Voluminous Beauty
Hydrangeas are the stalwarts of the summer and autumn garden, providing immense visual volume and structural interest long after their initial flush of color fades to parchment.
The Chemistry of Color
The most fascinating aspect of the Hydrangea macrophylla (Bigleaf hydrangea) is its ability to change color based on soil chemistry—specifically, the availability of aluminum, which is dictated by soil pH.
- Acidic Soil (pH < 6.0): Aluminum is readily available to the plant, resulting in blue flowers. Add elemental sulfur or aluminum sulfate to lower pH.
- Alkaline Soil (pH > 7.0): Aluminum is locked up, resulting in pink flowers. Add garden lime to raise the pH.
- Note: White hydrangeas (and H. paniculata) cannot change color regardless of soil chemistry.
Demystifying Pruning
Incorrect pruning is the primary reason hydrangeas fail to bloom. You must know which type you have:
- Blooms on Old Wood (Macrophylla, Oakleaf): These form their flower buds in the late summer for the following year. Do not prune in spring, or you will cut off all the flowers. Prune only immediately after they finish blooming in summer.
- Blooms on New Wood (Paniculata, Smooth/Annabelle): These form buds on the current year's growth. They should be pruned in late winter or early spring before new growth emerges. Hard pruning encourages larger (though fewer) flower heads.