Hydrangea macrophylla: Bigleaf Hydrangea
This is a rather large and confusing group of plants commonly found in shade gardens. Plants generally range in size from 4' tall to 6' wide. Plants are known for very showy flowers in the summer.
Flower color on many of the selections is influenced by soil pH. In very acid soils (pH 5 to 5.5) the flower color may be more blue. In a less acid soil (pH 6.5) the flower color may be more pink. In reality the flower color is determined by the amount of aluminum in the soil. If you have pink flowers and want blue, try watering your plants occasionally from the time flower buds are set until they open with a solution that contains ½ ounce of aluminum sulfate per gallon of water. If the flower color is blue/purple and you would like a pink flower color, try amending the soil with lime. Cultivars vary in their responsiveness to this soil pH/flower color treatment. Sepals when they first start coloring (pink or blue) may appear to have a white eye in the center. Color pigmentation seems to start at the edge and work toward the center. Many of the mopheads are great for dried arrangements (e.g. Hamburg').
Mophead or Lacecap
The many cultivars tend to be grouped into one of two categories: mopheads or lacecaps. 'Mopheads' typically have a dome-shaped inflorescence that is primarily composed of sterile flowers. These sterile flowers look like popcorn. With 'lacecaps', the inflorescence tends to have an outer ring of sterile flowers surrounding an inner circle of fertile (less showy) flowers. A hot area with the Bigleaf Hydrangeas is the selection of 'remontant' flowering plants.
Credit for the popularity of Bigleaf Hydrangeas should be given to Endless SummerR. which was introduced in 2004. Until that landmark introduction, most Bigleaf Hydrangeas were thought to bloom only on wood produced during the previous season. Plants that were accidentally pruned to the ground or killed by harsh winter conditions were less likely to produce summer flowers the following year. The recent introduction of remontant types, those that bloom on both old and new wood, enables plants to bloom regardless of winter damage or pruning mistakes. This remontant trait also makes it more likely that the bloom season will be extended. You can read more about remontant Bigleaf Hydrangeas at Dave’s Garden. http://davesgarden.com/guides/articles/view/2519/#ixzz2yVs5n05h
Mildew and Bigleaf Hydrangeas
Bigleaf Hydrangea is fairly susceptible to powdery mildew. A number of selections with thicker leaves (more substance) appear to show promise as being less susceptible to this aesthetic disease.
When dealing with the lacecap varieties, the name 'Teller' may come up. Breeding in Switzerland resulted in a series of 26 cultivars known as the Teller series. The word 'Teller' means 'plate' in German. Sadly, some of the cultivars have been lost so some nurseries sell generic plants called 'Teller Pink', 'Teller White' etc.. The original hybrid names have Swiss or German names for birds. For example, 'Fasan' is translated as 'Pheasant' and is synonymous with the cultivar name 'Twilight'.