Preemergence Herbicide Failure: Why?
The two biggest reasons for failure with preemergence herbicides are not getting the herbicide out before the weeds begin to germinate and lack of timely activation with at least 0.5 inch of rainfall or overhead irrigation. Activation should occur within one week of application. Sooner is better. If irrigation is available, water-in pre’s immediately after application. If the turf is mowed and bagged before it is watered-in, control will be reduced. Other important factors include choosing the right herbicide and applying the correct rate accurately and uniformly. The most common target weeds for preemergence herbicides in turfgrass are annual grasses. Crabgrass, goosegrass and annual bluegrass make up the big three in Arkansas. The most commonly used products for crabgrass control in Arkansas are prodiamine (Barricade) and pendimethalin (Pendulum). Typical rates for these products are 0.75 lb ai/ac for prodiamine and 3.0 lb ai/ac for pendimethalin. Timing for crabgrass control would be February 20 for south Arkansas, March 1 for central Arkansas and March 10 for the northern parts of the state. Split or repeat applications are applied about 60 days after the first treatment. If goosegrass is the only target, pre’s can go out 10 to 14 days later than the crabgrass timing.
For preemergence control of annual bluegrass apply apply either prodiamine or pendimethalin at the above mentioned rates on August 15. Annual bluegrass may germinate under the turf canopy in late summer or early fall causing September applications to be less than satisfactory. Many turf managers skip the August pre application for annual bluegrass because they plan to use atrazine or simazine, which provide postemergence control of small annual bluegrass, in November or December. The target weeds for atrazine and simazine are annual bluegrass and broadleaf winter annuals. Another option is to apply a tank mix of atrazine or simazine plus prodiamine or pendimethalin with a postemergence broadleaf product such as Trimec or Manor/Blade. This mixture is typically applied from December to early March. It will control broadleaf perennials, annuals, annual bluegrass and crabgrass the following spring. Golf operators often opt to use Roundup at 0.5 lb ai/ac applied to completely dormant bermudagrass in January or February as a cheap, effective means of controlling annual bluegrass.
Early is almost always better with pre herbicides. Many lawn care companies, due to time constraints, begin pre crabgrass applications in late January and early February. This practice is effective because herbicides degrade slowly during the winter. However, applying leachable, short residual herbicides such as simazine or atrazine too early in fall (September –October) may result in poor weed control because the herbicide often dissipates before weeds emerge.