A clump of irises (Iris spp.) Supply summer blooms as well as the big sword-shaped leaves provide dramatic greenery. Irises rise in U.S. Department of Agriculture plant hardiness zones 3 through 9, depending on the variety. Invading grass can ruin the otherwise stately look of this bed, but also a combination of manual and chemical controls can keep the garden tidy and grass-free.
Manual Removal
Grass roots may get entangled in the iris rhizomes, making it difficult to pull the grass effectively. Digging up the iris in late summer or early fall, after flowering, and removing all of grass at the bed can give the garden a fresh start. Rinse off the rhizomes with water to remove any soil and inspect the roots carefully for any grass rhizomes tangled in with the roots. Cut the rhizomes apart into two- or even 3-inch sections that each have a fan of leaves. Pull any remaining weeds or grass from the bed and replant the rhizomes to ensure the top of the rhizome is right above the ground. Rhizomes need dividing every three decades, so a thorough weeding in this time greatly reduces bud development.
Smother the Grass
Mulch suppresses weed growth but a heavy mulch layer on top of this rhizomes can cause iris roots to decay. Cover the bare soil around the plants with a 3- to 4-inch layer of straw or bark mulch, but do not place the mulch on top of the rhizomes. The mulch will keep weeds from growing across the iris plants, which can impede the grass’ spread into the iris rooting area. Mulch breaks down and decomposes over time, so replenish it one or two times annually to maintain its thickness.
Herbicide Options
Herbicides containing the compound sethoxydim can kill many grasses without damaging the iris plants if you follow tag mixing and application directions carefully. Wear eye protection, protective gloves and long sleeves when applying herbicides, and avoid inhaling the fumes. Many sethoxydim herbicides require dilution. For instance, mix 1.3 ounces of a concentrated 18 percent sethoxydim herbicide with 1 gallon of water at a lawn sprayer, and spray the solution onto the actively growing grass until it is evenly coated. Avoid working at the bed for five days following application, and then pull up the dead grass from across the iris plants.
Border Control
Grass will return, particularly when a lawn area borders the iris bed. Edging across the bed and keeping the yard mowed so the grass does not go to seed can keep it out. Straightforward trench edging prevents the grass roots from propagating into the bed. Dig a 6-inch-deep, 3-inch-wide trench around the perimeter of the bed and keep it clear of soil and debris. Otherwise, install rubber or vinyl edging around the perimeter of the iris garden. Bury the edging 6 inches deep with 1 or two inches protruding above the ground. Keep the grass trimmed near the edging so that it does not spread over the top and into the iris bed.