Hydrangea macrophylla is a deciduous shrub native to Japan, bearing large mophead or flat lacecap flower clusters in pink, blue, or purple. It is one of the most widely planted flowering shrubs in temperate gardens and reliably blooms from midsummer into fall. Flower color is directly tied to soil pH: acidic conditions below 6.0 make aluminum ions available to the plant, producing blue pigmentation; alkaline conditions above 7.0 result in pink. White-flowered cultivars do not change color regardless of pH.
Most bigleaf hydrangeas bloom on old wood — flower buds are set in late summer and fall on canes that must survive winter intact. Late pruning or hard freezes destroy those buds and eliminate the following season's flowers. Reblooming cultivars such as 'Endless Summer' set buds on both old and new wood, improving flowering reliability in zones 5–6. In colder zones, mounding mulch around the base after hard frost protects low canes and increases bud survival.
Within the broader Hydrangea genus, roots have been used in traditional Chinese medicine and by Cherokee practitioners as a diuretic and for kidney and bladder complaints. H. macrophylla contains hydrangin, a cyanogenic glycoside; all plant parts are considered mildly toxic if ingested and should be kept away from children and pets.
Powdery mildew
White or grey powdery coating on leaves — usually starting on older growth in humid conditions or when nights cool.
Improve air circulation by thinning plants. Apply neem oil or potassium bicarbonate spray at first sign. Avoid overhead watering.
Botrytis (grey mould)
Grey fuzzy mould on petals and stems, worst in cool wet conditions.
Remove affected parts immediately. Improve air circulation. Avoid overhead watering. Apply copper fungicide if severe.
Cercospora Leaf Spot
Small brown or purple spots on lower leaves that enlarge and turn tan in the center; heavy infection causes yellowing and leaf drop.
Remove and discard affected leaves. Avoid overhead watering and water at the base. Improve air circulation and apply a fungicide if spread is severe.
Cercospora Leaf Spot
Small round spots with tan or gray centers and reddish-purple borders on leaves. Spots merge in severe cases, causing leaves to brown and die back.
Remove infected leaves and avoid overhead watering. Space plants for airflow and rotate crops. Apply a copper-based fungicide if the infection spreads.
Spider Mites
Fine stippling or bronzing on leaves, faint webbing on undersides, foliage drying out during hot, dry weather.
Spray foliage with a strong jet of water to dislodge mites. Apply insecticidal soap or horticultural oil to leaf undersides; repeat every 5–7 days as needed.
Japanese Beetle
Metallic green-and-bronze beetles feeding on leaves and flowers, leaving lacy, skeletonized foliage and chewed petals.
Handpick beetles in early morning and drop into soapy water. Avoid pheromone traps, which attract more beetles. Treat soil for grubs if infestations recur yearly.
Aphids
Clusters of small soft insects on new growth and flower buds.
Knock off with a strong jet of water. Ladybirds and lacewings are natural predators. Insecticidal soap as last resort.
Phytophthora Root Rot
Wilting despite moist soil, yellowing leaves, stunted growth, and dark, decayed roots and crown tissue. Vines may collapse during hot weather.
Plant in well-draining soil and avoid waterlogging. Remove and destroy affected vines. Improve drainage with raised beds or amended soil and avoid overwatering.
Slug and Snail Damage
Large irregular holes chewed in leaves, with slime trails on foliage and soil.
Hand-pick at night, set beer traps, or apply iron phosphate bait. Remove debris and mulch where they hide.
Hydrangea Ringspot Virus
Irregular pale yellow or white rings, line patterns, and chlorotic spots on leaves. Affected leaves may pucker or distort. Symptoms intensify in warm weather and can resemble nutrient deficiency.
No chemical cure exists. Remove and destroy infected plants promptly. Disinfect pruning tools between cuts with a 10% bleach or 70% isopropyl alcohol solution. Control aphids, which spread the virus mechanically. Purchase only certified virus-free nursery stock.