What Interesting Effects Do Different PH Levels Have on Hydrangea Blossoms?

Hydrangea Species

• Bigleaf hydrangea (Hydrangea macrophylla) has two forms: hortensia, or mophead, and lacecap. Serrated hydrangea (H. serrata) has flowers resembling those on lacecap hydrangea, but its leaf margins are serrated. Smooth hydrangea (H. arborescens) is a southeastern native having smooth-textured bark. Panicle hydrangea (H. paniculata) and oakleaf hydrangea (H. quercifolia) have cone-shaped flowers, but different leaf shapes. Climbing hydrangea (H. petiolaris) is a clinging vine. Of all the hydrangea species, only some cultivars of bigleaf and serrated hydrangeas bear flowers that can be influenced by soil pH.

Influence of pH

• Aluminum is the key to producing different flower colors in bigleaf and serrated hydrangeas. Although most garden soils contain aluminum, plants cannot absorb it unless it is available to them. This availability is possible when pH levels are low -- from 5.0 to 5.5. At this level, hydrangeas can absorb aluminum, which makes their flowers bluer. At pH levels of 6.0 to 6.5, plants are unable to absorb as much aluminum, which makes their flowers pinker. Some bigleaf hydrangeas, such as Pia and Kardinal, are bred to have “non-bluing” flowers and they retain pink or red flowers.

Manipulating pH

• For color-change receptive hydrangea cultivars, manipulating soil pH requires the addition of soil amendments. Lime will lower soil pH to produce bluer flowers and sulfur will raise soil pH to produce pinker flowers. The University of Georgia’s Center for Urban Agriculture recommends applying 1/2 cup of wettable sulfur powder to soil per 10 square feet once a year to lower soil pH for bluer flowers. For pinker flowers, they recommend applying 1 cup of dolomitic lime to soil per 10 square feet once a year to raise soil pH.

Multi-Color Effects

• Changing soil pH is a gradual process and may produce interesting effects in some hydrangeas. Aluminum uptake is concentrated in blossoms, where it influences the anthocyanin pigments that are responsible for flower color. Where uniform concentrations are found, flowers display solid shades of blue, pink, red and purple. Where concentrations are erratic, different colors may appear simultaneously on the same flower cluster. If soil amendments are not maintained each year, flowers will revert gradually to their naturally occurring colors. Until their reversion is complete, usually within three years, they may bloom in different colors each year.