What is the long term impact of inputs on annual bluegrass in greens?

In the northern U.S., golf course putting greens often consist of mixed populations of annual bluegrass (ABG, Poa annua L.) and creeping bentgrass (CBG, Agrostis stolonifera L.) (21). Creeping bentgrass is the initial and desired turfgrass species in these mixed populations due to its tolerance of low mowing heights, relatively high disease resistance, and good traffic tolerance (8).

Often considered a weed due to its invasive nature (2,13), ABG establishes in CBG to become a mixed-species putting green. Annual bluegrass is undesirable due to poor heat and drought tolerance in summer and low cold tolerance in winter (4, 14, 21, 23). Additionally, ABG tends to have more chronic disease and pest issues than CBG (6).

With varying results, golf course managers try to suppress ABG from CBG putting greens using many chemical and cultural management strategies, such as adjusting soil pH, plant growth regulators (PGRs) and nitrogen fertilizer programs. However, annual bluegrass is challenging to control when established within other turfgrass species due to its prolific seed production, large seed bank and ability to germinate over a wide range of time and under diverse environmental conditions (12).

Fertility to minimize ABG

Past research found nitrogen fertility to be an effective strategy to minimize ABG competitiveness within CBG. Concentrating nitrogen fertility applications to the summer months favors the growth of CBG, as ABG growth is more vigorous in the spring and fall months (6). Creeping bentgrass has a lower nitrogen requirement than ABG, reducing annual nitrogen applications may further suppress ABG (22).

Managing soil pH within a range of 5.5 to 6.5 favors CBG growth (22). Iron treatments reduce ABG shoot growth more than CBG (24). Others found iron sulfate (FeSO₄) may or may not decrease ABG in CBG putting greens (7,17). Applications of methiozolin on ABG control were similar or increased when mixed with iron sulfate (9).

Additional research evaluated the influence of nitrogen fertility and PGRs as independent and combination treatment applications on ABG control. Golf course superintendents routinely apply trinexapac-ethyl (TE), flurprimidol (FL), and paclobutrazol (PB) to favor one species over another. Applications of TE improve both ABG and CBG turfgrass quality (15). However, repeated applications of FL and PB reduced ABG in mixed-stands with CBG (3,11). Ethephon in combination with TE reduced ABG seedhead cover (1).

Our earlier work (10) evaluated low and high nitrogen rates, iron rates and PGRs on ABG control in a CBG putting green from 2010 to spring 2012. In this short-term study, the percentage ABG was less under the lower nitrogen rate (0.5 lb. per 1,000 ft² annually). ABG populations decreased with iron rates under low nitrogen, but no differences among iron rates in plots receiving high nitrogen (3 lb. per 1,000 ft² annually).

Flurprimidol was the most effective treatment for ABG control in this study. However, turf treated with FL had reduced quality ratings on greater than 50 percent of the rating dates than turf treated with TE or no PGR.

The long-term influence of these strategies on ABG control in mixed-species putting greens is unknown. Long-term nitrogen applications impact soil organic matter (OM) development on golf course putting greens (18).

Cultural practices to manage OM such as vertical mowing and aerification could reduce the density of desirable turf, disturb the top-soil surface, and bring ABG seeds to the surface, favoring germination (6). Lower seasonal nitrogen fertility may slow OM development, reducing the necessity for disruptive cultivation practices, and thus result in a reduction of ABG (10).

We conducted this research to continue the work from 2010 to 2012, previously reported (10). The objective of this study was to determine the long-term influence of nitrogen, iron and PGRs on ABG populations, turfgrass quality and soil characteristics in a mixed-species putting green of ABG and CBG.

Material and methods

A 7-year field study was initiated from 2012 to 2018 at the Joseph Valentine Turfgrass Research Center in University Park, Pa. After publishing the results from the initial two years (2011 and 2012) of the study (10), we continued the trial for seven more years to evaluate the long-term treatment impact.

Initially established in 2010, the experiment is on an existing research putting green consisting of ‘L-93 × CBG (75 percent) and ABG (25 percent). The soil texture was sand with an initial pH of 7.2, 1.8 percent OM, cation exchange capacity of 15.3 milliequivalents per liter per 100 g. of soil, 5.5 lb. phosphorous per 1,000 ft² (Melich-3 test), and 2.9 lb. potassium per 1,000 ft².

We mowed the putting green 5 to 6 days per week to a height of 0.1 inches using a walk-behind greens mower (Flex 21, The Toro Company). Sand topdressing was applied every 1 to 3 weeks during the growing season at a rate of 80 lb. per 1,000 ft². We made an average of ten sand topdressing applications per year between 2012 and 2018.

