Turf experts Gary Beehag and Peter McMaugh AM look at the recent Research project investigating thatch-mat and root architecture beneath new bentgrass putting greens.
Accumulation of partly-decomposed organic matter beneath turfgrass is a normal phenomenon which progressively increases because of an imbalance between its rate of accumulation and decomposition. The biological causes, consequences and management of excessive organic matter accumulation beneath turfgrass swards is widely documented, particularly of aged golf greens.
The most widely published definition that describes thatch is probably “an intermingled organic layer of dead and living shoots, stems and roots that develops between the zone of green vegetation and the soil surface”. The combined term ‘thatch-mat’ describes the total accumulation of organic material above the original level of construction sand.
Superintendents comments and trial results has led to several authors stating the newer-generation, bentgrass cultivars have a propensity to produce much greater amounts of thatch accumulation particularly during the first 12-24 months after seeding. Other authors have further stated a time frame of a seeded bentgrass green to reach “maturity” ranges only from 2-3 years, largely based on a stabilised infiltration rate. Nonetheless, maturing bentgrass putting greens require adequate thatch-mat thickness and surface firmness to ‘hold’ a golf ball without undue plugging or bounce when playing an approach shot.
This is why the management of thatch-mat architecture becomes critical. Hence, two key questions of superintendents after establishing a new bentgrass putting green arise;
Currently, these two and related questions remain largely based on subjective assessments of firmness by feel rather than any quantitative measurements carried out by superintendents.
Understanding of the dynamics of bentgrass thatch-mat architecture and root growth during the first few years of a new green is a fundamental aspect of their management. Empowered superintendents are then able to make informed decisions about the scheduling of key cultural practices (e.g. topdressing) when bringing a new putting green into play and, to manage thatch-mat accumulation and putting green performance to acceptable levels.
Example of a moderately-stratified sample. Note sand-filled tine hole
For the purpose of this research project, the authors have adopted the combination term ‘thatch-mat’. The primary objective of this research project was to investigate and measure thatch-mat architecture and thickness on relatively new putting greens. The seeding date, type of construction and topdressing sands and grow-in practices was sought from the respective superintendents and recorded as management variables.
This study represents the first of its kind ever conducted in Australia. The authors are not aware of any similar published work conducted on new bentgrass greens in Australasia. Sampling was conducted on 30 bentgrass putting greens representing 14 golf courses around Sydney over an eight week period in June-July 2017. The courses represented private, semi-private and public courses each with 1-3 greens. Three samples from each green were taken using a split profile sampler to the maximum depth of 200mm.
The thatch-mat layer was defined as the vertical distance from immediately below the layer of green verdure to the original level of the construction sand. Thatch-mat thickness was measured using a digital readout, Vernier calliper. Thatch-mat values for each sample (front, centre and rear) were recorded separately and later averaged for each green. The construction sand in the lower section was further assessed for colour and particle shape. Each sample was assessed visually for the depth of primary root growth, the degree of secondary root development and lateral branching.
Clearly noticeable, structural and colour differences of the thatch-mat architecture were apparent between most samples. Microscopic examination of samples revealed the majority of bentgrass stems were horizontally-orientated whilst primary and secondary roots were largely vertically-oriented. In all samples from all courses, irrespective of the cultivar, age of the green and thatch-mat thickness, bentgrass stems were totally absent in the underlying construction sand.
Overall, the observed thatch-mat architecture between samples ranged from very distinct, stratification of partly-decomposed organic matter and topdressing sand (see main photo, page 50) to several layers of organic matter and topdressing sand (see photo above left) all the way to a relatively homogenous (i.e.; without stratification) organic matter/sand thickness (see photo above right).
The results, based on the nominated methodology, not unexpectedly showed that thatch-mat thickness varied widely between greens sampled. Thatch-mat thickness varied only to a slight degree (up to 5mm) within most individual greens sampled but varied significantly between most greens sampled.
In attempting to understand the environmental and management-induced dynamics at play contributing to the thatch-mat architecture and its thickness against the age of greens, the authors considered it appropriate to first consider thatch-mat thickness against age of all greens sampled then, secondly, consider the greens sampled by grouping 2-3 of the greens which were established over similar periods during a 3-4 month time frame.
