I'm confused - is your tree stump still alive and growing? Or is it actually dead - there's conflicting info in your question above.
If its still growing, then treat it - I have a short blog on tree stump treatments - click on my avatar and select blogs on my home page, then click on that one. Tells you exactly how to kill off the stump.
On the other hand, if the stump is long dead and not showing signs of life, it won't be using any water at all.

Depending on what it is the stump of you will possibly have a lot of roots in the soil which will be impoverished and need some good organic matter dug in before you plant the hydrangeas