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 (non-acidic) 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.l. 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.
Pink flowering Hydrangeas will remain pink in Bay Area soil because they need an alkaline environment that 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.’ When all of your plants have been installed, don’t forget to mulch the flower beds. You can use any of the products discussed as amendments or also the tiny or small sized fir bark. Avoid using shredded redwood bark (gorilla hair) for the numerous reasons we have listed in the past. The mulch should not touch the plant stems and should extend past the drip line of the plants.
PESTS AND DISEASES: Hydrangeas have the usual minor animal pests and diseases. We have found aphids, spider mites, scale, mealy bugs, slugs and snails on the plants. Deer also seem to relish the tips and flower buds. Among the few plant diseases are mildew, collar rot, grey mold, rust and ringspot virus. The virus is not fatal and the other diseases can be avoided by proper cultural practices. The usual treatments for the animal pests will suffice.
PRUNING: Most Hydrangeas bloom on “last year’s wood.” Flowers will come from the buds on the stems that grew last summer. Therefore, the plant must produce new, healthy stems each year. The plants should be pruned in January or early February and in somewhat the same manner as roses.
The best approach is to stand back eight to ten feet from the Hydrangea and mentally identify canes one-half to one inch in diameter with no more than one “dogleg” in its length. These are the canes you will keep. Cut off at ground level any dead canes, any old gnarly canes, any canes pencil thickness or smaller, and canes with two or more “dog-legs.”
If there are two reasonably good canes only one or two inches apart, remove the smaller of the two. If you haven’t removed at least one-third of the total growth, do that next. Make all cuts at ground level.
Decide how tall you want your Hydrangeas to become and cut the remaining stems 6 inches to 12 inches shorter than that. Make cuts within one inch above a pair of buds on the stem. Cut all other stems to about the same height above a pair of buds. The larger and plumper the buds, the more generous will be the blooms. If the bushes have been long neglected, they may bloom poorly after this treatment, but will have much new growth for the next year.
Recently, Monrovia growers have introduced a variety of mopheads called Endless Summer, Penny Mac and Blushing Bride will bloom on both last year’s wood and this year’s wood. In other words, they will continue to bloom all summer. As with other flowering shrubs, dead-head the Endless Summer mopheads to encourage continuous reblooming. Cut the spent mopheads back to stems with at least the diameter of a pencil.
CUT AND DRIED FLOWERS: If you are using Hydrangeas for cut flowers, harvest them in early morning or late evening when they have not been exposed to sun for between two and four hours. Water the shrubs well, the day or night before cutting. Drop the stems of the cut flowers immediately into a container of hot water with floral preservative and strip off any leaves which will be under water as soon as possible.
For dried arrangements, follow the same procedure but within an hour or so, remove the flowers and their stems from the container of water and hang them upside-down in a shaded, low light area. The cooler the area the better, but don’t refrigerate. A garage would be a suitable place. Let the plants remain upside-down until the blossoms have dried and they will be ready to use.
PROPAGATION: Most Hydrangeas are propagated from cuttings. Midsummer is usually recommended but Mr. Ed has used pieces from the annual pruning in January and had fairly good results. Tip cuttings at least one-half inch in diameter are best. Use a piece from an unbloomed cane with three or four pairs of buds. Strip off all but the top two leaves and cut those two in half. Dip the base in Rootone and plant in your rooting media (one-half Gardeners Gold potting soil and one-half Perlite) with two pairs of buds underground. Plant four in a four-inch pot with the expectation that one or two will root. Set the pots in a warm, brightly lit area and allow four to six weeks for new growth to begin. Keep the soil barely moist.
The above information has been directed to the care of Hydrangea macrophylla but all of the other Hydrangeas commonly found at Wegman’s Nursery should be treated the same. Only H. macrophylla comes in the various shades of pink or blue. All the other Hydrangeas have white flowers.
Yes, Hydrangeas can be grown in containers, but you will need at least a 15 gallon size. Remember that all deciduous container shrubs need to be fertilized monthly from Valentine’s Day to Halloween.