Hydrangea flowers are produced from early spring to late autumn; they grow in flowerheads (corymbs or panicles) most often at the ends of the stems. Typically the flowerheads contain two types of flowers: small non-showy fertile flowers in the center or interior of the flowerhead, and large, sterile showy flowers with large colorful sepals (tepals). These showy flowers are often extended in a ring, or to the exterior of the small flowers. Plants in wild populations typically have few to none of the showy flowers, while cultivated hydrangeas have been bred and selected to have more of the larger type flowers.

There are two flower arrangements in hydrangeas with corymb style inflorescences, which includes the commonly grown "bigleaf hydrangea"—Hydrangea macrophylla. Mophead flowers are large round flowerheads resembling pom-poms or, as the name implies, the head of a mop. In contrast, lacecap flowers bear round, flat flowerheads with a center core of subdued, small flowers surrounded by outer rings of larger flowers having showy sepals or tepals. The flowers of some rhododendrons and viburnums can appear, at first glance, similar to those of some hydrangeas.

Hydrangea flowers, when cut, dehydrate easily and wilt very quickly due to the large surface area of the petals. A wilted hydrangea may have its hydration restored by first having its stem immersed in boiling water; as the petals of the hydrangea can also absorb water, the petals may then be immersed, in room-temperature water, to restore the flower's hydration.

Hydrangea flower color can change based on the pH in soil. As the graph depicts, soil with a pH of 5.5 or lower will produce blue flowers, a pH of 6.5 or higher will produce pink hydrangeas, and soil in between 5.5 and 6.5 will have purple hydrangeas. White hydrangeas cannot be color-manipulated by soil pH because they do not produce pigment for color. In other words, while the hue of the inflorescence is variable dependent upon cultural factors, the color saturation is genetically predetermined. In most species, the flowers are white. In some, however, (notably H. macrophylla), they can be blue, red, or purple, with color saturation levels ranging from the palest of pinks, lavenders & powder blues, to deep, rich purples, reds, and royal blues. In these species, floral color change occurs due to the availability of aluminium ions, a variable which itself depends upon the soil pH. For H. macrophylla and H. serrata cultivars, the flower color can be determined by the relative acidity of the soil: an acidic soil (pH below 7), will have available aluminum ions and typically produce flowers that are blue to purple, whereas an alkaline soil (pH above 7) will tie up aluminium ions and result in pink or red flowers. This is caused by a color change of the flower pigments in the presence of aluminium ions which can be taken up into hyperaccumulating plants.

• Hydrangea anomala – (climbing hydrangea) Himalaya, southwest China
• Hydrangea arborescens – (smooth hydrangea) eastern North America
• Hydrangea aspera – China, Himalaya
• Hydrangea bretschneideri – China
• Hydrangea chinensis – China and Taiwan
• Hydrangea chungii – China
• Hydrangea cinerea – (ashy hydrangea) eastern United States
• Hydrangea coenobialis – China
• Hydrangea davidii – China
• Hydrangea febrifuga – central & southern China to Malesia and New Guinea
• Hydrangea glaucescens – China, Myanmar and Vietnam
• Hydrangea gracilis – China
• Hydrangea heteromalla – Himalaya, west and north China
• Hydrangea hirta – Japan
• Hydrangea hydrangeoides – Ulleungdo, Japan, Kurils
• Hydrangea hypoglauca – China
• Hydrangea integrifolia – China
• Hydrangea involucrata – Japan, Taiwan
• Hydrangea jelskii – Andes
• Hydrangea kwangsiensis – China
• Hydrangea kwangtungensis – China
• Hydrangea lingii – China
• Hydrangea linkweiensis – China
• Hydrangea longifolia – China
• Hydrangea longipes – western China
• Hydrangea macrocarpa – China
• Hydrangea macrophylla – (bigleaf hydrangea) southeast Japan, southern China
• Hydrangea mangshanensis – China
• Hydrangea paniculata – (panicled hydrangea) eastern China, Japan, Korea, Sakhalin
• Hydrangea peruviana – Costa Rica and Panama, Andes

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