Bigleaf Hydrangea

Bigleaf hydrangea is the hydrangea that changes color with your soil, the one with the big, rounded mophead or delicate lacecap flower clusters that shift from blue in acidic soil to pink in alkaline soil, a chemistry experiment playing out in the garden every summer. Hundreds of cultivars exist, divided into mopheads (dense, ball-shaped clusters) and lacecaps (flat clusters with showy flowers around the edge). It grows four to six feet tall with a rounded, dense habit and the large, coarse-textured leaves that give it its common name. Native to Japan and possibly Korea.

In Western Washington, bigleaf hydrangea is everywhere, and the most common complaint is that it does not bloom. The reason is almost always the same: it blooms on old wood, meaning next year's flower buds form on this year's stems. A hard winter, a late frost, or an untimely pruning removes the buds and you get foliage without flowers. Choose reblooming cultivars like the Endless Summer series if you want insurance against bud loss. For blue flowers in our naturally acidic soils, aluminum sulfate is rarely needed, the soil does the work. For pink, you need to raise pH with lime. Part shade and consistent moisture produce the best results. Several diseases are tracked, including powdery mildew in dry shade. The flower color trick works because aluminum availability changes with pH, and it is one of the most satisfying things you can manipulate in the garden.