Create riverside garden

There is  small area of land by the river in Aberdeenshire which we are trying to turn into a relaxing space for the community.  The River Don runs through our village but there are few spaces where it can be appreciated.  So last year we tried to reclaim a bit of the riverbank that was covered in rose bay willowherb.  I scythed it and then mowed it and put in a bench.  This spring we planted 10 trees.  Now what is growing up is nettles, tuberous comfrey and rose bay.  I hoped by constant mowing it would turn to grass but maybe I am being too hopeful.  

Could someone advise the quickest way to,turn this area into a grassy bank with with wildflowers.  

Thanks
Liz