Pruning Nikko Blue Hydrangeas
Your Nikko Blue should bloom just fine without pruning. If you must prune to control size, cut out the longest shoots to the base immediately after bloomtime. Leave the others untouched. It sounds as if the plants were hosts to aphids. These secrete sweet honeydew, which then attracts spores of sooty mold, which causes black spots on the leaves. In spring keep an eye out for aphids. If you see any give them a sharp shot of water from the hose to dislodge them. Be diligent about spraying the undersides of the leaves too. They will not kill the hydrangeas but are unsightly as are the black spots.
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