The surface and soil disruptions were limited to solid-tine cultivation to a depth of 2.5-inches in May and October of each year. Irrigation management did not allow the turfgrass area to wilt, and we implemented a preventative disease program.

The study was arranged in a two-by-three-by-three factorial with four replications in a randomized complete block design as previously described (10). Individual plots measured 3-by-6 feet. The main treatment factors included two levels of nitrogen, three levels of iron sulfate (FeSO₄) and three PGRs, including Primo MAXX (TE), Cutless MEC (FL), and no PGR (See Table 1 for rates and product information).

We used ammonium sulfate as the sole N source throughout the study. It was applied at rates between 0.5 and 3 lbs. per 1,000 ft² at various times to evenly distribute the total nitrogen for each treatment in nine applications throughout the growing season (See Table 2 application rates and timing).

Plant growth regulator and FeSO₄ treatments were initiated in late April or early May of each year and reapplied approximately every three weeks for a total of nine applications annually. All treatments were applied using a CO2 backpack sprayer equipped with a Teejet flat fan nozzle.

Percentage ABG and turfgrass quality were evaluated monthly from April through October. Percentage ABG was rated using a 3 × 6 foot rating grid with 253 intersections and recording the presence of ABG at each intersection for each plot. Percentage coverage was calculated by dividing intersections with ABG present by 253.

We visually assessed turf quality on a 1 to 9 scale, where 1 = brown or dead turf, 7 = minimum acceptable quality level for a golf course putting green, and 9 = optimal uniformity, density and green color. Monthly evaluations of percentage ABG and turf quality were combined and averaged each year from 2012 to 2018. Annual data also were combined to obtain a total average across the entire study.

We collected soil samples from each plot on 17 Nov. 2014 and 5 Oct. 2017, and soil characteristics, including pH, OM, and Mehlich-3 extractable elements, were determined (Brookside Laboratories). We estimated OM by weight loss-on-ignition (20). Soil pH was measured in a 1:1 (soil/water) solution using a pH meter (16). National Weather Service provides the meteorological data for the experiment location.

Data were analyzed using the MIXED procedure of SAS (19) and means separated according to Fisher’s protected LSD test (P ≤ 0.05). Nitrogen, iron, PGRs, and year are the main treatment factors, and their interactions are the fixed effects, with replication treated as a random factor within the model.

Results

A significant four-way interaction of nitrogen x PGR x iron x year influenced ABG populations throughout this experiment. Averaged across all treatments, annual ABG populations were highest in 2012 and 2015 (44 percent) and lowest in 2016 (23 percent) (Table 3). Generally, the greatest ABG population was observed in 2012 and tended to decrease until 2018 under each treatment combination, but there are several exceptions (Table 3).

A three-way treatment interaction (PGR x iron x year) and a two-way interaction of (N x year) occurred for turfgrass quality. For PGR treatments, FL resulted in lower turfgrass quality compared with plots treated with TE or no PGR across all FeSO₄ treatment rates in four of the seven years of this study (data not shown). In the other three years of the study, there were no statistical differences between PGR treatments. The influence of FeSO4 was minimal within PGR treatments for the study.

High N combined with TE or no PGR treatment resulted in a higher ABG population from 2015 to 2017 (Table 3). The highest ABG population (80.5 percent) was in plots treated with high N combined with TE and no FeSO₄ treatment in 2016, whereas the lowest ABG population (1.7 percent) was under high N combined with no PGR and no FeSO₄ in 2017.

Nitrogen influenced ABG populations in each year of the study. From 2012 to 2014, the percentage of ABG decreased from 38 to 14 percent under low N fertility and from 49 to 31 percent under high N fertility (Figure 1a). Nitrogen influenced turfgrass quality every year except 2013, with the high rate resulting in increased quality compared to the lower rate (Figure 1d).

Differences in percentage ABG between N treatments became much greater in subsequent years. From 2015 to 2017, ABG populations in low N fertility ranged from 14 to 16 percent, whereas high N rate treatments during this period had ABG populations from 42 to 49 percent (Figure 1a).

The high N rate resulted in greater ABG populations than the low N rate from 2012 to 2017, but these differences were reversed in the final year (Figure 1a). In 2018, the percentage of ABG in high-N-rate treatments decreased to 16 percent and was less than that in the low-N-rate treatment (22 percent, Figure 1a).

We observed slight differences in percentage ABG among FeSO₄ rates in most years (Figure 1b). ABG populations were generally similar or lower in plots treated with iron each year. The main effects of FeSO₄ had little impact on turfgrass quality, and differences among rates were only present in the final year (Figure 1e).

