"To mulch, or not to mulch . . . ."
The answer depends a lot on where you live.
Here in the hot, dry country the natural summer mulch formed by dried iris
leaves and drifted sand helps keep the soil around the rhizomes a bit cooler.
(Exposed rhizomes = cooked rhizomes.) Before the ladybug colony was
self-sustaining, I'd have characterized winter mulch as a recipe for disaster --
keeping the beds clean was the only way to spot an aphid outbreak in time to
control it chemically. Now that the ladybugs are established, though, I've
found that the beds in which tree leaves are allowed to accumulate in the fall
have the best protection. They provide winter shelter for the ladybugs, which
in turn protect the iris from the aphids and other pests.
Sharon McAllister (email@example.com)