I’m a lazy gardener and my experience is that most perennials (if not all) do very well on a rather limited supply of nutrients always assuming the soil is improving. Here’s my routine for feeding perennials in my garden:
I generally use as much compost as I can on the plants. A shovel full spread around most plants is adequate. It really is great for the soil.
If you don’t have compost, then yes purchasing it is a great idea and perfectly acceptable.
I find that organic fertilizers such as bone meal and blood meal work very well on perennials although they can sometimes attract dogs or raccoons. Update: I no longer use this material as I find it superfluous – compost does everything I need it to do.
I’ve had excellent results from fish emulsion type fertilizers and go through several large jugs a season spot-feeding my plants. I spot feed if I want to push the plant along or if it’s looking a bit “off”. But generally, the fish emulsion is used on roses, container plantings and annuals. Update: I’m still a huge fan of this material. 🙂
I’m not a big fan of chemical fertilizers as these tend to be like candy and kill off some of the beneficial soil microorganisms we’re trying to grow along with our perennials.
Once you have the garden established and the soil working properly, then you’ll find that replenishing the mulch with several inches of leaves or an inch of (insert your favorite mulch here) every fall will keep your plants growing well. If you are unhappy with a particularly plant performance, you can spot feed with fish emulsion and spread compost in the spring.
But chemical fertilizers are not necessary for great performance in the perennial garden if your soil is alive.