Grooming, verti-cutting, hollow-tine coring, and verti-draining as aids in fine-turf management for golf and bowls
These are procedures aimed at counteracting inevitable trends towards mat development and soil compaction which are inherent in a soil deprived of earthworm activity. They should be looked upon, not as alternatives, but as a combination of treatments which together can be used by the greenkeeper to maintain the surface fit for play.
F.1
Terminology
This subject is confused by the fact that, where earthworms are absent from the soil, very similar organic-rich surface conditions can arise in forest, pasture or sports turf, but their common origin has not been generally recognized. This is a pity, because all concerned could benefit their understanding by actively seeking out the additional knowledge on this subject that exists outwith their specialisms (Tables 6.1 and 6.2).
Foresters, agriculturalists and greenkeepers have developed their own terminology in isolation. Thus the forester's ‘mor’ is the equivalent of the agriculturalist's ‘mat’ and the green-keeper's ‘thatch’. The English words mat and thatch, however, have familiar meanings in other contexts and it would be helpful to our understanding if we now used these meanings to rationalize our terminology. Thatch implies something fairly loose, laid on top; mat (or matted), something held tightly together by the intertwining of threads. Thus, under grass, it would be appropriate to use ‘thatch’ to describe the fresh and decaying, fibrous debris that sprawls across or is deposited upon the surface from shoots and clippings. In the presence of burrowing earthworms this will be rapidly incorporated out of sight, into the underlying mineral soil. But in the absence of earthworms, it will remain to accumulate in a discrete, wholly organic layer, unless diluted with mineral topdressing. In the cool, moist climate of Britain, such a surface organic layer will not entirely disappear by decay in the course of a year. Annual increments will build up and in time become invaded by fungal hyphae and grass roots, so becoming stabilized like a mat. As the grass root system tends to die back and shorten in response to frequent close mowing, the matted condition can get progressively tighter and the competition for the water and nutrients within it, intense. This is the condition that can lead to a spongy, water-holding and slow-playing surface.
For the benefit of play, therefore, the green-keeper should do everything he can to remove organic debris that would otherwise accumulate as thatch, doing this while it can be fairly easily worked out from between the grass shoots. But to re-invigorate the root system and extract organic matter from within the mat is by no means easy if the integrity of the sward is not to suffer. Thus it would be as well not to confuse the two terms, ‘thatch’ and ‘mat’. Use ‘thatch’ when referring to the surface accumulation of loose, organic debris that can be removed continuously without harm to the aerial shoots of the grass. Use ‘mat’ for the root-bound sod that holds the surface together; it may lose its cohesion if sliced, or may rip out if snagged by a spike. If only one of these words has to be used for the combination of the two, then mat is probably to be preferred.
F.2
Mat development
Mat development is a natural and inevitable feature of any vegetated surface where earthworm activity is unavailable to incorporate organic debris, not otherwise removed off site. Microbial decomposition will achieve a certain reduction but, as with composting, much will remain to accumulate on the surface if not otherwise removed. Where earthworms are virtually absent, usually because of lack of appropriate food, acidity or other chemical irritants, a surface mat will develop in which a balance is eventually struck between the overall loss by microbial decomposition and input by litter deposition. In a cool temperate climate, this balance may only be struck, under natural conditions, when the mat is 100–150 mm (4–6 in) deep.
As such a mat accumulates, the development of new roots, and the disposal of the whole rooting system, becomes more and more confined to the older layers of composted material that lie beneath the latest increment of litter. Being ultimately dependent on such a rooting medium, the natural vegetation must be highly specialized in its feeding arrangements, and well adapted to cope with the twin stresses of periodic waterlogging and periodic drought (Table 6.1).
On worm-deficient sports turf the potential for mat development is much reduced because of the removal of clippings, but even here it can become over 50 mm (2 in) deep if other measures of control are not implemented. It is formed from clippings which escape the box, die-back from leaf bases, surface roots and moss, plus organic residues from inappropriate topdressings. At worst a mat becomes just a root-bound sod.
Whether earthworms are absent or not will determine whether organic matter deposited on the surface remains to form a mat or is intimately mixed with mineral soil and incorporated beneath the surface. In either case, decomposition will continue at a relatively fast rate, unlike in peat development where the persistence of anaerobic conditions, brought on by permanent waterlogging, effectively arrests decay.
