DO NOT TILL INTO THAT RED CLAY!!!!!!!
That Johnson grass spreads by seeds and rhizomes.
All you will do is make it worse.
They should dig that Johnson guy up and toss him in the ocean for bringing this stuff over here from Turkey.
The best you can do is mulch the bed and every time you see a blade come up cut it off.
I frigging hate the stuff.
I am slowly but surly getting rid of mine.
Worth