I use a "pea gravel mulch" (over a layer of sand ) in my aril bred area. *about* 2" of gravel, over about 2" of sand, over the "good soil".....and that seems to work pretty well
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If you dig a small hole......maybe 8" deep X 8" around, in your bed area, and fill it with water, and there is still water in the hole 45-60 minutes after you have filled it, I would say you need "better drainage". I f it has soaked away, in that amount of time, you are probably OK, but, as has been mentioned, keeping spent leaves removed, and any other stuff that can act as a "mulch" in the clumps, is VERY important. I have also used a "spading fork" to lift clumps that were "too deeply planted", without actually digging the clump completely out. Just push the fork diagonally under the clump, and "Lever " it up a few inches, then, try to "stomp" some soil under the plant. Soaking the plant with the hose, while it's "raised up" will also tend to wash the "too much" soil from around the rhizomes into the "empty spot" underneath.
Thing is, about rot...of ANY sort....it needs water, and the right amount of temperature, to thrive....and you can't do much about the temperature, so you'll have to work on getting rid of the water....drainage , airflow, and sunlight are the main ways to do it! Just remember that the ROOTS need moisture, but the RHIZOME hates it and try to act accordingly
