A mulberry field is a mulberry field.
These are both the kun'yomi readings, but you haven't learned this for 原 yet. It's actually the rendaku'd version, so let's just use mnemonic to remember it instead:
You know you're in a mulberry field when you see a bunch of capybaras (ばら) rolling around in there with you. Capybaras love mulberries! Picture a whole family of capybaras munching on mulberries. Mm Mmm!
People started calling this place "Kuwabara" because there used to be a lot of mulberry fields here.