spreading because they do not produce underground runners. The aggressive reputation primarily stems from two native species: Hydrangea arborescens (smooth hydrangea) and Hydrangea quercifolia (oakleaf hydrangea). Both species are known for suckering, using underground runners to produce new shoots that quickly expand their footprint into sprawling colonies.
Controlling Unwanted Growth in the Garden
Containment and routine maintenance are the most effective strategies for managing aggressive species like H. arborescens and H. quercifolia. Unwanted shoots (suckers) should be removed immediately by severing them at the soil line or tracing them back to the root. For permanent containment, physical root barriers can be installed during planting to block horizontal runners from establishing new plants. Pruning for size management requires understanding the plant’s bloom cycle to ensure flower production. Smooth and panicle hydrangeas bloom on new wood and tolerate pruning in late winter, while oakleaf and bigleaf hydrangeas bloom on old wood and should be pruned only after flowering ends in summer. Selecting compact, non-aggressive cultivars is a proactive step for gardeners with limited space.