The best way to amend soil in an existing garden is by top-dressing the bed with an inch or two of compost each season. The compost will break down, improving soil structure and fertility. Plus, you'll perk up your perennials. This is particularly helpful if you live in a cold-winter climate and apply the compost as mulch, after the soil freezes in winter. If you don't have access to compost, you could use shredded leaves or well-rotted manure. You also can use a packaged fertilizer such as 10-10-10 or fish emulsion.
Here's how to successfully incorporate fertilizer around existing perennials. Check your timing. The best time to apply fertilizer to most plants is early to midspring, after the plants have grown a few inches above the soil but before they have so many leaves that it is difficult to work the soil around them. You can fertilize later in the season, but most perennials should not receive granular chemical fertilizer after about August 1 in the North or 6-8 weeks before frost. Pull away any mulch. Add fertilizer or compost. When using fertilizer, carefully measure the recommended amount. Avoid overdoing it. Too much can be harmful. Sprinkle fertilizer evenly on the ground around the plants. Avoid getting fertilizer on plant leaves or stems, because it can burn them. Lightly work fertilizer into the soil. Replace mulch. Soak the soil thoroughly, gently washing off any fertilizer that may have gotten on the plants.