Answer: Under the circumstances, I'd use a vegetation killer on the area first. When the weedy growth is dead you might consider putting a weed barrier down and then cutting holes in it for your new plants. You can purchase rolls of weed barrier or you can simply lay dampened newspapers (3-4 layers thick) on the hillside and cover that with decorative mulch. Then move the mulch away, cut holes in the newspaper layer and plant your new plants. Either way, the weeds should not poke their heads up again.
Best wishes with your landscape!
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