You will eventually need a large pot, yes, very large indeed, but not initially - its always best to use a pot just a couple of sizes bigger than the rootball of the plant as it exists, then pot on as it gets larger.

Second, sinking pots into the ground does not mean the roots will stay in the pot - they usually escape out of the bottom and spread into the soil beneath. If you block up the drainage holes so that doesn't happen, then the plant will likely drown or die from 'souring' of the compost because there isn't free drainage.

It's probably best to think of another plant that won't mind not having acidic soil conditions and plant it straight into the ground.

We have four different hydrangeas, all in large pots, and they do very well year after year.We haven't sunk them into the ground, just stood them wherever we need them.Each year we take a few inches out of the pots and top them up, then feed In Spring, and they always look magnificent.

Hydrangeas do tend to do well in pots and can stay there for many years. The treatment suggested above is a good idea.