How to Protect Your Hydrangeas From Pests and Disease

The big, bold, beautiful blooms of hydrangeas that we all love come in a rainbow of colors and an array of shapes and sizes—but all are susceptible to damage from common pests and garden diseases. Aphids, scale, Japanese beetles, spider mites, slugs, and snails can all infest your plants, eating the sap and leaves that keep hydrangeas thriving. While gray mold, powdery mildew, root rot, and leaf spot can inhibit photosynthesis and weaken your beloved shrubs.

We spoke with experts to learn how to recognize, treat, and prevent these common hydrangea adversaries—and protect your hydrangeas all season long.

How to Protect Hydrangeas From Pests

Hydrangeas aren't immune to common garden pests. Here's how to protect them from each one.

Aphids

Hydrangea's sweet, sugary sap draws tiny aphids who use it as a food source. "These small insects can cause twisted, stunted, and distorted growth often with yellow leaves," says Andrew Bunting, vice president of horticulture at the Pennsylvania Horticultural Society.

While hydrangeas often outgrow the damage done by aphids on their new growth, you can bypass this problem by choosing a spot with the right amount of light and well-drained—not soggy—soil.

"To get ahead of an aphid infestation, encourage beneficial insects like ladybugs and lacewings, which are natural predators of aphids," says Kelly Funk, president and CEO of Jackson & Perkins. "To treat for aphids, spray affected plants with a strong stream of water to dislodge aphids and apply insecticidal soap or neem oil to the affected areas."

Scale

Scale insects are also inclined to take up residence on your hydrangea to get at the sap, says Bunting. "These insects appear as clusters, dome-shaped or oyster shell-shaped, mainly on the stems of hydrangeas," he says. "Hydrangeas are less likely to get pest problems if they are grown in conditions with proper light—not too shady and not too sunny—and have a well-draining soil that is not overly irrigated. Treatment options include physically removing the scale by hand, pruning the affected area, and applying horticultural oil."

Japanese Beetles

If you find your hydrangea leaves chewed down to skeletons, you're likely dealing with Japanese beetles. "Japanese beetles are attracted to the foliage and flowers of hydrangeas," says Funk. "They skeletonize leaves, leaving only the veins, and can cause extensive defoliation and weaken the plant."

Protect your plants with row covers during the beetles' busiest season—typically late June to early August. "To get rid of them, handpick beetles, drop them into soapy water, and apply neem oil or insecticidal soap to the shrub," says Funk.

Spider Mites

In hot, dry weather, spider mites infest hydrangeas to feed on the sap; look for them on the undersides of the leaves or for the evidence of their feasts: stippling, small yellow spots, or webbing. "Maintaining adequate moisture levels around the plants and increasing humidity by misting the plants during dry spells can help prevent spider mites," says Funk. "To get rid of spider mites that are already present, spray the undersides of leaves with water to remove spider mites and use a miticide or horticultural oil if the infestation is severe."

Slugs and Snails

Damp leaves and mulch surrounding the bottom of your plants create a prime spot for slugs and snails to take up residence. "Slugs and snails are attracted to the moist environment and decaying organic matter around hydrangeas, and they chew irregular holes in leaves and flowers, which can lead to significant damage," says Funk.

Clear debris that collects underneath the plants, and "use copper tape or diatomaceous earth around the base of the plants to deter slugs and snails," says Funk. "To remove slugs and snails, handpick them off in the evening or early morning and use bait traps or iron phosphate-based slug pellets."

How to Protect Hydrangeas from Common Diseases

Pests aren't the only enemy to hydrangeas—these common diseases can afflict them, too.

Powdery Mildew

Powdery mildew is identifiable by the white, powdery spots of growth it deposits on stems, leaves, and buds. The spots inhibit the plant's ability to perform photosynthesis, leading to yellowing and leaf drop. "While hydrangeas need to be irrigated, watering with an irrigation system—especially at night—and not allowing the soil to dry out between waterings will exacerbate the issue," says Bunting.

