Hydrangea macrophylla (Thunb.) Ser.
Blue hydrangea, hydrangea, bigleaf hydrangea
Remember litmus paper? It turns pink in acid and blue in alkaline solutions. So do bigleaf hydrangeas (except that it is the other way around: acid = blue, alkaline = pink). It is a little more complicated, but the principle is the same: there is a pigment in these hydrangeas whose color is determined by the pH of the soil (level of acidity or alkalinity), and by the presence or absence of aluminum. The color is also determined by the amount of pigment that is present in the flowers. If there is no pigment, the flowers are white. If, as with many cultivated varieties, there is a lot of pigment, the shade or pink or blue (or purple) is very intense. Here are some details.
Identification: This hydrangea forms a rounded shrub 3-6' (91-182 cm) around. Branches emerge from the ground and rarely branch. Leaves are opposite, 4-8" (10-20 cm) × 3-6" (7.6-15 cm), crinkly, with serrated leaf margins. Fall color is underwhelming. Flowers form huge, ball-shaped clusters, which may be white or cream-colored; or shades of pink or blue ranging from pale in natural varieties to bright in cultivated varieties. (The samples on this page are cultivars.) The striking flower clusters are popular with landscapers. See this Hydrangea comparison table.
Hydrangea macrophylla on floridata.com
Hydrangea macrophylla on Plants for a Future, a resource and information centre for edible and otherwise useful plants
Hydrangea macrophylla at the University of Connecticut Plant Database
Hydrangea macrophylla at the Virginia Tech Department of Forest Resources and Environmental Conservation
Hydrangea macrophylla cultivars on Wikimedia Commons
Hydrangea macrophylla description by Thomas H. Kent, last updated 25 May 2020.
Range: Zones 6-10: