By Michael Bekken and Trygve Aamlid, Norwegian Institute of Bioeconomy Research (NIBIO)
Annual bluegrass must be one of the greatest conundrums of the turfgrass world. It’s the undisputed nemesis of many golf course superintendents. It’s often a pain to manage, and yet it’s ever present. It dominates during the growing season but dies back in the winter. In most cool-season turfgrass climates, it’s the grass everyone has, but that few want to admit to having.
And yet, there are exceptions to these general rules. Many golf courses in Sweden and some in the northeastern and northwestern U.S. have entirely annual bluegrass greens. For these golf courses, annual bluegrass is a friend. But as winter injury on golf courses becomes more severe, annual bluegrass is becoming an increasingly unreliable and untrustworthy friend.
Researchers in the WinterTurf project wondered, if winterkill on an annual bluegrass green was sufficiently severe, would this provide the opportunity to establish a creeping bentgrass surface? A field trial was carried out at NIBIO Landvik south Norway (58°N) and several locations across the U.S.
The trial began with simulating winterkill by applying glyphosate to an annual bluegrass green in the late autumn of 2023 and 2024. In the following spring of 2024 and 2025, four different cultivars of creeping bentgrass were seeded, including Declaration, 007, Pure Distinction, and Pure Select. One variety of annual bluegrass, Two Putt, was also seeded as a positive control and a sixth and final treatment was unseeded as a negative control. In addition, three different seeding dates were tested to determine if seeding date would influence the success of creeping bentgrass establishment. In both 2024 and 2025, the first, second, and third seeding dates were determined by the soil temperature during the day exceeding 5°C, 7.5°C, and 10°C, respectively (Figure 1).
During the spring, photos and visual assessments of coverage were made weekly. In early July, all plots were sprayed with Focus Ultra (cycloxydim), a product that kills bentgrass but does not harm annual bluegrass. Two weeks later, because the bentgrass was brown and the annual bluegrass was still green, assessments could be made of the sward composition. These assessments were made on 24 July 2024 and 22 July 2025.
Somewhat surprisingly, given the generally poor establishment of creeping bentgrass at low soil temperatures, the first seeding date led to a higher proportion of bentgrass establishment than the second and third seeding dates. In the first seeding in 2024, the coverage of 007 bentgrass reached an average of 58%, followed by Pure Select at 51%, Pure Distinction at 38%, and Declaration at 30% (Figure 2). In 2025, the first seeding date also led to the best establishment of creeping bentgrass; however, bentgrass cultivars that were most successful in establishing where different than in 2024. In 2025, the coverage of Declaration and Pure Select were highest at 38%, followed by 007 and Pure Distinction at 12% (Figure 2). As such, the results of this study unfortunately cannot point to a specific cultivar of bentgrass that most successfully establishes into annual bluegrass putting greens. The study results do seem to suggest that seeding once daytime soil temperatures reach 5°C may increase the success of bentgrass establishment on a winter-killed annual bluegrass green.
The persistence of annual bluegrass is undoubtedly impressive. Even after the green had been sprayed with glyphosate in the autumn, and creeping bentgrass was seeded into the dead green the following spring, the average annual bluegrass coverage across the four treatments seeded with bentgrass was 56% in 2024 and 75% in 2025. The annual bluegrass seedbank is persistent!
Practically speaking, these results seem to imply that if a golf course superintendent would like to introduce bentgrass into their turfgrass sward, seeding into a severely winter killed annual bluegrass green may lead to the establishment bentgrass coverage on that green. However, with only one seeding, the coverage that will be achieved, at least according to the results of this study, will likely be at or less than 50%. As shown in an earlier ICE-BREAKER project (Melbye et al. 2024), repeated overseeding over years will be necessary to achieve a higher proportion of creeping bentgrass.
That said, if a golf course has sufficient funding and wishes to quickly and completely transition from an annual bluegrass monoculture to a bentgrass monoculture on greens, the ideal method of achieving such a transition would likely be to remove the rootzone, replace with clean green-mix, and then install creeping bentgrass sod.
We are in the process of writing up the results of this study for publication. For the golf courses managing annual bluegrass, we hope the research provides them with useful information about which strategies may be most appropriate for their particular situation moving forward.