White Cottony Bugs on Hydrangea Stems and Leaves
I love hydrangeas but this year I have white fungal stuff on stems and underside of leaves. I sprayed with oil and then tried white vinegar but maybe my timing was wrong. When should I spray and when do I prune?
From the images you supplied, it looks more like a sap-sucking insects such as cottony hydrangea scale or mealybug - both can be difficult to remove.
How to treat it
Spray the infected plants with horticultural oil, using the winter rate. Be fastidious when applying, paying particular attention to the base of branches and the underside of leaves to ensure you coat the insects well. Repeat the application two weeks later and keep checking the plant and nearby plants for any sign of reinfestation. If the problem persists, talk to your local garden centre about a suitable insecticide.
When to prune your hydrangea
To reduce reinfection next year, when pruning your hydrangeas in late winter/early spring, carefully collect all leaves and branches and remove them from the garden.
Do not prune hydrangeas too early (eg: late autumn/early winter), because if there is a warm spell, already pruned plants will suddenly spout new leaves. This burst of energy means that the plant has less "stored vigour” and therefore new growth and flowering will be considerably reduced.
