This is the first year in my house and I have two big clumps of Pampas grass. One of them just bloomed in the past three weeks and now it looks like all of the blades are turning yellow. Really more like the entire clump is dying. The other clump bloomed about two weeks later but still appears to be as green as ever. Are these supposed to turn yellow after blooming? When we moved in last fall I swear up and down they were both green.