Once established, Sibs can tolerate drier conditions, but they will always perform best where they get ample moisture (esp. in the spring up through bloom). They like full sun, acidic soil, high soil fertility, and consistent soil moisture. A thick mulch of pine needles will help to both retain soil moisture and acidify the soil pH. If you want Sibs to flourish, give them sun AND plenty of water.

Laurie

I've got mine in the absolutely wrong conditions and they are very happy. My soil is slightly alkaline clay that becomes dry as adobe bricks come August. I just re-read what Laurie wrote. Now that I think about it, where I live the spring is rainy so they must not care as much about the dryness during summer if there is ample moisture going up to bloom time.
Remy

We have actually made a "feature" of our drainage ditch with several kinds of water irises (blues and yellows) and some marsh marigolds, daylilies along the edge and also Joe Pye weed and those really tall impatiens. It looks great! In the spring, ducks drop by and we also have a muskrat that visits sometimes. The frogs are singing their hearts out at night - this time of year.

Mine are very invasive in sun and part shade and they bloom well without any additional watering.