About green hydrangeas.....
Hydrangea flowers produce spectacular and colorful blooms in hues of blue, pink, white and often in many shades in between. But what causes hydrangea flowers to turn green? Here are some answers.
Some Varieties Naturally Produce Green Flowers
Among the various cultivars, some naturally produce green hydrangea flowers. These include the Limelight hydrangea with its bright lime-green flowers. In Limelight, the older flowers turn from green to pink to white to burgundy – while new floral growth comes in the spectacular lime green. Limelight hydrangeas grow from 6 to 8 feet tall and wide.
Another variety is the green-flowering Annabelle. In fact, the Annabelle hydrangea, which initially blooms white, will always turn to green flowers after it has been in bloom for a period of about 2 weeks. Sometimes, however, the blooms stay white a bit longer.
Changing Colors With Age
Among hydrangea aficionados, the prevailing opinion is that all hydrangea flowers change color with age. Pink and blue hydrangeas most commonly turn green. In the South, where the climate is hot and sunny, these green-hued hydrangeas may begin to pick up shades of burgundy and pink.
Some hydrangeas begin in various shades of green, then turn a deep blue before finishing up in a maroon shade.
Experts also caution that some pink and blue hydrangeas may produce green flowers in some years for reasons as yet unknown. The consensus is that this condition lasts only for 1 to 2 years before the plant returns to its normal coloring. Home gardeners can help facilitate this return-to-normal flowering by adding fertilizer.
How Climate Issues Affect Color Of Hydrangea Flowers
Hydranges like morning sun and afternoon shade, and don’t do well in drying wind. That said, they also like relatively consistent temperatures – not too hot and not too cool. But as the blooms begin to grow, cooler temperatures encourage deeper colored flowers.
When a sudden cold snap, frost or snow or freezing rain, negatively affects a blooming hydrangea, all the gardener can do is to cut back the flowers affected by the change in climate. The following year, or the next growing season, the hydrangea should flower as usual.
Some varieties of commercial hydrangeas, including Lace Cap hydrangea, like cool conditions and a moist soil. For hotter climates, container-grown plants are best grown indoors and moved outside when weather turns cooler (but only on frost-free days).
Two species of hydrangea flower on current growth: Annabelle and paniculata. Most gardeners have the bigleaf hydrangea, or H. macrophylla, which can be severely affected by either a late season freeze or pruning. Where soil does freeze, mulch should be applied right after the soil freezes. Any mulch material can be used to protect the hydrangea, such as dried grass clippings, straw, evergreen boughs or tree leaves.
Green-Flowering Hydrangeas Generally Temporary
Unless the cultivar is specifically known to produce green flowers, the hydrangea that delights the eye with greenish blooms is usually a temporary phenomenon. It will soon either turn another color or it ends the color cycle with green hues.
In any event, many gardening experts consider the green-hued hydrangea flowers as yet another example of Mother Nature’s quixotic sense of humor and exquisite taste.
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As I said, I know central Illinois pretty well and recognized the remnants of coal mining. That is wonderful about your family preserving and creating such a wonderful spot. It preserves the history of the area, too. I remember driving between St. Louis and Chicago and seeing the huge mounds north of Bloomington. 
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