F.3
Why bother anyway?
As organic matter decomposes at the surface, a greasy residue may impede drainage and foster persistent wetness by holding on to water. Surface rooting will be encouraged and consequences of poor aeration, such as denitrification and toxin production, will adversely affect plant growth. At the other end of the moisture range, during drought the shallow rooting will severely limit the exploitation of residual water reserves at depth. In the organic mat, fungi and insects, including some that parasitize living plants, will thrive alongside the other, more benign organisms, involved in processes of decay. In the case of Fusarium, a major grass disease, the fungus can survive for much of the time, living as a saprophyte on dead organic matter, and then invading living tissue as a parasite, when the atmosphere is humid and the sward is moist.
But not only is a surface mat a problem for the well-being of the grass; it is also, for players, a major cause of slowness of pace, except when threatening desiccation. Should it be allowed to desiccate, then it will behave as if waxy, failing to re-absorb water and threatening sward survival.
Mat development is the main natural problem which the greenkeeper has to strive continuously to control.
F.4
Measures to control thatch, additional to boxing off clippings
1.Brushing, combing or the use of grooming reels, are all aimed at lifting sprawling growth to cutting height so as to provide the tight, vertical growth required for a true playing surface. This is particularly important for agrostis (browntop bent) which can provide an excellent playing surface but has a marked tendency to sprawl. Incidentally, these procedures, aimed primarily at improving the quality of the surface for play, will also make a contribution to the removal of debris sheltering within the stem bases. When first used on a neglected sward they will expose bare patches smothered by sprawl and will noticeably roughen the surface. As routine procedures they are probably best carried out just once a week during the growing season.
2.Verti-cutting is a more vigorous form of grooming. The machine is a mower whose blades have to be kept sharp enough to cut through sprawling growth and remove the freed clippings. It must be treated with respect as a vertical mower and not blunted by the blades being lowered to scratch the surface of the soil. It contributes to thatch control by removing clippings that might otherwise escape a horizontal mower, and like grooming, helps to maintain the tight vertical growth required for a true playing surface. As a routine procedure it is probably best used once a month during the growing season, and always immediately prior to topdressing.
3.Topdressing with uniform, fine sandy material is intended, like grooming and verti-cutting, not only to help true the surface for play, but also to further reduce any tendency to mat accumulation by diluting the decomposing surface debris that escapes the action of the mowing and grooming procedures. As sand sticking to leaves would blunt both the horizontal and vertical mowers, it should be applied dry so that it can be worked into the base of the sward, and then watered to ensure that it settles firmly into the soil surface, out of the way of cutting blades. It is best added frequently, in monthly increments after verti-cutting, when it will have a good chance of uniformly diluting any residue of organic matter. The alternative of a relatively large, once-a-year autumn treatment, tends to encourage the layering that interferes with uniform moisture and root distribution.
Topdressing, without prior removal of organic debris, risks burying trouble out of sight, for once below the surface, it will accumulate water and develop a sour smell as it continues to decompose anaerobically. In the past, most bought-in turf contained a rooting base of loam or peat, and most topdressings were similarly afflicted. The legacy for the greenkeeper has been the need to develop means to remove or bypass these materials so as to improve vertical drainage and prevent layers of anaerobicity developing as they sink out of reach from the surface, under successive layers of topdressing.
Providing the surface starts without a mat or soil-cap problem, the combination of horizontal and vertical mowing, plus boxing-off of clippings, grooming and sand topdressing should go a long way to controlling mat development without recourse to the remedial treatments discussed next.
F.5
Remedial treatments
These become necessary where continuous attention to thatch control has not been adequately implemented, or where former construction and maintenance faults are responsible for drainage problems built into the soil itself. Such problems may exist at the surface (mats and caps), or within the immediate topsoil (introduced layers of inappropriate material), and also at greater depth where the indigenous soil has become unduly compacted.
Initial poor construction, or subsequent differential settlement, may result in enclosed hollows where surface or subsoil drainage water may cause trouble by accumulating. This will further add to the problems associated with mat but will need more than standard maintenance techniques to remedy.
Soil examination in depth will be necessary before a logical decision can be made on the remedial action required.
