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Both Smooth and Mophead hydrangea have similar needs, well drained soil, regular watering particularly in summer, sun and partial shade. A position with morning sun and afternoon shade/ partial shade is perfect. The Mophead is likely to do better in partial shade but both will flower less without sufficient sun. The Eucalypts will take much of the water and nutrients, the only answer is water and fertilise more than you would plants that are not competing with large trees. Use a general fertiliser in Spring and Autumn. You will be able to tell if the plants are needing more water by the leaves which become flaccid if the soil becomes too dry.