The flower stems and petals have started to rot and gone all gross.
The leaves began to rot and became all yucky.
1. The flower stems and petals have started to rot and gone all gross.
花の茎と花びらが腐ってべちょべちょになってる。
花の葉っぱ/花びらを英語で"petals"と言います。
樹木などの時は葉っぱを"leaves"と言います。
go/become gross > べちょべちょになる
2. The leaves began to rot and became all yuck.
葉っぱが腐って、べちょべちょになってる。
go/become yuck > べちょべちょになる