There has been a clear trend in recent years of increased overseeding of Poa annua-dominated greens with bentgrass in the UK and Ireland. This has been driven predominantly by a desire for more sustainable turf management, particularly given current and future restrictions on fungicides and water use. The demand for improved performance, especially year-round playability also remains an obvious and significant driver.
Given this trend, and the investment involved, it seems prudent to look closely at bentgrass selection, at both species and cultivar level.
Overseeding of golf greens with bentgrass
UK Performance Data
The most comprehensive, reliable, and independent data on bentgrass performance for golf greens in the UK can be found in the BSPB Turfgrass Seed trials, run by STRI at Bingley. An ongoing set of biennial close-mown trials assess new and existing cultivars against established controls, which are published each year in Table G1 (see Figure 1).
Figure 1: Table G1; BSPB Turfgrass Seed 2024
Any Agrostis bentgrass can be submitted for trial – currently there are only two species listed, browntop (capillaris) and creeping (stolonifera), although velvet (canina) and Highland (castellana) have appeared in the recent past.
The data in Figure 1 summarises the performance of a range of browntop and creeping varieties, and the two species can be compared directly against each other.
Browntop varieties at the top of the list offer superior mean performance (shoot density and visual merit), as well as finer leaf texture. Recent cultivars such as Charles and Arrowtown are better than all the creeping bents in the BSPB trials, and even long-standing browntops like Manor and BarKing are comparable with the latest creeping varieties.
An additional point to note perhaps is the absence of certain other creeping bentgrass varieties in the UK market such as Crystal Bluelinks and Pure Distinction from the table.
Year-round Performance
A logical next question is why do browntop bentgrasses perform better than creeping varieties in the BSPB trials? One significant reason is the fact that browntop bentgrasses exhibit stronger year-round growth than creeping, which then translates into better recovery and performance during winter and spring. This can be seen by analysing monthly assessments from BSPB trials and is also illustrated in Figure 2.
Figure 2: Data from Golf Green Trial STRI/Barenbrug 2010
This data is from an independent UK trial run by STRI on behalf of Barenbrug. The trial was conducted on a sand-based green and also incorporated wear simulation throughout the trial.
It can be seen that browntop and creeping visual merit is broadly similar early on in the trial, but that creeping bentgrass as a species suffers a dip in the winter period 2008/09, and recovers slowly during summer 2009 before another drop off in winter 2009/2010 prior to the conclusion of the trial. The lower visual merit scores arise from lack of density, turf-thinning and subsequent ingress of annual medaowgrass.
At the conclusion of this golf green wear trial, browntop bentgrass plots contained an average of only 9% Poa annua ingression, compared to 21% in the creeping bents. Full details are available at request from Barenbrug as part of Golf Greens Trials Booklet.
Shade Tolerance and DLI data
One potential reason that browntop bentgrasses are better adapted for year-round growth in the UK versus creeping bentgrass may be their superior tolerance of low light conditions.
A recent publication by the USGA (Light The Way To Healthy Putting Greens (usga.org)) details relative shade tolerance of turfgrass species in terms of their approximate Daily Light Integral requirements for healthy turf. This is summarised in Figure 3.
Figure 3: DLI requirements of cool-season turf species. Adapted from USGA publication cited in text
It can be seen that browntop bentgrass requires less DLI than creeping bentgrass from the available species data. The numbers cited for the two species are approximately 24.5 mol/m2/day for creeping bent and 17 mol/m2/day for browntop. It should be stated clearly that cultivar differences among species are likely to differ widely, and that the subject needs more study to solidify conclusions.
However, if these DLI figures are broadly accurate, this difference in shade tolerance could contribute to around six to eight weeks of additional growing conditions in terms of light availability in the UK climate (Figure 4; data extrapolated from DLI App in Shiny Apps | Asian Turfgrass Center).
Figure 4: DLI measurements for central England using NASA data for 2022/23. Creeping bentgrass with a requirement of 24.5 mol/m2/day falls below the DLI curve for approximately six weeks longer than browntop’s theoretical requirement of 17 mol/m2/day. In other UK locations, this extends to eight weeks.
Disease Tolerance
A large driver behind many overseeding and species exchange projects is to improve disease tolerance of Poa annua-dominated greens in response to diminishing fungicidal control of turf pathogens.
Micodochium is a ubiquitous concern. It is fair to say that both browntop and creeping bentgrasses offer strong tolerance at the species level, particularly in comparison to Poa annua. A lack of reliable data exists for cultivar differences in UK conditions – this is being looked at by BSPB, and it is hoped that a change in Turfgrass Seed protocols will deliver data to the booklet in the not-too-distant future.
Overseeding with a mixture of cultivars/species, rather than a monoculture may be a precautionary step if Microdochium is a major reason for your decision to overseed. The additional capacity for growth and recovery of browntop in early spring should not be overlooked also.
The threat of Dollar Spot is fast becoming problematic in the UK, particularly in certain geographical areas. Dollar Spot has been an issue on bentgrasses in USA for a while, but variability of the disease in the UK market is not yet well-understood.
In general, browntop (colonial) bentgrass is thought to have superior tolerance to the pathogen in USA (Plumley et al., 2000 & Hempfling, 2024 (online)) compared to creeping bentgrass, although it should be noted that recent breeding advances in stolonifera has improved tolerance levels. Without extensive data, relying on monoculture overseeding, particularly with creeping bentgrass, may again not be prudent if Dollar Spot is a future concern.
Conclusion
Overseeding of golf greens with bentgrass is increasing in the UK and Ireland. Both browntop and creeping bentgrass mixtures and varieties are marketed for their suitability, yet available scientific data and research points to browntop being the more appropriate choice, in terms of turfgrass traits (density, fineness and aesthetics), year-round demands of the golf industry, shade tolerance and disease tolerance in UK conditions.
With browntop seed being more affordable than creeping also, it’s a wonder why all course managers aren’t banking on browntop.
References
Plumley, K.A., W.A. Meyer, J.A. Murphy, B.B. Clarke, S.A. Bonos, W.K. Dickson, J.B. Clark, and D.A. Smith. 2000. Performance of bentgrass cultivars and selections in New Jersey turf trails. Rutgers Turfgrass Proc. 32:1-21.
Hempfling, J. 2024. Dollar Spot demystified: Proven Strategies from the USA and UK for Effective Integrated Management. Envu/OAS. April 2024.
ALL BENT
A 100% browntop bentgrass blend designed for overseeding and species exchange in golf and bowling greens
