Alternatively, just keep it dry, hang it over a washing line and beat dust/dry mud out of it like a rug.

Re: garden uses: Don’t forget that although the grass will decompose and eventually rot down, there are probably artificial fibres holding all that grass together which will not. Don’t allow it to decompose in your garden if you don’t want hundreds and hundreds of tiny nylon threads woven thickly into your soil for years and years to come.

(This is the bitter, weary voice of experience speaking….)

If you know anyone with an allotment they’d be really good for excluding weeds – put cardboard underneath to exclude light completely. In fact, if you’re near Leeds I’d take them off your hands for that purpose!

You could also put them on the garage floor, under where your car sits when it is parked, to act as drip mats for oil, etc.