Suppression of Poa annua in bentgrass putting greens is an issue for turfgrass managers, with control often relegated to the use of plant growth regulators. Thus, any additional management methods that may help with Poa annua suppression are always of interest. These methods include fertilization practices, such as the use of iron.
To explore this idea of integrated annual bluegrass control, researchers at Virginia Tech studied combinations of iron sulfate (FeSO4) and plant growth regulators (PGRs) over a two-year period, applying the treatments to a 25-year-old Penneagle creeping bentgrass putting green.
Specific treatments were rates of FeSO4 (which was 19% Fe + 11% sulfur) applied at total rates of 0, 0.25, 0.5 and 1 pound/1,000 square feet (0, 12.2, 24.4 and 48.8 kilograms/hectare) per year (note that these rates are FeSO4, not Fe), with the FeSO4 applied biweekly in March-October.
Growth regulators were: (1) seaweed extract (SWE); (2) paclobutrazol, applied at 0.007 pound a.i./1,000 square feet (0.36 kilogram a.i./hectare) in March-June and September-October, and 0.004 pound a.i./1,000 square feet (0.18 kilogram a.i./hectare) in June-August (note that these rates are much higher than label recommendations); or (3) no PGR. The PGRs were also applied biweekly.
All materials were applied as foliar sprays and allowed to remain on the foliage for 24 hours, after which the turfgrass was watered. The nitrogen source was ammonium sulfate applied uniformly every two weeks (March-November) at 0.1 pound nitrogen/1,000 square feet (4.8 kilograms/hectare).
Collected data included percent annual bluegrass infestation — using a grid to count plants in each plot — and turfgrass quality.
At the beginning of the study, the creeping bentgrass green had a natural infestation of about 45% annual bluegrass. Over the two years of treatment applications, the collected data began to show an interaction between the PGRs and the Fe treatments. Basically, application of paclobutrazol reduced annual bluegrass populations to less than 10%, and this happened no matter how much FeSO4 was applied. So, adding FeSO4 provided no additive benefit to the paclobutrazol for annual bluegrass control. Application of seaweed extract failed to reduce annual bluegrass, with or without FeSO4 in the mix.
And applications of FeSO4 by itself, without any PGRs? In those treatments, the infestation of annual bluegrass was reduced to an average of 21%. However, it should be noted that annual bluegrass infestation also naturally declined in every plot (even the controls) to an average of 37%, likely a result of competitive creeping bentgrass in a low-phosphorus environment.
So, the addition of FeSO4, by itself, did reduce populations of annual bluegrass a little bit. Should you consider using FeSO4 as part of your bentgrass management program? Well ... maybe.
First, adding FeSO4 to the aggressive paclobutrazol program did not improve annual bluegrass control. Second, the medium rate of FeSO4 often provided the highest turfgrass quality — a result of the dark green color. The highest rate of FeSO4 was too much, and plots were too dark.
Before definitive recommendations can be made about the inclusion of FeSO4 for annual bluegrass control, it probably needs to be examined further, in combination with lower rates of paclobutrazol that fit within label recommendations.
Source: Ervin, E.H., N. Reams, X. Zhang, A. Boyd and S. Askew. 2017. An integrated nutritional and chemical approach to Poa annua suppression in creeping bentgrass greens. Crop Science 57:567-572.
Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and the 2019 president of the Crop Science Society of America. She is a 20-year member of GCSAA.