They all benefit from the occasional foliar feed with a high potash fertiliser such as Phostrogen or Tomorite, to encourage the formation of flower buds. Hydrangeas, with the exception of the paniculata varieties are better not to be pruned unless the branches get overcrowded or untidy, in which case they should be taken out at the base in late spring after the danger of frost has passed. They should never be pruned in autumn, and the old flowerheads should be left on to protect the plants in the winter. They will grow better with feeding and mulching and regular watering in dry weather. Paniculatas, except Praecox, should be spring pruned and mulched with compost or well rotted manure.
Blue flowers are only possible on acid soil, on plants that carry blue flowers.
All young blue flowered plants carry pink flowers, and it may take between one and three years for them to turn blue. If the plant is moved, even in the same garden, the flowers will be pink and then turn blue again with time.