Prevent powdery mildew with proper spacing, pruning, and watering techniques. "Ensure good air circulation around plants by spacing them appropriately and pruning to reduce overcrowding, avoid overhead watering to keep foliage dry, and plant in a sunny location," says Funk. "If powdery mildew is found, apply fungicides labeled for powdery mildew according to the manufacturer's instructions. If treatment is not successful, remove and destroy infected plant parts."

Cercospora Leaf Spot

The small purple, reddish-brown, and tan spots on your leaves are the result of leaf spot, a fungal disease. Like powdery mildew, the expansion of these spots can inhibit the health of your plant and cause leaves to fall prematurely, says Funk. "Severe infections can cause defoliation, reducing the plant's vigor and flowering potential," she says. "To prevent leaf spot, water plants at the base to avoid wetting the foliage, mulch around the base to reduce soil splash onto leaves, and clean up and dispose of fallen leaves and debris around the plant. To treat an infected hydrangea, improve air circulation by proper spacing and pruning, remove and destroy infected leaves, and apply fungicides containing chlorothalonil or thiophanate-methyl if the disease is severe."

Botrytis Blight 

Botrytis blight is a fungal disease that presents as gray mold on leaves, buds, and flowers, causing them to die and turn brown before blooming, says Bunting. "Discolored leaves with spotting is often an indicator, too." Since this disease prefers cool, humid conditions, moving your hydrangea to a drier area is one solution. "Additionally, removing infected plant parts and monitoring irrigation will help to prevent this issue," says Bunting.

Funk recommends watering the base of the plant and implementing proper spacing and pruning techniques to allow airflow between the plants. "For treatment, apply fungicides such as those containing captan or copper-based products according to label directions, remove and destroy infected plant material to reduce spore spread, and keep the garden clean of plant debris," she says.

Root Rot

Hydrangeas grown in over-wet soil are susceptible to root rot. "Root rot causes wilting, yellowing leaves, stunted growth, and brown or black rotting roots, and damages the root system, leading to poor water and nutrient uptake, and can be fatal if not addressed," says Funk. To prevent root rot, plant your hydrangeas in their preferred soil: one that's rich in organic matter and well-draining, says Bunting.

"If the hydrangea shrubs are affected by root rot, improve soil drainage by amending with organic matter, apply fungicides labeled for Phytophthora root rot to the soil, following the manufacturer's instructions, and remove and destroy severely affected plants," says Funk.

Tips For Protecting Your Hydrangeas

Treating your hydrangeas with the proper care techniques and planting them in a spot where they can thrive allows you to prevent damage before it happens. "Essentially, anything that puts the hydrangeas under stress will make them more prone to pest and disease issues," says Bunting.

Water Carefully

"Avoid overhead watering, as wet foliage can promote fungal diseases," says Funk. "Instead, water the soil directly at the base of the plant. Keep the soil consistently moist but well-drained. Hydrangeas do not like to dry out completely, nor do they like to sit in waterlogged soil," Funk.

Encourage Airflow

Spacing the plants apart to encourage airflow lowers the humidity around the plant, decreasing the moisture that can support the development and spread of disease. (Choose a spot with "part shade or dapple shade," says Bunting.) "Be sure to promptly prune dead or diseased branches and thin out dense growth," says Funk. "This improves airflow and light penetration, reducing the likelihood of disease."

Boost the Soil

Choose a planting spot with well-draining soil and nutrient-rich soil. "Amending the soil with organic matter can improve drainage and soil structure," says Funk. "Apply a layer of mulch around the base of the plants to help retain moisture, regulate soil temperature, and reduce weeds. Mulch also prevents soil splash, which can spread disease."

Compost and a balanced fertilizer can provide necessary nutrients throughout the season. "Incorporate organic matter, such as compost, to improve soil health and provide a slow-release source of nutrients," says Funk. "Excessive nitrogen can promote weak, disease-prone growth."

Watch Closely

Check your plants regularly to ensure they are getting the right water and nutrition and to catch any pests or diseases before they take hold. "Frequently inspecting the plants for signs of pests and diseases can help with early detection, allowing for prompt action, reducing the severity of infestations or infections," says Funk. "Remove and dispose of any diseased or infested plant parts immediately to prevent the spread of pathogens or pests to healthy parts of the plant."