Resembling the Hydra, a multi-headed aquatic monster, or a simple water vessel (hydor-angeon in ancient Greek), the name “hydrangea” originates from the shape of its seedpod. However, it’s not the seeds that make this plant a desirable garden shrub. It’s the large, rounded clusters of long-blooming flowers that gardeners value. While it is typically a hardy plant, with several pest-resistant varieties, hydrangea does have its share of pest bugs.
Native to Japan, there are about 23 hydrangea species, five of which are common in the United States. Depending on the species and cultivar, they thrive in Sunset Climate Zones 7 to 9 and 14 to 17, in full sun to partial shade. Hydrangea is a deciduous shrub that grows easily in containers and as vines. With some varieties, aluminum content and soil pH determine the floral color. Acidic soil with available aluminum produces blue flowers while soil with a higher pH produces pink flowers.
Tiny and often wingless, aphids are pear-shaped insects with two fingerlike protrusions near the tip of their abdomen. They range in color depending on the species and their food source. Aphids feed on hydrangea by sucking the plant juices, causing the leaves to curl and wilt. Several natural predators of aphids usually keep their population from becoming destructive. Where no ladybugs, lacewings or other predators are available, treat the infested plant with a strong spray from the garden hose or wipe the leaves with rubbing alcohol.
Large infestations of scale insects cause yellowing, wilting and dropping of the plant’s leaves. The female cottony hydrangea scale, Pulvinaria hydrangea, is a flattened, oval insect usually covered with white wax. The oystershell scale, Lepidosaphes ulmi, hides under the armor of an oyster-shaped, dark gray cover. Both species are pests of hydrangea. Like aphids, beneficial insects naturally manage scale problems on the plant. Horticultural oil--a plant or petroleum based product--is the preferred treatment for problematic infestations. To avoid injury to the plant, ensure it is well watered and there is no threat of rain before spraying it with horticultural oil. Also avoid using oil during foggy periods or extreme cold or heat (under 32 or over 90 degrees Fahrenheit).
Impossible to see without a magnifying glass, mites are recognized by the damage they cause to plants. Spots on the leaves, yellow or red discoloration and easily visible silk deposits are evidence of a mite problem on hydrangeas. Adequate watering helps prevent infestations as mites typically appear during dry, hot periods. A spray of water or insecticidal soap helps remove these pests from the plant.
The caterpillars of several moth species protect themselves from predators by tying leaves together. They deposit silk on two parts of a leaf and as it dries it pulls the sides together, forming a pocket. The caterpillar hides inside while feeding. Clip rolled or tied leaves from hydrangeas and dispose of the caterpillar far from the plant. Use an insecticide with the active ingredient Bacillus thuringiensis to treat large infestations. This bacterium, when sprayed on the plant and ingested by the caterpillars, causing the pests to stop eating and die.
Your Planting Articles
- University of California Agriculture and Natural Resources: Pests in Gardens and Landscapes -- Hydrangea
- University of Idaho Extension: Hydrangea
- Auburn University: Hydrangea
- University of Georgia Cobb County Extension Service: Hydrangea
- United States National Arboretum: Hydrangea Questions and Answers
- University of California Agriculture and Natural Resources: Pests in Gardens and Landscapes -- Scales
- University of California Agriculture and Natural Resources: Pests in Gardens and Landscapes -- Foliage Feeding Caterpillars
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