The ideal time to apply control is in mid to late fall just after the weeds have germinated and emerged. By May it will be too late. Plants will already have flowered and will be completing their life cycles by dispersing seeds, rendering herbicides ineffective. Mowing can be an effective cultural control in early spring when weeds are taller than turf. But mowing does not remove the entire plant and may actually aid in seed dispersal when the weeds have reached full maturity.
Many gardeners have problems keeping grassy weeds out of iris.
This means that an applicator can spray the top of an iris bed without hurting the iris. Poast and Fusilade II are relatively slow acting, and it may take more than a week for grasses to start dying. There may be other trade names, also. Fortunately, the product may be identified on the label by the common chemical name. Fusilade's common chemical name is fluazifop.