Smooth crabgrass (Digitaria ischaemum) is a common summer annual weed found throughout much of the United States. Its long leaves and prostrate growth give it the potential to become a serious problem in both closely mowed areas such as greens or tees, and in higher mown areas such as home lawns. This is especially true of thin and high-trafficked areas. Typically, smooth crabgrass encroachment can be prevented, or greatly diminished, by application of a preemergent herbicide such as dithiopyr, pendimethalin, or prodiamine when there is a 24-h period of soil temperatures greater than 10 °C (McCarty et al., 2001). Multiple applications may provide even greater crabgrass control, as Derr (2004) reported with prodiamine.
While accurate timing of preemergent herbicides can provide excellent crabgrass control, severe infestations can still occur if the herbicides are applied after seedlings have germinated. Postemergent herbicide options such as fenoxaprop, fluazifop, and quinclorac are all labeled for crabgrass control in turf (BASF Corporation, 2012; Bayer Environmental Science, 2011; Syngenta Crop Protection, 2014). Mesotrione, a newer active ingredient in the turfgrass industry, has postemergence activity on crabgrass that is enhanced when nitrogen is applied within 3 d before application (Beck et al., 2015).
Despite the existence of postemergence options, control may still not be 100% or may require multiple applications for good control (Dernoeden, 2001). The bleaching potential of mesotrione on certain desirable turf species also limits where it can be used for crabgrass control. In addition to these issues, the recent restrictions placed on monosodium methanearsonate, an effective postemergent crabgrass herbicide, shows the environmental concerns with herbicide applications (Environmental Protection Agency, 2016). Environmental issues and desire for more sustainable practices are both a growing concern turf managers face when using pesticides. One method for reducing environmental effects from chemical crabgrass control is to reduce usage rates. Dernoeden (2001) examined reduced preemergent herbicide rates over a 3-year time period and found only benefin with trifluralin and prodiamine alone still provided acceptable control in the 3rd year, among the various herbicides tested. This indicated reducing application rates of several preemergence herbicides may not be appropriate for controlling crabgrass.
In addition to reducing current chemical rates, other studies have been conducted examining natural crabgrass control methods. McDade and Christians (2001) examined humic acid and soybean oil to improve the efficacy of corn gluten hydrolysate, which has been reported to have preemergent crabgrass activity. They reported no beneficial response from the additives but concluded corn gluten hydrolysate may still be beneficial. Earlywine et al. (2010) tested oriental mustard seed meal as a soil treatment to suppress crabgrass germination. Treatments were effective but negatively affected desirable turf species. Mahoney et al. (2014) used liquid carbon dioxide in a study on crabgrass in established turf species and found high usage rates to be effective in controlling juvenile crabgrass plants, with desirable turf species temporarily affected.
Another technique of reducing herbicide usage is through cultural control methods. One of the most studied cultural control methods for crabgrass is mowing. Dernoeden et al. (1993) found a mowing height of 8.8 cm provided good crabgrass control compared with shorter mowing heights and also provided the best turf quality on one variety of tall fescue. The authors also reported that nitrogen applications helped reduce crabgrass control in plots not treated with herbicides. Similar results have been reported for chewings fescue (Festuca rubra ssp. commutata), kentucky bluegrass (Poa pratensis), perennial ryegrass (Lolium perenne), blends of tall fescue, and a mixture of cool-season turf species (Busey 2003; DeBels et al., 2012; Kowalewski et al., 2010; Voigt et al., 2001). However, Hoyle et al. (2014) reported that increased mowing heights on bermudagrass (Cynodon dactylon) did not decrease crabgrass incidence when no pre- or postemergent herbicides were applied. In a related study, Gannon et al. (2015) reported better crabgrass control on bermudagrass areas treated with preemergent herbicides when mowing heights were higher (3.8 vs. 1.5 cm of cut) in one of their two evaluation years.
While the previous studies conducted on cool-season turfgrasses have shown decreased crabgrass incidence at higher mowing heights, they all used a consistent height of cut for the entire growing season. Also, increased mowing heights typically result in better quality, but lower heights of cut within a species tolerance range can result in increased plant density over higher heights of cut (Turgeon, 2005). Therefore, the first objective of this study was to determine if lower heights of cut during spring and fall for cool-season grasses would increase stand density sufficiently to limit smooth crabgrass encroachment. The second objective was to determine if seasonal changes in mowing heights on tall fescue would allow a reduced rate of prodiamine to provide acceptable smooth crabgrass control.
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