Perennials need to be watered well immediately after planting. For about 2 weeks after that, you should check plants daily to be sure the ground around them has adequate moisture. For the next 2 weeks at least, be sure your new perennials receive water approximately every other day. Water, but not too much and not too little. Overwatering probably is the most common cause of death for newly transplanted perennials. WATCH FOR WILTING PLANTS!
Following the initial two weeks perennials should be watered approximately 1″ of water every week (either by rain or you). During dry and/or windy spells you will have to water more frequently. Let the soil and the plants be your guide. By watering thoroughly, deeply, and allow the soil to dry out in between watering you will encourage a strong root system and a healthy plant.
Deadheading improves the look of most plants and many perennials will re-bloom after the spent flowers are removed.
Wait until perennials are well situated in their new environment, at least one year, before attempting to fertilize. After that, if you wish to fertilize your perennials apply a granular (dry) slow-release fertilizer according to package directions.