Clumping plants merely spread shorter distances each time than those that creep or run.

If left in place indefinitely, most clumping grasses DO continue to grow, slowly expanding their width over time. And the new growth does tend to be concentrated at the periphery. In time, they will often develop that 'dead' center, some species more obviously than others. Miscanthus sinensis is well known for this growth habit.

Pampas **ass is also good at this. God , I hate that stuff. It gets rotted and dead in the center and is a BYATCH to divide or rip out and remove in disgust. Bleeding arm syndrome from knife edged blades.

Hate Pampas Grass. It is very invasive in my area and has ruined the natural California landscape in an area I drive frequently. Some plants shouldn't be allowed to be sold, at least not in certain geographic areas. Usually, by the time someone figures this out, it's too late anyway.

Just today in my area we are calling for volunteers for the annual broom pull. No, that's not a grass, but an example of how darn hard it is to keep the bad guys from taking over.