Hydrangeas are popular garden shrubs, cherished for their large, showy flowers that provide color throughout the summer and fall. The genus Hydrangea contains several distinct species, each with unique characteristics and care requirements. Proper identification of the specific type of hydrangea you have is necessary because care tasks, particularly pruning, must be tailored to the species for a successful bloom cycle. Understanding the differences among the most common types is the first step toward successful cultivation.
Starting the Identification Process
Before examining the details of the plant, begin with an assessment of its location and overall growth pattern. Observe the mature size and shape, noting whether it grows as a multi-stemmed bush, a tree-like specimen, or a clambering vine. This observation can narrow the possibilities; a large, upright, tree-form shrub is likely a Panicle Hydrangea (H. paniculata), while a smaller, dense, mounding shrub suggests a Bigleaf or Smooth Hydrangea. Note the time of year and whether the plant is flowering, as this provides a major clue about its blooming habit. Examine the environment, noting the amount of daily sun exposure it receives, as some types are much more sun-tolerant than others.
Decoding the Leaves and Stems
The foliage and wood structure offer defining characteristics. Oakleaf Hydrangea (H. quercifolia) is easily distinguished by its lobed leaves, which resemble those of an oak tree and can grow up to 12 inches long. This species also exhibits unique, cinnamon-tan-orange bark that naturally peels from the woody stems, providing winter interest. The leaves often turn rich shades of bronze, red, and purple in the autumn.
Smooth Hydrangea (H. arborescens) has large, heart-shaped leaves with a matte green, often floppy texture. The stems are generally smooth and bear opposite leaves, forming a rounded, colonial clump. In contrast, Bigleaf Hydrangea (H. macrophylla) has thicker, glossy leaves that are coarsely toothed along the edges. Its stems can sometimes show a reddish or streaked coloration.
Panicle Hydrangea (H. paniculata) tends to have more elongated and pointed leaves, frequently arranged in whorls of three at the nodes. This species develops woody, persistent stems that allow it to be trained into a small tree form.
Analyzing Bloom Structure and Timing
Flower characteristics, especially shape and timing, offer the most definitive clues for identification. Bigleaf Hydrangea (H. macrophylla) is known for two distinct flower forms: the Mophead (large, spherical clusters of sterile flowers) and the Lacecap (a flat cluster of fertile flowers surrounded by showy sterile florets). Bigleaf flower color is pH-dependent, turning blue in acidic soil and pink in alkaline or neutral soil.
Panicle Hydrangea blooms appear in large, cone-shaped or pyramidal clusters, referred to as panicles. These flowers typically start creamy white and transition to shades of pink or reddish-pink as they age throughout the summer and fall, a color change that is not influenced by soil pH. Oakleaf Hydrangea also produces cone-shaped flower clusters, but they are looser and more elongated than those of the Panicle Hydrangea.
A fundamental distinction is whether they bloom on “old wood” or “new wood.” Old wood bloomers (Bigleaf and Oakleaf Hydrangeas) set flower buds on the previous year’s growth in late summer or fall. New wood bloomers (Smooth and Panicle Hydrangeas) produce flowers on the current season’s growth, typically in mid-to-late summer. This difference is critical for determining the correct pruning time.
Why Knowing the Type is Essential
Knowing the species directly impacts care and maintenance, particularly the pruning schedule. Old wood bloomers (Bigleaf and Oakleaf Hydrangeas) must be pruned immediately after flowering in summer to avoid removing the buds set for the next year. Pruning these types in late fall, winter, or early spring removes the dormant flower buds and results in a year without blooms.
New wood bloomers (Smooth and Panicle Hydrangeas) can be pruned aggressively in late winter or early spring before new growth begins, as their flowers form on the fresh stems of the current season. Identification also informs hardiness and sun requirements. Panicle Hydrangeas are the most cold-tolerant and sun-loving, thriving in full sun, while Bigleaf Hydrangeas often require partial shade and winter protection in colder climates. Choosing the correct pruning time and providing the appropriate environment are entirely dependent upon accurately identifying the hydrangea type.