I've spent a lot of time and effort trying to dig couch grass up and I have found that covering the soil for a good length of time and/or using glyphosate has been much more effective against couch grass. The problem lies with its root system.
If you think neither treatments have not worked, there may be several reasons:
- when using an appropriate weedkiller, you may need to do more than one treatment (the first treatment might knock it back but couch grass have an extensive root system and parts of it may survive and throw up shoots which a second treatment should kill)
- when covering the soil with black plastic, total exclusion of light is imperative (a tiny sliver may just be enough to keep it alive) Also be aware that due to its sprawling root system, some of the plants sharing the same root may be in full sunlight and be keeping the whole thing alive until favourable conditions return
- when covering a bed, be prepared to keep it covered for a long time. All plants need some light to survive, and if excluded, the food in the roots will keep it alive until it returns. However, even this is a finite source and it will eventually die. I'd like to say 12 months minimum, maybe 2 years at most ... if this is too long, you could cover the bed with plastic and plant crops such as squash, sweetcorn or tomatoes through it (I chose these because they'll also benefit from the soil being warmed up by sunshine on black plastic)
- when ground has been cleared, this will encourage seeds to germinate and grow giving the impression that none of the above treatments actually worked
Talking of tomatoes, I read somewhere that growing these in a bed will prevent couch grass from growing. I'm still unsure of the results but this year I grew tomatoes on a new bed that had been originally infested with couch grass and it would be true to say that after clearing the tomato plants, there were no couch grass (but plenty of marestail
)