Hydrangea macrophylla

In many cultivars of Hydrangea macrophylla, flower color depends on soil pH and aluminum availability—acidic soils (pH below 6.0) tend to produce blue flowers, while alkaline soils (above 7.0) produce pink flowers. Adding aluminum sulfate can lower pH and promote blue coloration, while lime raises pH and shifts flowers toward pink.

Most Hydrangea macrophylla cultivars bloom on old wood formed during the previous season, so they should be pruned immediately after flowering in late summer. Pruning in spring or fall removes the flower buds and can prevent blooming entirely the following year.

Mophead hydrangeas produce large, rounded clusters of mostly showy sterile flowers, while lacecap hydrangeas have flatter flower heads with small fertile flowers in the center surrounded by a ring of larger sterile florets. Both are forms of Hydrangea macrophylla and have similar cultural requirements.

The most common reason Hydrangea macrophylla fails to bloom is winter damage to flower buds, which are formed on old wood the previous season. Late spring frosts, improper pruning, or planting in zones too cold for reliable bud survival (typically zones 5 and colder) can all prevent flowering.