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Is there any chemical I can use that will kill invading Couch grass but leave the Kikuyu grass alive?

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No, there is no simple chemical treatment. So it will be necessary to consider other approaches to your problem, and for this we need to dig deeper into the comparative characteristics of the two grasses.

They are both rapid growing species from rhizomes (roots below ground) and or stolons (above ground). In my area couch grass is valuable because it will survive in sandy loam which becomes too dry for finer grasses and recovers quickly from extremes of drought and temperature. However in low lying moist areas it is crowded out by the finer grasses which prosper in moist soil conditions.

Kikuyu can do well in moist conditions, so one possibility is to create a clean starting point with neither grass present and then suddenly introduce kikuyu and keep it moist. If the area is small you can kill off all grasses with a couple of layers of landscape fabric left on for a year or more to exclude all light which will certainly remove the couch grass and likely the kikuyu as well. Otherwise if you have plentiful irrigation available then it is possible to water consistently so that the kikuyu overwhelms the couch grass.

In addition, Kikuyu flowers and sets seed at a much lower height than couch grass, so a mowing programme which allows Kikuyu to set seed but prevents couch from doing so will give the Kikuyu a self-seeding advantage.

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