Failure to Bloom
Frost or Pruning - Because many hydrangeas are florist plants, bred for holiday gifts and not entirely cold hardy, their stem tips where blossoms appear are often nipped by frost when they are planted outdoors in the yard. Blossoms are forestalled although the shrub is healthy and vigorous. Also, winter pruning of the stem tips where blossoms will appear removes buds, so no blossoms appear. For more details click on button to left of this screen "More Details On Bloom Failure".
But there are other reasons for lack of bloom including lack of sun, pruning too late and excessive pruning.
Leaves Skeletonized, Flowers Damaged
Rose Chafers - Rose chafers are grayish or fawn-colored beetles about 1/2 inch long. These slow-moving insects chew holes in leaves and damage flowers, especially white ones, by feeding on the petals and soiling them with excrement. Control them by hand-picking. For major infestations, spray beetles with a pyrethrum spray.
Leaves Curled And Distorted
Aphids - Aphids are soft-bodied, pear-shaped sucking insects, about the size of the head of a pin. They cluster on tender new shoots and leaves, sucking plant sap. They retard and distort plant growth. Under their attack hydrangea leaves may turn yellow or brown, wilt under bright sunlight, or sometimes curl and pucker. Check leaf undersides for small groups of the pests. Hit them with a forceful spray of water 3 times, once every other day, in the early morning. If that does not work, spray them with insecticidal soap every 2 to 3 days until they are gone. A neem insecticide sprayed on shrub foliage 2 or 3 times at 7 to 10 day intervals will also control aphids.
For more information see the file Controlling Aphids
Leaves Webbed Together
Leaftiers - Hydrangea leaftier larvae are green caterpillars, 1/2 inch long, with dark brown heads. These caterpillars protect themselves while feeding by binding adjacent leaves together. Hydrangea foliage becomes ragged and unsightly, turns brown and dies. For minor infestations, crush the larvae in their rolled hideouts. To counter major attacks, spray the foliage of vulnerable shrubs with Bt (Bacillus thuringiensis) just as the caterpillars begin feeding. Repeat every week or 10 days while the caterpillars are feeding. They will ingest the bacteria as they feed and die in a day or two.
Leaf Margins Burned
Mites - Mites are about 1/50 inch long, barely visible to the unaided eye. They may be yellow, green, red or brown. Two-spotted spider mites cause damage on hydrangeas resembling sunscald. The leaves look burned, especially along their edges. Start control measures as soon as you notice evidence of burning on the leaves or delicate webbing near leaf stems. Spray the shrubs in the early morning with a forceful water spray to knock the mites from the leaf undersides. Repeat the water spray daily for 3 days. If that does not do the job, spray the mites with insecticidal soap every 3 to 5 days for two weeks. Spraying the bare canes with dormant (heavy) oil spray in early spring before leaves emerge destroys many over-wintering mites. For more information see the file on Controlling Mites
Small, Round Bumps On Twigs
Scale Insects - Oystershell scale sometimes attack hydrangeas. They usually appear on the upper ends of the stems. Scale insects are covered by rounded waxy shells, which protect them while they feed. The shells may be white, yellow, or brown to black, and are about 1/10 inch in diameter. Spray infested shrubs with light, or "superior" horticultural oil which will coat the scale and smother them. For more information see the file on Controlling Scale
Leaf Undersides Covered With White Powder
Powdery Mildew - Powdery mildew is caused by a fungus that covers leaf undersides with white mold. The upper surfaces of the infected leaves may stay green or turn purplish brown in color. Buds and new growth may also be attacked. Spray affected shrubs thoroughly with a wettable sulfur fungicide once or twice at weekly intervals starting as soon as the whitish coating appears. Collect and discard all aboveground refuse in the fall to prevent the fungus from overwintering on yard debris. Mulch shrubs to prevent rain splashing fungal spores up onto leaves and completely change the mulch each spring. Recent research suggests that spraying foliage vulnerable to mildews with anti-transpirant spray helps it resist infection, as the spores have difficulty adhering to the coated leaves. For more information see the file on Controlling Fungal Disease
Leaves Spotted With Powdery Brown Pustules
Rust - A rust disease caused by a fungus attacks certain hydrangea varieties. The leaves become brittle and spotted with many yellowish to rusty brown pustules, especially on the undersides. The disease overwinters on old plant parts. Prune out and destroy any affected branches and spray with wettable sulfur at weekly intervals until the symptoms disappear. For more information see the file on Controlling Fungal Disease