Answer: Viburnums do best in full sunshine to partial shade. It sounds as though yours might not be getting as much sunshine as required. Too much shade can make them prone to mildew. Inspect the leaves to make sure no disease is present, and be sure to water regularly throughout the growing season. (A deep soaking once each week if your soil is well-draining, less often if soil holds lots of moisture). Other than providing additional sunshine, I can't think of anything else your viburnums might need to help them thrive.
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