In my experiences with a similar situation the only resolve that worked was to dig up the switch grass and split the "clump" and remove the crab grass.

Do all this by hand. Power tools will not help. Weed or grass killer will not help.

To prevent this from happening remove any and all grass and weeds from the intended planting area prior to planting your ornamental grass. Make sure your ornamental grass clump is clean to start with.

Jake

Thanks Jake,
How do I know which is which? I'm very new to this. Some of the crab grass is getting plumes that almost look like switch grass.

If " .. they almost look like.." then you can tell the difference so you already know more than you think.

I have never seen crab grass that gets as tall as switch grass. That would be scary.

Crab grass from my experience will bloom or get plumes sooner than switch grass, at least in our area.

Jake