Grasp the crabgass firmly at the base and pull up all the roots. Loosen the patches of crabgrass with a garden trowel if necessary.
Soak crabgrass that can't be pulled up or whose roots remain in the ground with a herbicide such as monosodium-acid-methane-arsonate that is safe for flowers and grass.
Cut the crabgrass from around the flower bed if the two areas connect.
Apply a 2-inch layer of mulch around the flower garden to block crabgrass growth.
Apply a pre-emergent herbicide with active ingredients of siduron or dithiopyr during the spring season to block any crabgrass seeds from germinating. Make the application before the crabgrass appears.