Question: We have a border of six-foot blue-flowered hydrangeas that bloomed three years ago and again this past summer (but not in between). To diddle with the flower colors, I fed each one a tablespoonful of ground limestone. Disaster! The flowers were mottled green and brownish, and looked like dried bouquets. Should I cut back the bushes? Should I feed them? What must one do to have glorious hydrangeas?
Answer: Assuming that your hydrangeas are bigleaf or florist hydrangeas (H. macrphylla, sometimes called H. hortensis), the biggest problem with flowering is winter temperatures that are too low. The flower buds, formed in the fall, are inured at 27 degrees Fahrenheit and killed at 20 degrees. Such temperatures do occur occasionally in your area. If you cover the plants with burlap in late fall or early winter you can offer some protection.
Improper and untimely pruning also will cause flower failure if the buds are accidentally removed. Shape the hydrangea border when the flowers have faded. You may prune lightly in early spring, but cutting back too severely will again remove the buds.
It is doubtful that one tablespoon of limestone per plant had any effect on the flower color. But in the future, have the soil tested for pH and texture before trying to change the color of you hydrangeas’ flowers. These plants grow best in slightly acid soil (pH 5.5 to 6.5). To get pink flowers you must raise the pH to about 7.0. Begin the treatment the year before the color change is desired. The amount of limestone you have to add depends upon the existing soil pH and type. For instance, a sandy loam takes four pounds of agricultural limestone per 100 square feet of soil to raise the pH by one point (6.0 to 7.0); a silty loam takes seven pounds, a silty clay loam 10 pounds.
Hydrangeas are apt to develop chlorosis in soil with a higher pH. More nitrogen (a spring feeding with a 15-10-15) is needed for the production and maintenance of good green foliage. Bear in mind, though, that high nitrogen interferes with a good blue flower color; keep this feed away from those which are to remain blue (use a 5-10-10). As for mottled green or brownish flowers (probably the dried sepals of the inflorescences), these may be cut off at any time.