Royal Purple Smokebush should not be overwatered once established. The species is intolerant of flooding and will do better from mulch rather than excessive irrigation.
As this shrub flowers on new growth, pruning should be completed during the dormant season (winter or spring).
This shrub can be maintained in two ways depending on your goals. If you prefer the deep purple foliage and do not care for the flowers and plumes, you may consider pruning Royal Purple Smokebush to the ground each spring, allowing new growth to shoot upwards with healthy, vigorous leaves. This heavy pruning can be done after the plant has established (typically 3-5 years in the landscape). However, this pruning method will cause reduced flowering as the shrub will be more focused on vegetative growth.
Alternatively, if you prefer the look of the flowers and feathery plumes, and are not concerned with keeping the plant smaller, you can use rejuvenation pruning to keep new growth healthy while maintaining the large form the plant.