Answer: It's almost impossible to amend soil once trees and shrubs are planted, but you can dig your perennials, amend the spots where they were growing, then replant. Some of the nutrients from the decomposing organic amendments will trickle down to where the deeper tree and shrub roots are growing.
Fall is a great time to dig and divide perennials. Once you've dug the plants, either pull them apart or slice them into divisions, making sure each division has both roots and stems or foliage attached. Amend the planting area by spreading 2-3 inches of organic matter over the top and digging it in to a depth of 8-10 inches. This is there area where the roots of most perennials will be. Then replant and water well to help settle the soil.
Best wishes with your garden!
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