I've had an ongoing battle for two years with the roots left over from removing a crêpe myrtle. In removing the tree, I removed the main root underneath it. But the outlying roots keep sprouting.

I thought that over time, spraying the sprouts with Roundup would eventually kill the root. This summer, I tried trimming the sprouts to within an inch of the ground, then applying Roundup just on the cut stalks. This seemed to have no effect.

Please, does anyone have an effective way of killing these outlying roots without having to dig them all up? Thanks very much for your responses.