Actually, you DON'T need to use mulch. I've grown the maximum number of Berries on all of my plants without using mulch. You CAN use mulch to help it out, but what is most important is making sure you water it consistently throughout its growing cycle.
Water it when it is planted, water it again when it's starting to grow, water it again when it's sprouting, water it again when it's flowering, etc.
Watch the soil! Don't let the soil become orange/tan colored/too light colored. Keep the soil moist and dark and you should be fine.
:)