www.uky.edu/hort
May - July
2018
Hydrangeas
– Blue or Pink? You Decide. By: Kelly Jackson
Hydrangeas grace the landscape with beautiful flowers in the spring and summer. The most colorful hydrangeas are bigleaf hydrangeas, Hydrangea macrophylla. Their flowers are usually either pink or blue. Flower color depends on the pH of the soil, a measure of soil acidity. Soil pH can be raised by applying lime. Some hydrangeas will respond to a higher pH (between 6.0 and 6.5) with pink flower color. To lower pH, apply aluminum sulfate. A lower pH (between 5.0 and 5.0) often results in blue flower color. A soil test will determine the existing pH and you can change your soil with the appropriate amendment to get the resulting flower color you want. Hydrangeas grow best in moist, well-drained soil and partial shade. When planted in full sun in our summer heat, they have a tendency to wilt. The north side of a house or a location where the plant receives afternoon shade is best. There are two flower forms of bigleaf hydrangea mophead and lacecap. Mophead types make a full, large, round ball of sepals. The showy parts of the flower are the sepals, which surround the flower bud. The lacecap type has flower buds surrounded by showy sepals along the outside edge of the flower head. The inside of the flower head has buds but lacks the showy sepals. The type of flower head a plant produces depends on the cultivar. Most bigleaf hydrangeas bloom on previous season’s growth. Part of last year’s growth may be injured by cold weather during winter. As buds begin to show green in the spring, you will be able to determine how much wood is still living. The dead wood should be pruned out. Stems can be pruned lower as long as there are still some live buds below the pruning cut. Flower buds are more sensitive to cold temperatures than leaf buds. Until your plant begins to flower, it will be hard to know if all the flower buds are living. Recently new cultivars called re-blooming hydrangeas have been released with the advantage of blooming on old
wood and new growth. Endless Summer® (Hydrangea macrophylla ‘Bailmer’) (released in 1998) is a true re-blooming hydrangea. It has a mophead flower type up to 8 inches in diameter with pink or blue color, depending on pH. The plant is 3 to 4 feet tall. Another new hydrangea is ‘Lady in Red’. This cultivar has three seasons of color. The stems and leaf veins are red. In the fall, the foliage turns a reddish-purple. It’s lacecap flowers open pinkish white or bluish white, depending on pH. As they mature, they become deep burgundy rose. This plant is compact forming a 2 foot wide and 3 foot high mound. A popular cultivar for blue flowers is ‘Nikko Blue’. This plant grows 5 to 6 feet in height and spread. Although its mophead flowers will be blue in lower soil pH, they still become pink if pH is higher. ‘Blue Wave’, a lacecap flower type with lilac to pale blue sepals in low pH, also has sepal-less fertile dark blue flower buds in the center of the head. This plant reaches 4 to 6 feet tall and wide. Remember that most big leaf hydrangeas bloom on old wood. Pruning should be delayed until new growth appears in the spring. However, as we discovered this year, sometimes the worst injury can occur during a late frost in March or April. If your hydrangea doesn’t bloom, its sensitive flower buds were likely killed by cold temperatures.
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