Hydrangea Scale Infestation: White Clusters on Leaf Undersides

Identifying the Pest

Tiny white clusters on hydrangea leaf undersides—especially if stationary and slightly raised—are scale insects (likely cottony or soft scale). They suck sap, weakening the plant and excreting honeydew that attracts ants and sooty mold.

Systemic Solution

Imidacloprid soil drench (Bayer Advanced Tree & Shrub) provides season-long control. Mix per label for shrub size, pour at base. Translocation takes 7–14 days; protection lasts 12 months. Treat all nearby hydrangeas preventively.

Supplemental Measures

• Scrub accessible scales off with soft brush and soapy water.
• Apply horticultural oil in dormant season for overwintering stages.
• Monitor new growth for crawlers (mobile nymphs) in spring.

Early systemic treatment prevents spread to other plants.