Apply a pre-emergent herbicide labeled to kill grasses before the grass and irises begin to grow. When to apply depends on your climate, usually sometime from fall until late winter -- iris foliage emerges in very early spring and even during winter warm-ups. Pre-emergent herbicides help prevent the grass from germinating. It is not effective once the grass is growing. According to Louisiana State University, pre-emergent herbicides that contain pendimethalin (e.g., Pendulu) or s-metolachlor (e.g., Pennant) work well in preventing grass from growing in an iris bed.
Apply a herbicide that will kill the grass, but not harm the irises. According to North Dakota State University, herbicides that contains the active ingredient sethoxydim is safe to use in an iris bed and is effective in killing grass.
Remove grass manually. If you have quack grass, this may be your only solution since herbicides often do not work. It's easier to pull grass out with the soil is slightly moist. You can use a weeding tool, such as a three-prong hand tool, that you can forcefully insert into the ground to help loosen the roots. Be careful not to harm the iris rhizomes in the process. You can also dig up the entire bed, removing the grass, and start over -- an option that gives you the opportunity to divide your irises as well.
Install edging around your garden bed if it is planted next to a lawn. Plastic edging that is inserted a few inches in the ground works well. You can also use landscape rocks, bricks or another edging. Bury some of it in the ground a couple inches deep to keep it firmly in place and to prevent the grass from spreading via underneath the edging.