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Pennsylvania Turfgrass – Pete Landschoot, Ph.D. Professor of Turfgrass Science, Penn State

During late spring and summer, summer annual grasses can be an unsightly weed problem in many Pennsylvania lawns, sports fields, and golf course fairways and roughs. Reasons for severe infestations include high seed populations in soil, thin turf canopies allowing space for weeds to grow, mistimed preemergence herbicide applications, and premature breakdown of preemergence herbicides. Once summer annual grasses begin to emerge, they generally grow faster than cool season turfgrasses and will gradually takeover large sections of turf. If not controlled before producing seed, another large weed crop will emerge the following year.

The most common summer annual grasses likely to infest turf in Pennsylvania include crabgrass, goosegrass, and foxtail species. Other summer annuals that may be problematic in certain areas are barnyardgrass and Japanese stiltgrass. Seeds of summer annual grasses don’t all germinate at the same time; instead, germination and seedling emergence is staggered over several weeks or months. This timing mechanism ensures that a late frost or some other weather-related event won’t kill off the entire population. Depending on where you live in Pennsylvania, you may see crabgrass seedlings emerging from mid-April to mid-July. Also, seed of various species of summer annual grasses germinate at different times of the growing season. For example, goosegrass seed usually germinates and emerges 3 to 4 weeks later than crabgrass. The long germination/emergence period makes control of summer annual grasses challenging.

The most effective means of controlling summer annual grasses is through cultural measures that create a dense turf and applications of preemergence herbicides. When these measures fail, postemergence herbicides can be used. Postemergence control of crabgrass and other summer annual grass weeds involves the use of products that kill growing plants after they have appeared in the turf. Postemergence herbicides can be used to treat only those areas where summer annual grass weeds have emerged. Preemergence herbicides, on the other hand, are usually applied over the entire turf area since the applicator does not know where seeds are located, or if they are present.

Stage of Growth and Postemergence Herbicide Applications

Knowing the growth stage of the target species and the corresponding herbicide rate are important for successful postemergence control of summer annual grasses. Stage of growth can be determined by examining individual plants and observing the number of leaves on the main shoot, or how many secondary stems, called tillers, have emerged from the crown. Generally, summer annual grasses with 1 or 2 tillers are easier to control with postemergence herbicides than plants with 3 or more tillers. As the number of tillers increases, the product rate will likely increase, or a different herbicide may be needed.

Because summer annual grass seedlings emerge in turf stands at different times during spring and summer, their growth stages will vary. Be sure to examine several plants in different locations of the turf stand to get an adequate assessment of plant maturity. If early in the growing season, the stand may be composed mostly of non-tillered or 1 or 2-tiller plants. In this case, choosing a herbicide product and rate that provides effective control and is safe on turf may be easier. Adding a preemergence herbicide with a postemergence product early in the growing season will prevent late-germinating summer annuals from infesting the stand. Later in the summer, annual grasses may have 4 or more tillers along with some newly emerged plants. In this case, you may need to apply a herbicide at rates that will control weed plants with the higher tiller numbers. Postemergence applications at this stage of growth may require more than one application for complete control. By the time seedheads are produced in late summer, control with postemergence herbicides becomes inconsistent and not very practical.

Postemergence Herbicides for Control of Summer Annual Grasses

Dithiopyr is mainly used as a preemergence herbicide, but products containing this active ingredient (Dimension 2EW, Dimension Ultra 40WP, Dithiopyr 40WSB, and others) have information on labels concerning early postemergence control of crabgrass. When used alone on cool season turfgrasses, dithiopyr can provide postemergence control of crabgrass prior to tillering and when plants have fewer than 5 leaves. Dithiopyr does not provide postemergence control of goosegrass. Some dithiopyr product labels allow for mixing with other postemergence herbicide products, such as Acclaim Extra, for control of crabgrass up to the 3-tiller stage. Most cool season turfgrasses are tolerant of dithiopyr, but product labels caution against use on certain cultivars of creeping bentgrass and fine fescues.

Fenoxaprop‑p-ethyl is a postemergence herbicide that con­trols tillered crabgrass and other summer annual grasses with a single application. Products containing fenoxaprop‑p-ethyl (Acclaim Extra and Last Call) are labeled for control of crabgrass, goosegrass, barnyardgrass, foxtail species, Japanese stiltgrass, and panicum species. Fenoxaprop‑p-ethyl is rela­tively safe on most cool season turfgrasses but may injure some Kentucky bluegrass cultivars, especially at high tempera­tures. Rates of Acclaim Extra range from 3.5 to 39 fl oz/acre, with suggested use rates increasing as the number of tillers on summer annual grasses increase up to 4 or 5 tillers. Efficacy of Acclaim Extra improves with the addition of a nonionic surfactant. On Kentucky bluegrass the product rate should be limited to 28 fl oz/acre per application. This herbicide should not be applied if cool season turfgrasses show signs of drought stress. Fenoxaprop‑p-ethyl is less effective when tank‑mixed with phenoxy herbicides such as 2,4‑D and MCPP but can be mixed with preemergence herbicides for improved control of crabgrass. Last Call contains fenoxaprop‑p-ethyl and the non-phenoxy herbicides fluroxypyr and dicamba for control of summer annual grasses and some broadleaf weeds.

