Don't let mulch pile up against the crown of the plant, which could cause crown rot. Use water permeable landscape cloth or black plastic covered with rocks. River rock seems natural for most areas of California, and it has many advantages. You can clean it with rake or blower if it's large enough. A rock covering keeps cats away too.
Volcanic rock is very light.
It's too easy to rake or blow onto the lawn, and unless you live by Mt Shasta, it looks out of place. Most of the plants that begin blooming in March are still blooming in April. The first and best bloom of roses is in April. Rose bushes have not yet been attacked by insects and fungus diseases. It's time to begin a program of fertilizing and spraying to keep the roses healthy all spring, summer and fall.