The Kentucky Hydrangea

The following Horticulture article also featured in the May/June edition of the Kentucky Horticulture Newsletter.

Growing Hydrangeas in Kentucky

Primarily known as a source of summer color, hydrangeas’ interesting bark and flower heads can also provide winter texture when left untrimmed until spring. Their overall texture is coarse, so they work best in less formal gardens, possibly bordering the property.

Depending on the species, hydrangeas range from three to fifteen feet in height, four to ten feet in width. They grow in full sun to part shade in rich, well-drained, moist soil. Hydrangeas flower June through August — long after most shrubs have finished flowering — in large white, pink, or blue panicles, or clusters. Four species are commonly used in Kentucky landscape plantings.

KY Hydrangea Species – Big Leaf Hydrangea

KY hydrangea macrophylla

Hydrangea macrophylla or Big Leaf Hydrangea is the most widely used hydrangea species in Kentucky. Its large flowers range from white to pink to blue. While white cultivars remain white, pink or blue cultivar color is determined by soil pH and availability of the micro-nutrient aluminum. A soil pH range between 5.0 to 5.5 will generally produce the blue flowers, and a pH of 6 and above inspires pink flowers.

Certain cultivars of Big Leaf Hydrangea are grown as potted plants in a greenhouse to bloom for late winter and spring holidays like Easter. These House Hydrangeas are selected for flower size, ease of flower forcing, but not hardiness. Most house hydrangeas will not produce flowers dependably when planted in Kentucky landscapes without special winter protection. The color of the potted hydrangea’s flowers is pink unless aluminum sulfate is applied to the potting media or landscape soil which lowers pH and turns the flowers blue.

Most cultivars of this species bloom on the previous season’s wood. If temperatures drop too low, the flowers for the next season will be lost. Kentucky is considered zone 6 with possible low temperatures to -10 degrees, and occasionally these low temperatures kill shoots or flower buds, preventing any flowering that year. Breeding efforts to improve blooming characteristics have found several new selections like “Endless Summer” that will bloom on current season’s growth, providing blooms even if flower buds are killed by late spring frosts.

KY Hydrangea Species – Smooth Hydrangea

KY hydrangea-arborenses

Hydrangea arborenses or Smooth Hydrangea is popular for its large, white blooms from June to September every year on new growth. Removing the flowers as they turn brown will encourage a second flush of flowers in August. It grows in full sun to part shade in moist, well-drained soil. Part shade is best in locations where the weather is generally hot and dry. This hydrangea grows three to five feet tall, making it a possible choice in smaller landscape spaces. There are several good cultivar selections, but the most popular is “Annabelle.”

Smooth Hydrangea is a Kentucky native plant.

KY Hydrangea Species – Panicle Hydrangea

KY hydrangea-paniculata

Hydrangea paniculata or Panicle Hydrangea is one of the larger shrubs growing six to ten feet tall and six to ten feet wide depending on the cultivar. This plant will grow best in full sun. Enjoy the white to purplish-pink flowers from June to September. Blooms can be pruned when they turn brown or during the winter. One popular selection of this species is “Limelight” with large, light green flowers that mature to white.

KY Hydrangea Species – Oak Leaf Hydrangea

KY hydrangea-quercifolia

Native to the southeastern United States, Oak Leaf Hydrangea (Hydrangea quercifolia) is known for its large, oak leaf-shaped foliage, is a popular landscape choice for areas with part shade. The white to purplish-pink flowers are four to twelve inches long with three- to four-inch wide panicles. The flowers are abundant and fragrant. In the fall, the foliage turns to shades of red, orange-brown, and purple, adding additional color to the landscape. The Oak Leaf Hydrangea works best in a partly shaded, border planting.

Growing Hydrangeas in Kentucky

To get the most from a planting of these four popular Kentucky hydrangea species, provide a location that will have full sun to part shade. Afternoon shade reduces summer heat and drought stress. Avoid pruning in the fall. Prune after the shrub has bloomed during the summer, giving the shrub time to grow new flower buds for the next season before winter sets in.

A good resource for additional information is Michael A. Dirr’s Hydrangeas for American Gardens, Timber Press (2004).

Written by Michael Boice, Oldham County Horticulture Assistant. Edited by Lauren State, Oldham County Extension Staff Assistant.

Photography by KENPAI, Frank Vincentz, and Anne Norman. Used under the Creative Commons Attribution license.

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