Phosphorus and soil pH affect annual bluegrass (Poa annua L.) competitiveness in creeping bentgrass (Agrostis stolonifera L.)
Creators: name: McNally, Brandon Cole
Publisher: name: No Publisher Supplied
Year: 2022

description: Annual bluegrass (Poa annua L.) is a problematic winter annual weed that has poor tolerance to biotic and abiotic stresses. It is also a high maintenance turfgrass that requires high nutrient inputs to survive and is difficult to control in a mixed sward with creeping bentgrass (Agrostis stolonifera L.). It is problematic in the mid-Atlantic and Northeastern US both in mature turfgrass swards and when establishing a field through seed. Previous research indicates annual bluegrass growth, establishment, and infestation is favored in high phosphorus (P) soils and germination is reduced in acidic soils. Creeping bentgrass has demonstrated sufficient growth in low P soils in monoculture, however, it is unclear what soil test P value may provide creeping bentgrass an advantage over annual bluegrass when these species are grown as a mixture during seeded establishment or from tillers. Two greenhouse studies were conducted in 2020 and 2021 to determine P effects on sward composition of annual bluegrass and creeping bentgrass. The first study evaluated the effects of P when annual bluegrass and creeping bentgrass were seeded as a mixture. Phosphorus applications (12.5 to 100 kg ha⁻¹ P) at seeding did not influence annual bluegrass cover at the conclusion of the experiment; however, annual bluegrass cover was reduced when no P was applied. Similarly, creeping bentgrass cover was unaffected by P application at seeding, but less was found in the treatments without P fertilization. Green cover, turfgrass quality, and biomass all increased with P application, but little to no differences were found among 12.5 to 100 kg ha⁻¹ P treatments. The 0 kg ha⁻¹ P treatments failed to reach 7% green cover, had poor turfgrass quality (3.0), and the least dry aboveground shoot biomass production. The second study evaluated P effects on annual bluegrass and creeping bentgrass when tillers were planted in competition at a low (5.6) and high (7.1) soil pH. In this study, annual bluegrass response was greatest at the highest P rate (12 kg ha⁻¹) in the high pH treatment; this resulted in a Mehlich-3 P soil test value of 10 mg kg⁻¹ at 10 days after planting. Creeping bentgrass cover was greatest in the 3 kg ha⁻¹ P, low pH treatment; this treatment resulted in five times as much creeping bentgrass cover as annual bluegrass cover. All P-receiving treatments (3 to 12 kg ha⁻¹) in the low soil pH had similar green cover, but the greatest competitive advantage for creeping bentgrass was observed at 3 kg ha⁻¹ P. A competitive advantage for creeping bentgrass was also observed in the 0 kg ha⁻¹ P, low soil pH treatment, but green cover and turfgrass quality were poor. This experiment found a narrow soil test P value range where annual bluegrass was less competitive than creeping bentgrass (3 to 6 mg kg⁻¹ Mehlich-3 P at 10 days after planting), with slightly higher Mehlich-3 P soil test values required for adequate turfgrass quality and green cover in the low pH soil (≥ 4 mg kg⁻¹). However, the competitiveness of annual bluegrass with creeping bentgrass increased with P rate in low P soils. Future research should further examine the lower range of soil test P values that may reduce the competitiveness of annual bluegrass in creeping bentgrass during seeded establishment. Evaluating a range of soil test P values in other P-depleted soils that are finer-textured than the sand used in these experiments is also warranted.