The bigleaf hydrangeas, also called French, mophead or snowball hydrangeas, are the most popular and well known of the hydrangeas grown in our area. They bloom profusely during the early summer months with pink to dark blue globular clusters of flowers and have thick medium to dark green leaves.
The plants are attractive in the landscape and can be used as a specimen, group of plants or in containers. They are also used in flower arrangements. More than 500 varieties are available in the nursery trade, and they can be divided into two types. The hortensia hydrangeas have large snowball like blooms. The flat lacecaps have their blooms in an outer ring of large flowers and a mass of tiny fertile ones on the inside.
Bigleaf hydrangeas are deciduous shrubs and have a rapid growth rate reaching several feet in height. They should be planted in a partly shaded area. The plants prefer a couple of hours of early morning sun, but are unable to tolerate hot dry sites that receive the full afternoon sun. However, avoid deep shade since they require at least three to four hours of morning sunlight daily. Excessive shade will reduce their production of flowers.
Hydrangeas do not perform well in our heavy clay soils. Add 50 pounds per 10 square feet of organic matter such as peat moss, topsoil or compost to the soil in the area where they are to be planted. This will loosen the clay soil by helping to improve aeration and drainage as well as helping retain nutrients and water.
Dig the hole the depth of the rootball and at least twice as wide. When planting, make sure the crown of the plant is level with the top of the hole and no deeper. Avoid fertilization until the plant is established, around four to eight weeks after transplanting. During the growing season, apply a general purpose fertilizer, such as 10-10-10, in March, May and July.
The bigleaf hydrangea has shallow roots and a high water requirement. It should be planted in moderate water-use landscape zones. Excessively dry conditions will cause them to wilt, so the soil needs to be kept evenly moist. When the flowers begin to turn green or brown in color, remove them. Prune other shoots as necessary to encourage branching and fullness.
Any pruning should be finished by Aug. 1. The new buds will begin forming in late summer for the following year. If the plant has sustained winter injury, prune out the dead or damaged branches. This action will encourage more vegetative growth at the expense of flowers, but will be beneficial for the overall health of the plant. Sometimes the plants fail to bloom. The cause is most likely freeze damage, the application of excessive nitrogen fertilizer or pruning at the wrong time of the year.
The level of aluminum compounds in the plants is the mechanism that determines the flower color. Higher levels of aluminum cause the flowers to be a deep blue. The color is a light blue when only minimal quantities are present. If aluminum is absent, then flowers are pink. Soil pH has an indirect impact on flower color. It determines the level of aluminum available for plant absorption. At a pH of 5.0 or lower, aluminum is more soluble and thus the roots can absorb more of it. At a pH of 7.0 or greater, aluminum levels decrease.
If you want blue flowers, you can acidify the soil by adding a half cup of wettable sulfur per 10 square feet. For pink flowers, apply one cup of dolomitic limestone per 10 square feet to raise the pH. It may take a year or more before color changes are observable.
Bigleaf hydrangeas are definitely a worthy addition to the garden. If given the right growing conditions, the plants will put on a show with their colorful blooms.
For more information, refer to the website of the American Hydrangea Society at www.americanhydrangeasociety.org.
Timothy Daly, is an Agricultural and Natural Resource Extension Agent with Gwinnett County. He can be contacted at 678-377-4010 or firstname.lastname@example.org.