Mesotrione is the active ingredient in Tenacity herbicide and can be used as a preemergence and postemergence herbicide for controlling crabgrass, barnyardgrass, and yellow foxtail, but only provides postemergence control of goosegrass. Postemergence control of crabgrass and goosegrass is most effective when applied to young plants with fewer than 4 tillers. Repeat applications are usually necessary for complete control of summer annual grasses. Rates of Tenacity range from 5 to 8 fl oz/acre and should not exceed a total of 16 fl oz/acre over the course of the growing season. Tenacity is relatively safe on most turfgrass species, but cannot be used on bentgrass, and may cause superficial whitening of perennial ryegrass foliage. Tenacity can also control or suppress several species of broadleaf weeds, as well as creeping bentgrass, nimblewill, and nutsedge in turf. This herbicide can be used for controlling newly emerged crab­grass in new seedings of Kentucky bluegrass, perennial ryegrass, and tall fescue. It should not be used in new stands of fine fescues. Label instructions sug­gest spraying only after newly emerged turfgrasses have been mowed twice, or 4 weeks after emergence. Tenacity can be mixed with prodiamine, dicamba, triclopyr, fluroxypyr, or carfentrazone when used on established turf. It is most effective in controlling summer annual grasses when mixed with a nonionic surfactant. Susceptible weeds turn white before they die, creating a highly visible treatment effect for 2 to 3 weeks following application.

Quinclorac is the active compound in Drive XLR8, Quinclorac 75DF, and several other products. This herbicide is primarily a postemergence herbicide for control of crabgrass, foxtail, and barnyardgrass, but not goosegrass. It also controls a number of broad­leaf weed species in established turfgrass. It controls newly emerged (up to 1 tiller), as well as mature annual grasses with more than 4 tillers. How­ever, this herbicide may not pro­vide complete control of annual grasses with 2, 3, or 4 tillers. Quinclorac can also be used as a preemergence and postemergence herbicide in new turfgrass seedings. The Drive XLR8 label suggests different appli­cation intervals following emergence of specific turfgrass species. For example, tall fescue seedlings can be treated 7 days after emergence, whereas Kentucky bluegrass should not be treated until 28 days after emergence. The label rate of Drive XLR8 is 64 fl oz/acre and should be combined with a methylated seed oil for optimum efficacy.

Topramezone is the active compound of Pylex herbicide and its mode of action and systemic activity is similar to mesotrione (Tenacity). However, Pylex offers some important differences in weed species controlled or suppressed. Like Tenacity, Pylex is labeled for postemergence control of crabgrass, barnyardgrass, foxtails, and goosegrass (Pylex is particularly effective on goosegrass) in newly established or established turf. It can be applied any time before seeding, or 28 days after seeding tolerant turfgrass species. Pylex controls numerous broadleaf weed species, as well as nimblewill and dallisgrass in turf. This herbicide is also labeled for selective suppression/control of bermudagrass in cool-season turf. Unlike Tenacity, Pylex is not labeled for control of creeping bentgrass. Best postemergence control of summer annual grasses occurs when Pylex is applied to plants with fewer than 4 tillers. Rates of Pylex range from 1.0 to 1.5 fl oz/acre per application for most turf stands, but applications up to 2.0 fl oz/acre can be made on pure stands of Kentucky bluegrass. The total seasonal amount of Pylex is restricted to 4.0 fl oz/acre. It is most effective in controlling summer annual grasses when mixed with a methylated seed oil or crop oil concentrate. Susceptible weeds turn white before they die, creating a highly visible treatment effect following application. The whitening effect can be reduced somewhat when triclopyr is mixed with Pylex. The addition of triclopyr also aids in suppression/control of bermudagrass.

A few other postemergence herbicides provide control of annual grasses but have limited species tolerance or use allowances. These include amicarbazone (Xonerate), fluazifop (Fusilade), MSMA products, quizalofop (Assure II), sethoxydim (Segment II), and sulfentrazone products. Be sure to carefully follow label directions when using these compounds for postemergence control of annual grasses.