You could cover with thick cardboard or newspaper to exclude the light, wet it well,then put on as much compost/well rotted manure as you can, plant straight into itthe cardboard should let you through if it’s very wet, try & cover back up to the stem with the paper,cardboard or even put more around to exclude the light so the couch grass doesn’t grow through, then by next year couch should be gone, you could even put weed suppressant over the top of the compost/manure & cut crosses to plant through, definitely no couch next year.