Pardon the pond mess - it was under reconstruction, but this is how the pampas grass looked in late June of 2013.

Pampas grass dies out in the center as it ages. I see them all over the city looking exactly like yours, that center dead area will just get larger each year. You have to dig & divide the plants when this happens or core out the center with a saw below the soil, add some soil and let the plants fill back in with new growth. Good luck, its a real bear trimming or dealing with this grass because of the sharp leaves.

It looks like center die-out. It's common with older grasses. Pampas Grass is best divided every 3-4 years to avoid it.

You can take divisions from the outer, healthy perimeter, dig out the dead section of the crown, and plant the divisions in its place.

It also looks like you have some other plants invading. Is that Vinca I see, among others?

Thanks everyone for the replies. I remember last year before they began to grow, we saw what appeared to be a dead place about 18" circumference in the middle but they filled in OK. I'll take the advise advise and try to save them.

Thanks again.