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