Hydrangeas, belonging to the genus *Hydrangea*, are a diverse group of flowering shrubs highly valued in horticulture for their impressive and long-lasting blooms. Botanically, what is often perceived as a single large flower is typically a composite inflorescence, a structure composed of many smaller flowers. The genus exhibits significant variation in inflorescence morphology, flower type, growth habit, and cultural requirements, which are key to distinguishing between the popular types.
Mophead hydrangeas are the most recognized form of *Hydrangea macrophylla*. Their most striking botanical feature is a large, spherical inflorescence (a corymb) that can exceed 8 inches in diameter. This rounded shape is achieved through a predominance of large, sterile florets, each with four or more showy, petal-like sepals. These sterile florets are designed to attract pollinators to the few fertile, but inconspicuous, flowers hidden within the structure. A unique botanical trait of many *H. macrophylla* cultivars is their ability to alter sepal color based on soil pH, a result of the mobile aluminum availability influencing anthocyanin pigment expression.
Lacecap hydrangeas represent the wild-type inflorescence form of *Hydrangea macrophylla* and provide a clearer view of the plant's true flower structure. The inflorescence is a flat or slightly rounded capitulum. It features a central core of tiny, fertile bud-like flowers that are capable of producing seeds. This fertile center is surrounded by an outer ring of larger, sterile florets with prominent sepals. This arrangement is highly efficient for pollination, as the showy sterile florets effectively guide insects toward the nectar and pollen-rich fertile flowers in the center.
Panicle hydrangeas (*Hydrangea paniculata*) are distinguished by their large, conical or pyramidal flower panicles, which can be up to 18 inches long. This panicle is a branched inflorescence that contains a mixture of both fertile and sterile flowers. The plant's growth habit is often more tree-like than other hydrangeas, with a woody, sturdy structure. A key botanical advantage of this species is its exceptional cold hardiness and its tendency to bloom on new wood grown in the current season. This means flower buds are formed in spring and are rarely lost to winter kill, making it a reliable bloomer in colder climates.
The smooth hydrangea (*Hydrangea arborescens*) is a deciduous shrub native to eastern North America. Its most popular cultivar, 'Annabelle', is known for producing enormous, symmetrical globular inflorescences, often reaching 12 inches across. These are composed almost entirely of sterile white florets. Unlike *H. macrophylla*, its flower color is not affected by soil pH. Similar to *H. paniculata*, it blooms on new wood and can be pruned back hard in late winter or early spring without sacrificing that season's flowers. Its stems are also notably more flexible than other types.
Oakleaf hydrangea (*Hydrangea quercifolia*) is easily identified by its foliage and flower form. Its leaves are deeply lobed, resembling those of an oak tree (*Quercus* spp.), and provide brilliant red and purple fall color. The plant produces elongated, conical panicles of white flowers that gradually fade to pink and then to a persistent tan. The individual sterile florets are larger and less densely packed than those of panicle hydrangeas. This species is also valued for its exfoliating bark, which peels back on mature stems to reveal a rich cinnamon-colored inner layer, providing winter interest.
