Botanical note: A 'bent' is a type of grass, as in 'Creeping Bent' (Agrostis stolonifera) although, I suppose, the 'folk' could have applied it to any type of grass, sedge or rush (and may still do in certain parts of the country).
I have encountered both the magical charm and 'amatory' explanations for the 'lay the bent to the bonny broom' refrain.