Posted: 03/09/2014 at 15:17
It dépends on what kind of hydrangea they are. If they are the usual mophead/lace cap types they will produce next year's flowers on old wood so you need to remove the dead flower heads once spent but taking care not to spoil any bud forming behind them. Flower colour will depend a lot on teh type of soil you have - blue for acid soil and pink for alkaline. There are feeds you can buy in garden centres to enourage the blue flowers. Follow the instructions on the pack.
If they are the paniculata forms, they flower on new wood so can be left now as they are and then pruned back to a pair of buds on each stem next spring. How high or low these buds are will depend on teh severity of the winter which may kill off the ends of stems and also your choice as to teh eventual size you want the shrub to attain each year.
There is also a quercifolia form but you'll have to Google those or wait for someone else to reply.
Either type may appreciate a feed of bonemeal forked in around their roots in autumn as this helps root formation over winter when the rest of the plant is dormant. Both types wil appreciate a generous mulch in autumn and again in spring with a dollop of either pelleted chicken manure or blod, fish and bone. For extra flower power you can also give them a liquid feed of rose or tomato food.