When we've done the same thing, the roots have still been active, and need to be removed when preparing the soil for planting. Same goes for bindweed, whose roots seem to be able to survive for a long time.
We have had success over this last winter in killing couch including the roots with a combination of glyphosate applied in the autumn and membrane over the winter, to tackle a new piece of allotment that we'd taken over. I know some people don't like to use chemical weedkillers, but this really needed some serious reclamation.