Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
2/6/03
FOR IMMEDIATE RELEASE
Contacts:
Linda Naeve, Reiman Gardens, (515) 294-2710, lnaeve@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Bigleaf Hydrangeas Present Magical Color
By Linda Naeve
Extension Coordinator, Reiman Gardens
A lot of magic and "hocus pocus" can be seen in several movies currently showing at the theaters. Some of the magic is performed by a simple wave of the wand, while other tricks require some knowledge of chemical combinations. The use of computerized special effects in the movies makes the magic seem very real. Gardeners and greenhouse growers use their knowledge about plant nutrition and chemistry to perform a real trick, not an illusion, that can change the flower color in this week's Reiman's Pick - bigleaf hydrangea, Hydrangea macrophylla.
The bigleaf hydrangea is sometimes referred to as a French hydrangea even though it is native to Japan and Korea. Although it is a shrub that will grow to up to six feet tall, it is a hydrangea species that Iowans see more often in florists' shops than in the garden. Bigleaf hydrangea is hardy from zones 6 to 9 and produces flowers on the previous season's growth. Unfortunately, in our climate, most bigleaf hydrangea plants may survive a mild winter, but cold temperatures will kill the flower buds. An exception is a new cultivar, 'All Summer Beauty,' which has proven to be hardy at Reiman Gardens.
As its name implies, the bigleaf hydrangea has large, broad leaves that are 4 to 6 inches long. The flower heads are also so large they appear out of proportion to the size of the potted plant. There are two types of bigleaf hydrangea cultivars that are grouped according to their flower form. The "hortensias" have sterile flowers in large, rounded heads and the "lacecaps" have flat flower clusters. The lacecaps flower head consists of showy sterile flowers in a ring around a center of fertile, bead-like flowers, giving it a lacy appearance.
In most plants, the color of the bloom is determined genetically and can't be changed. However, the color of the bigleaf hydrangea flowers can be altered, almost magically, using a bit of chemistry. When grown in an acidic soil with a pH between 5.2 and 5.5, the flowers are blue. In an alkaline soil, they are pink. Researchers have determined that the actual cause of color variation is due to the presence or absence of aluminum compounds in the flowers. When aluminum is present within the plant, the flower color is blue. When it is absent, the flowers are pink.
Aluminum is a plant nutrient that is only available to plants when the soil pH is low and it becomes unavailable at higher soil pH levels. Greenhouse growers apply a specific rate of aluminum sulfate to the soil to lower the pH and produce blue flowers. Adding lime to the soil to increase the pH produces pink flowers. Growers may also apply high levels of phosphorus fertilizer to slightly acidic soil. The phosphorus works to prevent or limit the plant's uptake of aluminum. If the soil pH is somewhere in between, the blossoms may be dull green or purple in color. Strangely, the bloom color of some white cultivars of Hydrangea macrophylla cannot be changed by modifying the soil pH.
Hydrangeas are great flowering houseplants and make excellent gifts. The flowers last a long time in the home when kept in a bright location and away from heat sources. Check the soil often because a potted, flowering hydrangea will require frequent watering. This may be due to pot-bound roots or a very light, well-drained soil mix. A large plant may need to be watered every day or two.
Hydrangeas are often purchased from florists with decorative foil or plastic pot covers. Although it dresses up the container, decorative covers should be removed. If the pot covers are left on, the excess water that drains through the container will collect in the liner and may cause root rot.
Consider a potted hydrangea an extended flower arrangement. Once it is through blooming, discard it or place it in the compost pile.
You can see beautiful white, lacecap 'Lybella' hydrangeas blooming in the white winter display in the Conservatory at Reiman Gardens. The beauty is truly nature's magic.
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Editors: Two color photos, suitable for publication, are available at right. Click on each thumbnail photo to go to the fullsized photo. The top picture's fullsize photo is 224K and the bottom picture's fullsize photo is 288K. Caption: Bigleaf hydrangea (Hydrangea macrophylla) 1. |
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| Caption: Bigleaf hydrangea (Hydrangea macrophylla) 2. |  |
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