I Need a Mulch!
Many thanks for your recent enquiry regarding the couch grass which is invading your fruit cage. I'd probably recommend Strulch as a mulching material, mainly because I've used this material to good effect in our Kitchen Garden - also in a fruit cage planted up with raspberries!
However, my fruit cage wasn't overrun with couch grass, so it might be worthwhile trying to dig out (for one last time!) the majority of the underground roots or rhizomes and laying a cover of the weedblock biodegradable mulch down first for extra protection. This material breaks down after roughly 12 months and, when covered in a healthy layer of Strulch (about 1-2" deep), should see an end to your problem. You can of course top up the strulch layer when required, and the material will also enrich the soil as it degrades, along with helping to retain moisture.
Best of luck with your raspberries this year.