Joyce, Tardiva is the latest to bloom of all paniculatas and in your zone you'll have a much shorter season than in higher zones.
If there is a possibility to buy a different cultivar or exchange yours if you already bought it, to say 'Kuyshu' (similarly looking, but much earlier blooming), you'll have a much more bangs for your bucks.
Sunwise, it need to be watered regularly in its first year in a ground, but after that I think in z5 it would need only ocassional watering during dry spells.

I would also recommend another cultivar. While Tardiva is beautiful, it is so late in Z5, you don't get to enjoy it as long as others. Kyushu, as mentioned above is a very similar alternative, but earlier blooming. Also, if water was a concern to me I would plant it in a little shade. This will help keep it from drying out as quickly, thus using less water, and mulch well. Any new hydrangea will need to be well watered until established, but after that paniculatas are quite drought tolerant. That's not to say you don't get bigger and more flowers if you water and fertilize regularly.

My Tardiva in zone 5 is blooming nice now, the flowers are just started to pinkin. It has been blooming for 3 weeks or so, it is next to my Limelight which started 3 weks before it. It is in dappled shade, but I do water it when there is no rain in summer. Good soil and some shade should lighten your watering some.

Mine has been blooming for 3 weeks also, I like the late bloom.