Rosie, Cyperus rotundus is certainly one of the most difficult weeds to control in agriculture let alone in the garden. It is a perennial sedge. There are selective herbicides which are used but these are generally not available to the gardener. For example I could control CYPRO (this abbreviation is often in industry. It comprises the first three letters of the generic name follwed by the first two letters of the specific name. Try it. It works 99 times out of 100) in maize/corn, peanuts/groundnuts, turf, etc.
The best product and safest in glyphosate but it takes time and patience. What I do is to mix up a container-full, fill up an empty squigy bottle (old washing-up liquid bottle) and simply squirt the offending weed. Adding some surfactant-washing-up liquid will help. The problem with CYPRO is that there is virtually no target to aim at. Remember that glyphosate will kill anything green. Several follow-up applications will be required.
Wear gloves and wash afterwards. David
