With these plants, mulching is important, especially if they are planted where they will get some direct sun. Bark chips or any other good moisture-retaining top mulch will be very helpful in maintaining the constantly-moist ground they prefer.
They need no more than the usual feeding, like most flowering perennials. For feeding perennial gardens, my favorite perennial expert, Frederick McGourty, recommends familiarizing yourself with standard perennial flower fertilizers, which are always labeled with three numbers in a sequence, such as 5-10-5 or 5-10-10, or 10-10-10. These numbers on the bag refer to percentages of Nitrogen, Phosphorus and Potassium, in that order. McGourty explains that the most important element here for flowering plants is the middle number, Phosphorus, so he recommends choosing a fertilizer with the middle number no lower than the other two.
If your garden is new, try to rake in your fertilizer a week or two before planting. And in later years, one feeding in spring (remove the mulch, and rake it in) should do. If you use dry granular fertilizer, remember to scatter it when the soil is wet and the foliage of your plants is dry. Dry fertilizer sticking to stems and leaves can burn the plants.
If you're new to the neighborhood, it's always good to give your local county extension agent a call, and discuss soils and feeding with him or her. That's what they're there for. It's great local advice, and it's free. You'll find the number in the phone book under US Government, Cooperative Extension Services.