Mulch Mulch shrub beds to keep roots cool and to supress weed growth. Use at least 2 inches of an organic compost. Renew this mulch each spring bu digging the old mulch into the soil and adding new mulch on the top. 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.