It’s better to water deeply, and less often, than giving plants a light watering each day. Deep watering encourages plants to grow deep roots that are better able to withstand periods of drought. The best method is to use a soaker hose, which places water right at soil level and minimizes the water wasted through evaporation. The idea is to soak the soil down to the depth of the root ball. So, if your perennials have 6-inch deep roots, try to water the plants so the soil 6 inches down is moist. The best way to test the depth the water is reaching is to simply dig a hole with a trowel—without destroying your new plants’ roots, of course.
In hot, dry weather, you might need to water two or three times a week until plants settle in. Then, once a week usually suffices—unless nature provides.