September Turf Tips - Commercial

Jeffrey Derr and Adam Nichols

Turf Management Commercial Lawn Care Operators

As our temperatures decrease as we move into fall, there are several items to consider for lawn maintenance. Winter weed control, fertilization, and overseeding are topics to consider this time of year.

Once temperatures cool in September, coupled with rainfall to trigger germination, winter annual weeds will start to emerge. Most germination of winter annuals occurs in September to November. One weed of concern is annual bluegrass, often called poa due to its scientific name Poa annua. Annual bluegrass will be most noticeable in spring when it produces whitish seedheads. One way to address control of annual bluegrass would be to apply a crabgrass preventer, such as prodiamine (Barricade, others), dithiopyr (Dimension, others) or pendimethalin (Pendulum, Pre-M, others), prior to annual bluegrass germination. Ideally this should be done sometime between mid-August and early September. In most cases, though, one would not be able to overseed the lawn as the crabgrass preventers will also prevent successful germination of tall fescue and other turf species. If overseeding/establishing a cool-season lawn is the priority, applying a product such as Tenacity or Sublime at the time of seeding has shown to help with the establishment through the elimination of most winter annuals.

There are some options for control of emerged annual bluegrass in tall fescue, including ethofumesate (PoaConstrictor) and bispyribac (Velocity). Repeat applications of ethofumesate will be needed during the fall months. The Velocity label limits applications when growth slows in the fall or when temperatures are consistently below 60 oF. We have investigated use of repeat treatments of Velocity for roughstalk bluegrass control with mixed results. Another potential use of Velocity is in dormant bermudagrass overseeded with perennial ryegrass. Velocity cannot be used in actively growing bermudagrass but products such as Revolver, Monument, or Katana could be used in bermudagrass that is not overseeded.

Another advantage of applying a crabgrass preventer in late summer or early fall is that they will also control certain winter annual broadleaf weeds like common chickweed and henbit. Although these broadleaf weeds can be controlled postemergence, one has to live with dead or dying broadleaf weeds as the product works. A weed like henbit can take a while to die after a postemergence treatment, while with use of a preemergence product the weed never appears in the lawn.

Although not effective on annual bluegrass, isoxaben (Gallery) can be applied for preemergence control of a wide range of winter annual broadleaf weeds. Isoxaben could be applied with a crabgrass preventer, or there are products such as Crew that contain both isoxaben and a crabgrass preventer. Crew, a granular product, contains both isoxaben and dithiopyr. Again, in most cases, one would not be able to overseed after applying such a product.

One winter annual broadleaf weed that has increased in the area is lawn burweed (Soliva sessilis). We receive calls in spring about this weed as it forms sharp spines on the seed, which form in the center of the rosette. The spines prevent walking barefoot in the lawn. Isoxaben will control this weed if applied before emergence. Emerged plants of lawn burweed can be controlled with common three-way postemergence herbicides containing dicamba or triclopyr, but the weed needs to be treated in fall or winter before it has produced spines.

Fall is the time to fertilize cool-season turf such as tall fescue. September through November is the time to apply most, if not all, of the nitrogen to a tall fescue lawn. Fall fertilization does not stimulate excessive shoot growth, while spring fertilization can push shoot growth, resulting in increased mowing, and potentially increased disease and less turf root development. Fall is not the time to fertilize warm-season grasses like bermudagrass, zoysia, and St. Augustine. Warm-season grasses should be fertilized primarily during the summer months.