Plant growth regulators influenced ABG populations throughout the trial. Applications of FL significantly lowered ABG compared with the TE and no PGR treatments each year (Figure 1c). The greatest differences in percentage ABG occurred between 2015 and 2017, where populations in FL-treated plots averaged 5-7 percent ABG, whereas plots treated with TE averaged 41-48 percent. Some differences among PGRs were present for turfgrass quality, and plots treated with FL generally resulted in reduced quality compared with plots treated with TE or no PGR (Figure 1f).

We observed a significant three-way interaction of N × iron × PGR for ABG data pooled across all years. Under low N treatments, TE applications resulted in 6 percent higher ABG compared with the nontreated (Figure 2).

Regardless of the N rate, FL consistently produced the lowest ABG populations. We did not see differences among FeSO₄ treatments within each PGR treatment under low fertility. However, under high fertility, FeSO₄ lowered ABG populations within each PGR.

We observed slight differences in OM among the N main effects (Figure 3a). Soil nitrogen rate influenced soil iron, and there was an iron x year interaction. For the main effect of nitrogen, soil iron was lowest within plots treated with high N (Figure 3b).

Nitrogen rates also influenced soil pH with a two-way interaction of N × year. In 2014, soil pH was higher within low nitrogen plots (7.39) compared with high nitrogen plots (7.26), but there were no differences in 2017 (Figure 3c).

In general, soil iron increased with increasing iron rates in 2014 and 2017. However, in 2014, no differences were observed between plots receiving FeSO₄, or in 2017, plots receiving 0.25 lb. per 1,000 ft² applications and plots receiving no FeSO₄ (Figure 4).

Discussion

In this long-term study, the lower nitrogen rate and FL applications had reduced ABG populations. The result agrees with the findings from the initial two years of the study (10). The long-term impact of these two treatments demonstrated that ABG populations decreased to a low level around 2014 or 2015, where it remained stable for 3 to 4 years.

These results concur with previous studies that demonstrated lower ABG populations in CBG putting greens with low nitrogen fertility and FL applications (3,6). The lowest ABG population in this study was found within turfgrass stands receiving the low N rate in combination with FL.

Although the application of FeSO4 resulted in decreased ABG populations in the initial two years of the study (10), its influence was less impactful long-term. This finding indicates the contribution of iron to ABG control may occur in the first few years of application but that the long-term impact may be less influential. Regardless, the effect of iron supports previous findings that high rates provide more reduction of ABG shoot growth than for CBG (24).

Annual bluegrass populations varied from year to year. After several years of applications, ABG populations remained relatively stable until 2018, during which treatments generally resulted in higher ABG populations (e.g., high N and the PGRs TE and none) exhibiting a sharp decline in percentage ABG. Weather conditions may explain some changes in ABG populations at the end of the experiment.

Higher precipitation in 2018 may have caused nutrient leaching within plots, thus creating lower N availability within the high nitrogen treatments. However, we did not notice any significant decline in turf quality, scald in the summer months, or that possible occurrence of ABG winterkill. Despite these potential environmental factors, their role for reduced ABG populations in 2018 remains unclear.

Turfgrass quality was generally lower under low N and FL treatments. Low N fertility could reduce turf quality for both ABG and CBG (2,22). Additionally, the negative impact of FL on turf quality is well documented and agrees with previous reports (3,5).

In the 2010 and 2011 research, turfgrass quality was acceptable (≥7) but decreased to between 5 and 6 in 2012 (10). This decline in quality may be due to a reduction in soil OM in the first two years of this study, from 1.8 percent in 2010 to 1.6 percent at the end of 2011. Poor turfgrass quality within the low N plots may have been due to the mineralization of nutrients in the soil in the first two years of the study. Exhausting these resources may have taken 2 to 3 years before negatively impacting turf quality. We did not see soil iron accumulate in plots treated with 1.0 lb. FeSO₄ per 1,000 ft² when sampled in 2017.

In summary, our results confirm that varying N, PGRs, and iron rates influence ABG populations within mixed stands of CBG. Over many years, the influence of repeated FeSO₄ applications became less influential to ABG populations.

The most significant impact on ABG populations was from applications of FL in combination with relatively low seasonal N rates. Still, we observed reductions from FL under the higher seasonal N treatment. Superintendents should use caution when initiating these programs, as turfgrass quality decreased in the third year of this long-term study.

These findings agree with anecdotal reports of turfgrass decline 3 to 4 years into similar programs. However, later in this study, turfgrass quality was at or below an acceptable level within treatments that effectively reduced ABG populations.

Superintendents may be willing to accept slight reductions in visual quality to eradicate ABG. Regardless of the influence of treatments in this experiment over nine years, other factors such as weather influenced seasonal fluctuations in ABG populations.