an unintentional action that leads to an unexpected positive result; literally meaning 'planting a willow branch without intent and seeing it grow into a shade tree'

unintentional actions may bring unexpected results; literally to idly poke a willow branch in the ground and have it grow into a shade tree

intentional efforts may fail while unexpected actions lead to success; literally: planting flowers with care but they do not bloom, while sticks of willow planted without thought grow into shade

Literally "to plant flowers with intent and have them not bloom, while sticking a willow branch in the ground without thought and having it grow into shade"; describes how well-laid plans may fail while success can come unexpectedly from casual actions.