An “old wood hydrangea” is a shrub that develops flower buds on the stems, or “wood,” grown during the previous summer and fall. These buds survive the winter and open the following season, typically in early summer. This growth habit makes pruning timing critical. The term primarily applies to the popular Bigleaf Hydrangea (Hydrangea macrophylla) and the closely related Mountain Hydrangea (Hydrangea serrata).
Overall Size and Woody Stems
Old wood hydrangeas are deciduous shrubs with a generally rounded or mounding growth habit that contributes to a dense, full appearance in the landscape. Mature Bigleaf Hydrangeas (H. macrophylla) typically reach a height and spread of between three and six feet, depending on the specific cultivar. The Mountain Hydrangea (H. serrata) is often more compact, usually staying in the two to four-foot range.
The defining feature is the stem structure, which is noticeably woody and stiffer than the new growth that emerges green and flexible in the spring. Older stems are tough, thicker, and present a distinctive gray or brown coloration, often resembling slender tree branches. These perennial stems carry the dormant flower buds through the winter, allowing them to bloom in the new season. This rigid, established framework gives the mature plant its durable shape throughout the year.
Leaf and Foliage Characteristics
The foliage of old wood hydrangeas is bold and substantial, contributing significantly to the plant’s coarse texture. The leaves are arranged oppositely on the stems and are generally large, particularly on H. macrophylla, measuring between four and eight inches long. Each leaf is typically ovate, or egg-shaped, with a prominent pointed tip.
The leaf surface is often a deep, dark green with a slightly glossy sheen, especially on Bigleaf Hydrangea varieties. The leaf edges are distinctly serrated or finely toothed, which is a trait particularly evident in Hydrangea serrata, giving the species its name. While the foliage remains green throughout the growing season, some cultivars can exhibit a subtle color change in autumn, developing tones of burgundy or reddish-purple before dropping.
Flower Forms and Coloration
The flowers are the most complex and variable feature of old wood hydrangeas, appearing in two primary forms: Mophead and Lacecap. Mophead blooms are large, dense, and rounded clusters composed almost entirely of showy, sterile florets, creating a pom-pom effect that can reach the size of a grapefruit. Lacecap blooms are flatter clusters featuring a central mass of tiny, fertile flowers surrounded by an outer ring of larger, more decorative sterile florets, resembling a delicate doily.
For both H. macrophylla and H. serrata, flower color depends on the soil’s pH level and the availability of aluminum ions. In highly acidic soils (pH below 6.0), aluminum is readily accessible, resulting in vibrant blue or lavender flowers. Conversely, in alkaline or slightly acidic soils (pH above 6.0), aluminum is bound up and unavailable, causing the flowers to display shades of pink or red. White-flowering varieties are an exception, as their pigments do not react to the soil’s pH and their blooms remain white.