Planting Tiger Lilies in Ditch

I have a 150 foot draining ditch that is filled with weeds and underbrush. It is impossible to mow and nearly impossible to weed eat. It looks even worse in the winter when the weeds die and the clay dirt shows through. I have decided to line it with Tiger Lilies to make the ditch more attractive as well as protect it from erosion. Should I spray the whole thing with roundup before planting the Lilies or plant the Lilies amongst the weeds and let them naturally choke the weeds out?
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