During my "Tropicalesque" garden period, I planted petasites in a raised bed so I could have large tropical leaves. So now I have petasites everywhere; they are very invasive. My question: I've read to never plant dahlias in soil with weed killer. I would like to use Roundup or its equivalent now to kill the petasites (they are runners from tubers and almost impossible to dig out). If I used weed killer now, would it be inactive by May, when I would plant dahlias? I'm planning a whole new large bed of nothing but dahlias. The rainy season has started here in Oregon, so that might help diffuse the weed killer. Or would it inactivate it? Any thoughts?