The greens sampled within the one course were reported to have undergone identical or a very similar key cultural programmes (i.e.; nutrition, irrigation, topdressing etc) from the time of their respective establishment and opening periods. Thus, trends of thatch-mat development should be similar.
Overall, the general trend of greens sampled was increased thatch-mat thickness over time. The greatest thatch-mat thicknesses were 30-39mm and 23-27mm and the least was 7mm.
For the purpose of this investigation, thatch-mat data was extrapolated against the time of establishment of greens sampled as a possible indication of thatch-mat thickness attained after one year and the length of time to attain a thickness of 5mm in each case indicating a range of time frames.
Greens sampled have been grouped (Table 1) according to their month/year of establishment. Within each group, there are some similarities but also differences of the rate of thatch-mat development; thus the extrapolated thickness at one year and time to achieve 5mm.
Whilst the figures for one year and thickness have been extrapolated, they can be useful to demonstrate that environmental (e.g. construction sand), key cultural management (e.g. nutrition, irrigation and topdressing) for each green sampled would have partly contributed to the range of thatch-mat thicknesses and associated stratification.
Example of bentgrass root growth sample greater than 200mm
Primary root depth in the majority of samples was observed in the range 100-150mm. The shortest root growth was 50-100mm and maximum root length was 200-250mm. There was no apparent correlation between measured root growth and age of greens sampled except to say that the greens sampled having greatest root and branching were between 1-1.5 years in age. The observation of the wide discrepancy of primary and secondary root growth among samples prompted a physical analysis on selected samples of the underlying construction sand.
Visual observations conducted of the construction sand in all greens sampled indicated a range of textures. In total, eight construction sand types were analysed for particle size distribution (PSD) using the NATA-registered laboratory Ground Science Pty Ltd. The PSD (Table 2) of greens sampled varied between narrow to wide distributions among most samples.
In the majority of cases, the tested sands recorded a PSD in accordance with the current USGA (United States Golf Association) specifications for putting greens. Sand grain shape (roundness and sphericity) varied between sand types. Clearly, differences in sand colour and grain shape indicated the construction sands were sourced from numerous sand quarries. We hypothesise that the pore size geometry of sands is one factor to have influenced primary root growth and secondary root branching.
|Very Coarse Sand||1||2.4||9.6||0.1||0.5||0.7||4.4||0.1||0.1|
|Very Fine Sand||0.053||11.1||3.8||1.5||8.9||2.6||9.4||3.1||0.3|
|Silt Plus Clay||<0.053||3.7||2.8||1.4||6||1.5||2.9||1.2||0.3|
Responses to a Superintendents’ questionnaire revealed different choices of bentgrass cultivars and key, cultural practices adopted during the grow-in (i.e.; establishment to opening) and the maintenance (i.e.; opening time to sampling date) phases.
The reported grow-in phase time of greens sampled ranged from as short as four weeks (one course) to 8-16 weeks (majority of courses) all the way through to 19-21 weeks.
Most greens sampled were reported to have been tined (i.e.; hollow and/or solid) during the grow-in phase from as few as 1-2 times (two courses) to 4-5 times but as numerous as 12 times (two courses) during this period. The number of reported sand topdressings varied from 2-3 (five courses) up to 6-9 times (two courses).
The maintenance phase time of greens sampled varied significantly, depending on time of opening and sampling dates. All greens sampled were reported to have been topdressed during the maintenance period. The number of topdressings varied from 1-21 times but mostly 5-7.
Sand types used to topdress greens sampled were reported to range from relatively fine (dune sand) either as part of a frequent (2-3 weeks) topdressing programme or a medium-coarse (river sand) twice yearly.
Example of an homogenous sample
The results and questionnaire responses from this investigation of thatch-mat architecture of relatively new bentgrass putting greens clearly indicated that the physical form and rate of bentgrass architectural development is a dynamically-complex phenomena and which occurs rapidly and is influenced by a multiplicity of environmental and management-induced factors specific to each green.
