Seems as tho this grass on land is hard to get rid of so under your water will be really hard. They say at the bottom of the post to cover the ground up w/ something after doing all the other work so maybe the only way to get rid of it will be to cover the bottom of the pond w/ something like liner or heavy black plastic after digging it deeper. Sounds like a real pain to me to do.
Here is a message from the net talking about getting rid of it on land:
How to Remove Couch Grass
without Weed Killers
Let's make our position clear at the outset. We are fully aware that the quickest, easiest way to clear couch grass (Elymus repens) is to trim the grass to a height of about 2cm (1 in) and then spray with the weed killer glycophosphate. It will kill the grass stone dead in 2 weeks and if it doesn't, a 2nd spraying certainly will. If you believe claims that glycophosphate is safe then by all means use it.
But maybe, like us, you have doubts over the environmental safety of a chemical that can kill a virulent weed like couch grass stone dead. Maybe you don't have 100% confidence in the manufacturer's claims or those of governmental agencies. Maybe you remember those reassuring claims that beef was safe around the time of the BSE outbreak and the subsequent findings on Creutzfeldt-Jakob Disease.
These pages are for those who want to clear couch grass without using chemicals or at least using them very sparingly.
Is It Couch Grass?
Click picture to enlarge The picture on the right (click to enlarge) shows the roots of couch grass. They are very dense in the soil and go down about 15cm (6in). The other key identifier is that couch grass will not be killed by burying it under ground as happens with normal grass.
Click picture to enlarge
Strong Points of Couch Grass
1. Burying or turning couch grass will not kill it. The roots will spread and shoots will reappear. 2. Digging the roots of couch grass is hard work, they are dense and bind even the lightest soil. 3. Couch grass grows extremely quickly.
Weak Points of couch Grass
1. Couch grass is shallow rooted, no long taper like roots going deep into the ground. The roots of couch grass typically go down about 10cm (4in) and rarely more than 15cm (6in). 2. If the soil is well-dug, the roots of couch grass can be pulled out with relative ease. 3. Digging of the soil weakens couch grass. 4. Couch grass produces less seed than most grasses
Our Non-Chemical Clearance Strategy for Couch Grass
1. Cut the couch grass to about 2cm (1in) high with a trimmer. Alternatives are to burn it off with a flame thrower or cut it down with a scythe. 2. Rake up the cut grass, don't use it on the compost heap unless you plan not to use the heap fro 9 months. 3. Rotovate then dig the soil or simply dig the soil. Use a fork not a spade to dig, this will keep the non-rotovated lower roots of the couch grass uncut and easier to remove in long strips. 4. Whilst digging, remove as many of the couch grass roots as possible. A sieve will make a thorough job and give you the crumbliest soil ever. 5. Dig a trench at least 15cm (6in) deep around the cultivated bed. This will stop the couch grass re-infesting the cleared area. 6. Over the next few months, weed out any couch grass which reappears from roots left in the ground. This is an easier task than most think because digging the soil will have opened the structure of the soil. We reckon about half an hour a week for the next 4 months will be enough to remove the remaining couch grass on a half allotment. 7. Once step 5 has been completed, any soil which is not to be used immediately can be covered in mulch (black plastic, carpet etc). This will clear any remaining couch grass in six months and will seriously weaken the grass after only one month.
