If you look outside and find that your hydrangeas have opened green blossoms, it doesn't necessarily mean you ended up with Dorothy in the Emerald City. Hydrangeas (Hydrangea spp.) are known for blue and pink flowers, but some have green flowers when immature; some have flowers that mature to green, and some cultivars simply produce green blossoms.
Hydrangea Colors Evolve
The best-known and most popular hydrangeas, bigleafs (Hydrangea macrophylla), have blue flowers if planted in acidic soil and pink if planted in alkaline soil. They thrive in U.S. Department of Agriculture plant hardiness zones 4 through 11. However, hydrangea blossom colors also evolve as they mature, totally independent of the soil pH. A flower may begin as one color, then deepen or lighten or even move into an entirely different range of color over time.
Some hydrangeas start life green, then change shades as they age. The cultivar "Jane" (Hydrangea paniculata "Jane") has blossoms that open in a soft green but mature into a lovely shade of burgundy. It grows best in USDA hardiness zones 3 through 8. For truly immense green blossoms, try the hydrangea known as Incrediball (Hydrangea arborescens "Abetwo). Its 12-inch globular flowers open lime-green before maturing to white in USDA zones 3 through 9.
Maturing into Green
You'll find another color variation in the smooth hydrangea (Hydrangea arborescens), a United States native that grows wild in the eastern part of the country. It's hardy in USDA zones 4 through 9. The round globes of blossoms are a pure white when young, but they mature into a pale green. The flowers of the blue enchantress hydrangea (Hydrangea macrophylla "Monmar") are blue or pink depending upon the acidity of the soil, but as the blossoms age, they turn green with white splashes. The cultivar thrives in zones 5 through 9. The flowers of midnight duchess hydrangea (Hydrangea macrophylla "Hymmadii") grow in pale pink but mature to green. It grows in zones 6 through 9.
Green From the Get-Go
Some hydrangeas just bloom green from the beginning. One cultivar whose name reveals its color predilections is "Limelight" (Hydrangea paniculata "Limelight"), a shrub that grows in USDA zones 4 through 8. It produces large, lime-green blossoms in midsummer, and they stay solidly green until late fall when they develop a pink cast. The shrub grows to 8 feet tall and 6 feet wide, flowering from midsummer to fall.