Turf Tips

By Jeffrey Derr and Adam Nichols

As our temperatures decrease as we move into fall, there are several items to consider for lawn maintenance. Winter weed control, fertilization in cool-season lawns, overseeding of both warm and cool-season lawns, and fall fungicide applications of warm-season lawns 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. For homeowners there is little available for selective control of emerged annual bluegrass and even commercial lawn care companies have few options for controlling established plants in cool-season turf. 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.

So one needs to decide what is more important, overseeding or applying a preemergence to stop annual bluegrass germination. If the cool-season turf is thin, it probably would be better to put the money towards purchasing and applying turf seed. In one study we conducted, the annual bluegrass population decreased as the overseeding rate increased due to crop competition.

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, but the weed needs to be treated in fall or winter before it has produced spines.

If overseeding is the priority for the fall in either a cool-season or warm-season lawn, products with the preemergence effects previously discussed cannot be used. Fall is the best time to establish and strengthen cool-season lawns. Starting in mid-September through early November, tall fescue, Kentucky bluegrass, and fine fescue can be seeded to bolster those cool-season lawns. At the time of seeding, these cool-season turf species can be treated with products containing mesotrione. Mesotrione is an herbicide that provides preemergence control of many winter annual broadleaf weeds, as well as suppression of annual bluegrass. When used, the cool-season grasses establish better without the hindrance of weeds being present to compete with the turf seedlings. Tenacity and Sublime both contain mesotrione in a liquid formulation while many starter fertilizers contain mesotrione in a granular form. Mesotrione also provides postemergence control of certain annual and perennial weeds, but generally two applications two or three weeks apart are required.

When overseeding a warm-season lawn for winter color, perennial ryegrass and Kentucky bluegrass are the recommended species. Neither preemergence products nor mesotrione should be used at the time of seeding these lawns as they can not only prevent establishment of the cool-season grass from the crabgrass preventers, but can also damage the warm-season grass from the mesotrione application.

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.

When maintaining a warm-season lawn, fall is the best time to apply needed fungicides to prevent and cure the worst diseases that affect these turf species. Both large patch (caused by the fungus Rhizoctonia solani) and spring dead spot (caused by the fungi (Ophioshpaerella sp.) infect warm-season grasses as both air and soil temperatures drop below 70 °F. Applications of azoxystrobin-containing fungicides in late September to early October, and 30 days after the initial applications on all warm-season species where known infections of large patch have occurred will stop this disease from causing problems. Applications of isofetamid (Kabuto), penthiopyrad (Velista), propiconazole (Banner MAXX), or azoxystrobin + propiconazole (Headway) are all effective against spring dead spot when applied in the same time frame as the large patch applications. Both large patch and spring dead spot are both soil borne fungi, and need to have their respective fungicide control treatments watered-in to move the active ingredients where they will be most effective.

June Turf Tips

Tall fescue:

For those maintaining tall fescue, keep an eye out for brown patch disease. This fungus infects tall fescue especially under warm, humid conditions we see during the summer. Preventative applications of a fungicide containing azoxystrobin during the summer can help limit the amount of overseeding that may need to be done in the fall. Avoid frequent irrigation as this can create conditions for the disease to build up. Limit nitrogen applications during the summer. Most, if not all of your nitrogen should go down in the fall – September, October, November. If annual bluegrass (commonly called Poa) is a concern, one control option is to apply a crabgrass preventer, such as pendimethalin, dithiopyr, or prodiamine, in mid- to late-August, as annual bluegrass usually starts germinating in early September. The downside of applying a crabgrass preventer in August is that in most cases you will not be able to overseed, as these chemicals not only prevent successful germination of annual bluegrass, they also prevent establishment of tall fescue from seed. Decide which is more important – overseeding or applying a crabgrass situation. If the turf is thin, consider focusing on overseeding.

Warm-season grasses (bermudagrass, Zoysia, St. Augustine, Centipede):

Now is the time to fertilize since these grasses are actively growing during the summer months. It also is time for such cultural practices as dethatching and aeration. Move plugs to fill in bare spots. Bermuda grass, zoysia , and centipede can be seeded in June or July. A crabgrass preventer can be applied in August for annual bluegrass control in established warm-season grasses, but there are additional options later in the year. Do not apply a crabgrass preventer to newly seeded turfgrasses.

Both tall fescue and warm-season grasses:

For both cool- and warm-season turf areas, keep an eye out for yellow nutsedge and kyllinga, two common perennial sedges in our area. Spot spray a chemical such as halosulfuron, sulfentrazone, or bentazon, among others, to sedge leaves for control. Apply a product containing quinclorac if you have crabgrass escapes in tall fescue, bermudagrass, or zoysia but do not apply a quinclorac product to St. Augustine or centipede. For broadleaf weeds, apply an amine formulation of a three-way broadleaf herbicide. Do not apply a triclopyr-containing product to bermudagrass. For St. Augustine, use a broadleaf herbicide for sensitive southern grasses.