I find myself recommending adding compost to almost everyone I work with because their soil could use it. Healthy soil is the best thing you can do for your plants and if yours aren't thriving, I would consider adding some. Compost is partially decomposed organic matter. You can also add cow manure, grass clippings, leaves, stems, small twigs, etc. When I cut back perennials I often tuck the clippings under the plants where you can't even see them (this is referred to as "chop and drop"). Think

October Garden Transformation: Transforming Small Spaces into Stunning Landscapes