Hi, Evelyn, Down here in Cornwall we grow lots of Hydrangeas very successfully, you don't say what type or colour it is but if you want blue flowers, then add alluminium sulphate to the soil, this can also be done by burying anything alluminium in the soil in the tub, a very good source is to add lots of alluminium roofing felt nails, which you can buy very cheaply. If you want bright pink flowers then make the soil alkaline, this can be done by adding calcium in any form, ie. beach sand, egg shells, cushed oyster shell,
limestone chippings etc. Also remember that if you prune in the winter, you will be removing the flowers for the comming year as they are already present in the terminal buds, the best way to prune is to remove 1/5th of the bush down to six inches (so in effect the whole bush is pruned every 5 years)
Best wishes Lannerman.