Grass growing in flower garden
I suspect you are battling something other than crabgrass, which is annual and does not produce an extensive, perennial root system. Preen would have no effect on a perennial grass. Perennial grasses are really tough, but you can apply grass killer (make sure it's selective grass killer, not roundup -- one brand is Grass B Gon) and it should kill the grasses and leave most of your ornamentals alone. Be sure to read all label instructions.