The results of the principal focus of this investigation leads to the interpretation that the rate, amount and physical nature of organic accumulation to primarily be a management-driven response and interpreted largely as a reflection of the respective tining and especially of sand topdressing programmes between greens sampled.
The homogenous thatch-mat architecture, as observed beneath the two intensively-managed sampled greens at the one course, is a reflection of the very frequent sand topdressing (dusting) concept. This Californian-developed topdressing strategy reportedly resulted to adding 13.5mm of sand per year with an overall increase (i.e.; total thatch-mat) of about 20mm per year to a topdressed green.
The impact of tines of different diameters and spacing on bentgrass greens reported by numerous authors obviously reduces the volume and density but not thatch-mat thickness. Generally speaking, the results of most investigations indicate a combination of tining (solid or hollow) and sand topdressing obtains the best results in managing thatch-mat accumulation. Tining provides an avenue to introduce topdressed sand albeit with varying effectiveness.
Several authors have questioned whether the reported reduction of organic accumulation from sand topdressing on bentgrass greens is the result of organic biodegradation by microbial stimulation or simply a dilution effect from the applied sand. The results of one recent investigation into the probable effects of tining and scarifying on microbial activity lead the authors to conclude that irrigation water management rather than cultivation had a greater impact on microbial activity.
Soil-borne, microbial communities are known to occur in aging putting greens as attested by the recently reported phenomena ‘thatch collapse’ caused by a soil-borne basidiomycete fungi. Commercialisation of numerous products purported to increase organic matter degradation and continuing investigations of microbial inoculants may be effective under certain conditions provided the microbial population remains sufficiently high.
Generally speaking, thatch-mat management programmes on bentgrass putting greens in Australia have largely been based on historical rather than scientifically-based, agronomic reasons. A multitude of mechanical cultivation (e.g. hollow and solid tining, air, water and sand injection) technologies are available, albeit at varying capital and on-going costs.
Overall, the results of this field investigation has demonstrated under a range of circumstances, the newer-generation bentgrass cultivars have the propensity to attain a thatch-mat thickness between 5-10mm at some time during the first two years after establishment. Thus, it could be interpreted that the rate of accumulation and thatch-mat thickness beneath bentgrass putting greens will be governed primarily by the intensity or otherwise of the cultural management objectives on the part of the course Superintendent.
An example of a highly-stratified sample taken from a putting green as part of the thatch-mat and root architecture research project
Under actual putting green conditions, as opposed to experimentally-controlled conditions, the complex interaction of climatic, environmental and management-induced factors, let alone club committee, player expectations and resources, come into the equation of how best to manage bentgrass thatch-mat architecture.
A question for each superintendent, based on the results of this real-life investigation is, what physical form and total thatch-mat thickness best serves the performance requirements, groundstaff and equipment resources for each golf course. An intensive, thatch-mat management programme is a huge commitment and one not to be taken lightly by superintendent or golf club.
The multiple variables of environmental and management-induced factors that have influenced the rate, production and architecture of bentgrass thatch-mat during this field study leads to the conclusion they are interacting and far more complex than occurs under controlled greenhouse studies, hence further work.
Three broad-based, take-home messages become obvious from this investigation in an attempt to attain and manage a balance between the rates and physical architecture of organic accumulation against the rate of decomposition on ‘maturing’ bentgrass putting greens.
• Progressive accumulation of thatch-mat, irrespective of its architecture and thickness, beneath relatively new, let alone aged bentgrass putting greens, is inevitable;
• Developing thatch-mat architecture will progressively impact on various agronomic and playing surface properties, perhaps to the point of being detrimental to playing surface quality, if improperly managed.
• Recognising that thatch-mat architecture and its rate of accumulation varies between individual greens established at the same and other golf courses during the same short time period, implementation of an early monitoring by periodic sampling and recording programme is mandatory.
Thus, each club committee and superintendent must co-operatively decide the required standard of playing surface presentation of their greens knowing the underlying architecture to be dynamic, but which can be managed given adequate resources from the club and commitment on the part of the superintendent and groundstaff.