General Information: There are at least 80 different species of Hydrangeas but only five or six are commonly grown or available. The most frequently seen is the Bigleaf or Florists Hydrangea (Hydrangea macrophylla). H. macrophylla has the huge blue, pink or white flower clusters and is further subdivided into two subgroups: the mopheads and lacecaps, both of which can reach a size of 8 feet wide by 8 feet tall. The mopheads have 50 to 200 individual flowers (all sterile) which make up the flower cluster. The lacecaps have a ring of showy sterile flowers similar to mopheads and then a mass of tiny fertile flowers within the ring.
The colors of H. macrophylla are genetically determined except for the new “Endless Summer” series. The color of a blue H. macrophylla can be intensified by treating the soil with aluminum sulfate. The aluminum imparts the blue color and the sulfur acidifies the soil so that the aluminum can be absorbed by the plant. The soil in the Bay Area and our water is very alkaline (unacidic) and so our blue Hydrangeas tend to lose their blue color and drift toward a greenish white color unless they are treated with aluminum sulfate. The aluminum sulfate must be applied before the flower buds are set so about four applications are necessary; one each in November, December, January and February. You will need to apply between one-half cup to one cup per plant depending on the nature of your soil. Probably start low and see how it goes. Aluminum sulfate is not a fertilizer so you must maintain your regular fertilizing schedule.
Pink flowering Hydrangeas will remain pink in Bay Area soil because they need an alkaline environment which our soil and water provide. To intensify the pink color and bring it to an almost red color, the soil must be made more alkaline. Alkalinity can be increased by adding agricultural lime to the soil. Oyster shell lime is first choice and Dolomitic lime is second choice. Aluminum is not a factor in the pink color. Apply the lime on the same schedule and in the same amounts as the aluminum sulfate. You cannot change a genetically pink flowering plant to blue by applying aluminum sulfate or blue to pink with lime.
However, two recently introduced varieties of H. macrophylla, “Endless Summer” and “Penny Mac,” can have their colors shifted from pink to blue or blue to pink by applying aluminum sulfate or agricultural lime as described above. These are the only exceptions. Obviously, you should not plant pink flowering and blue flowering Hydrangeas in the same bed.
Planting: Hydrangeas prefer a soil rich in organic matter which drains well. Add a couple of inches of Gold Rush, Bumper Crop or redwood compost plus gypsum (10 pounds per 100 square feet) and sulfate of ammonia (one-half pound per 100 square feet) to a new bed and dig it in or roto-till to a depth of 6 to 8 inches. For individual holes, use two-thirds soil and one-third organic matter as back-fill. Make the hole two times as wide and one inch less deep than the soil in the container.
Fertilizing: For white and blue flowering plants, fertilize with an acid Camellia, Azalea fertilizer. For pink to red flowering plants, use Rose and Flower fertilizer. Apply fertilizers twice a year about Easter and the 4th of July.
Watering: Water the plants the same as for most other shrubs; keep the soil moist but not soggy. Let the top 2 to 3 inches dry between watering. On hot days, your mopheads may wilt during the afternoon but they will perk up in the evening when it cools off.
Light: To bloom well, Hydrangeas need a moderate amount of sun. If they are growing under thinly foliaged trees and gets filtered sun all day that is desirable. Otherwise, three to four hours of morning or late afternoon sun will be sufficient. Full sun between 11AM to 3PM will cook your plants and produce burned leaves. Good ‘bright light’ will not work. Remember, ‘no sun, no flowers.’
