Be careful. Any weed killer designed to kill grass (monocots), will likely also kill iris (also monocots). Some weed killers are designed to kill only dicots, and may therefore spare your iris, but also won't kill grass!
I use this Japanese hoe, called a tomita.
https://www.amazon.com/dp/B002SW31YA/ It's wicked sharp, stainless steel, and can handle all the rocks in our very rocky, and often very hard clay soil. I can just whack it into the soil and draw it toward me to lift out weeds, or re-edge the garden. I also have a similar pointed long-handled hoe. Both are wonderful in our hard, rocky soil. It took me most of my life to find weeders I liked. These are it. The fine point lets you weed carefully around those iris rhizomes without disturbing them. Love it. And it doesn't rust.
