Hydrangeas, an old-fashioned garden favorite, have made a comeback in gardens as hybridization has tamed their tendency to grow out of bounds. They bloom all summer long, making them reliable background plants for the perennial garden and attractive hedges. Many hydrangeas, no matter what other colors they sport, go through a green phase, either at the beginning or end of their bloom.
Hydrangeas are summer-blooming herbaceous perennials grown in most of the U.S., from hardiness zone 4 to zone 9. Unlike their spring-blooming cousin, peony, they are woody and can form sturdy shrubs and hedges. Unlike their woody cousin, rose, their bloom period lasts for months instead of just a few weeks. What makes hydrangeas especially attractive in the perennial garden is that the blooms change color as they age. Shades of white, pink, purple blue and green are common. Many varieties go through multi-colored stages or have bicolor blooms. Blue and pink colors can be forced in some types of hydrangeas. Green blooms are natural, however and typical in two types of hydrangea.
Hydrangeas are native to North America and Asia. Early cultivars were shipped to England and France during the 18th century colonial period. Botanists adopted the fascinating hydrangea and French hybrids of the native plant were re-introduced to American gardens at the beginning of the 20th century. Japanese hybrids appeared beginning mid-century. Interest in native and old-fashioned perennials has led to the "rediscovery" of a plant that is the result of more than a century of research and hybridization.
Descendants of the native hydrangeas, arborescens (Annabelle) and macrophylla (mophead) bloom green and age to white---or vice versa; mopheaps often trend through pink or blue phases before ending as green blooms. Lacecaps, (h. macrophylla normalis) most resemble the original native plant in the form of flowering, beginning around the outside and blooming slowly inward. The Asian native serrata (or mountain hydrangea) thrives in woodlands, tolerating more shade than American forms. Pee gee (h. paniculata) are hybrids bred for their small size. Oakleaf hydrangea (quercifolia) sports leaves shaped like its namesake that turn brilliant colors in fall and thrives in heat sunny conditions. Arborescens, macrophylla and paniculata all have forms that mature to green blooms.
The mechanism that sets off the color progression in hydrangea's long period of bloom is not well understood. Mophead (macrophylla) responds to the presence of aluminum sulfate in the soil with blue coloring. Generally speaking, hydrangeas living in soil with extra phosphorus with a pH near 6.0 (often established by adding dolmitic lime) will tend to show more pink in their coloring. Hydrangeas that consume aluminum in their diet and live in soil that is rich in potassium with a low pH (5.2 to 5.5), often show more blues in their progression. Greens, like whites, seem to be naturally-occurring coloring in any soil.
Hydrangeas begin blooming in late June or early July; the blooms mature and often stay on the shrub throughout winter until they are cut back. Green blooms signal maturity on many varieties in autumn but a few, like the old-fashioned "snowball" or modern "abetwo", both arborescens cultivars, bloom first in shades of green and mature to white. After the first hard freeze, most varieties dry on the branch to a pale green or manila brown.