Selective control of annual bluegrass in turf is a big challenge in the Pacific Northwest. There are no tried-and-true procedures. Every breakthrough reported in another region of the United States is less effective in the PNW than in climates with greater environmental stress. Current strategies involve selective preemergence herbicides, selective pre- and early postemergence herbicides, and selectively suppressing growth with turfgrass growth regulators.
Selective preemergence herbicides prevent annual bluegrass seed from germinating. In general, while most available preemergence herbicides are effective in killing germinating seedlings, they have not provided effective annual bluegrass control. Part of the problem is that there are annual and perennial types of annual bluegrass in irrigated turf. In addition, annual bluegrass can produce seed any time of the year, but seeds most heavily in spring, with a second, less heavy flush of seeds in fall. Finally, annual bluegrass seed can germinate nearly any time of year. These factors make it difficult to target applications. Repeated treatments of preemergence herbicides to maintain herbicide activity year-round may stunt root growth of desirable grasses and predispose them to diseases or reduced stress tolerance.
Selective preemergence and early postemergence control is viable with ethofumesate applied to pure seedling stands of perennial ryegrass. Ethofumesate is active on young annual bluegrass up to about the four-leaf stage. In mature perennial ryegrass, Kentucky bluegrass, or tall fescue, annual bluegrass can be controlled with three applications of ethofumesate in fall. Start in mid-October and reapply at 3- to 4-week intervals. Activity generally is not apparent after the first application, but yellowing begins between the second and third applications. In western Oregon, the death of the treated annual bluegrass may not be apparent until March, depending on the year. Kentucky bluegrass and tall fescue may be slightly injured or discolored. Some tests achieved 90% to 100% control after one series of treatments. Annual bluegrass may reinvade from seed once the herbicide dissipates.
Selectively suppressing annual bluegrass growth has been successful in many parts of the country. The object is to retard annual bluegrass more than desirable grasses so the balance of competition shifts in favor of the planted grasses. This strategy seems to work best in areas with cold winters and hot, humid summers. Annual strains of annual bluegrass are more sensitive than perennial strains. Reducing annual bluegrass to acceptable levels may take several years, and may require ongoing spring and fall treatments.