F.5.1
Scarification
Where organic debris has not been adequately removed during the growing season, scarification will be required to remove it before the onset of winter restricts sward recovery. The tines of the scarifier are designed to cope with soil abrasion but this does not mean that they should be set to penetrate so deeply that the roots of the living grass are put at risk. Better to proceed cautiously, by means of several passes in different directions, but even then the treated sward may require extensive over-seeding.
To be consistently successful as part of an annual programme of maintenance, the work should begin early in September. This does not fit well with the requirements of year-round golf, nor with bowlers reluctant for the season to end.
F.5.2
Hollow-tine coring with sand infill
This is required when mat development has reached the stage of being root bound, or when an inappropriate layer of loam or peat enrichment has been introduced during turfing, or in subsequent topdressing. The aim must be to remove a substantial amount of the offending material and bypass the remainder with the sand infill; doing all this with minimum loss of use. Note that 12 mm 11/2 in) diameter cores every 75 mm (3 in) or 15 mm (5/8 in) diameter cores every 50 mm (2 in) will remove only 5–8% of the offending layer in one pass, but the sward will require time and good growing conditions to recover.
To be successful, the coring must take place when the surface is just moist enough to allow for easy penetration and efficient core extration. Then, after core collection and removal, to achieve a satisfactory infilling of the core holes with tightly packed sand, the topdressing must be a dry, free-pouring sand, brushed into the core holes over a closely mown, clean, dry surface. Finally, to settle the sand firmly into the core holes, irrigate. When the surface is dry again, over-seed and apply a further topdressing of sand, then mat to level off the core holes and re-true the surface. All this points to treatment well before the end of the growing season, and greens put temporarily out of use to accommodate.
If the infill within the core holes is texturally similar to or coarser than the underlying material to which it is to form a drainage link, it will tend to empty as the final stage of drainage is reached, leaving the surface dry. However, if at least one particle-size category finer, the infill will tend to remain persistently saturated after rain. This may usefully enhance drought resistance on high spots, but more generally will be seen to discourage a thoroughly soaked surface from rapidly drying out (section 4.2.1).
Because of the importance of timing for successful coring and sand infilling this is not an operation that should be delegated to a contractor, or made dependent on hiring the necessary equipment. It requires the timeliness of astute greenkeeping, backed up by ready access to the necessary equipment and materials.
Note, even at St. Andrews, where there is an established programme of weekly grooming plus monthly verti-cutting and topdressing throughout the growing season, still six greens each year are given the hollow-tining treatment to guarantee effective mat control. If the management for mat control throughout the growing season is inadequate, then all greens may require the hollow-tining treatment annually. American literature suggests that hollow-tine coring may be carried out more than once a year, typically in spring and autumn. At this frequency there must be appropriate equipment on hand to take full advantage of ideal weather conditions, there must be adequate staff, and there must be alternative greens, or acceptable alternative arrangements, to allow play to continue.
F.5.3
Verti-draining
Where there is soil compaction impeding drainage at a depth beyond normal solid- or hollow-tine reach, it may be necessary to use the greater depth of action made possible by a verti-drainer. With this, the combined effect of penetration, lateral displacement and heave will tend to crack open the compaction and raise the surface uniformly so as to accommodate the extra pore space. If the problem is simply confined to an impeding layer which the vertidraining tines can bridge through to a free-draining material below, then this may be all that is necessary, at least to achieve a significant, temporary benefit. However, if the subsoil beyond the depth of tine penetration is not free draining, then it may be necessary to follow up with some form of continuous slitting to provide for the positive clearance of excess water sideways to underdrains.
Disruption of an unstable soil will alone achieve only a temporary effect. The rate of resettlement will depend on the binding strength provided by the clay content. Only in a strong, clay soil could the beneficial effect of mechanical disruption be expected to last for more than one or two years. However, verti-draining equipment has now been adapted for deep hollow tining. Thus, positive steps can be taken to progressively remove an unfavourable material from depths down to 300–400 mm (12–16 in) and replace with sand. But, as with the normal, more shallow hollow-tining treatment, the condition of the infilling sand and the surface must be right, if the necessary substitution is to be wholly effective.
F.6
General hints on diagnosis and treatment
Be guided at first by the general performance of the greens in play, and the organic content of the top 50 mm (2 in) of the soil. If generally black in colour, sour in smell, and not very sandy in appearance when examined closely through a times 10 lens, then suspect an inadequate mat-control regime. Scarification and/or hollow-tine coring may well have to be introduced as an annual treatment until the surface of the soil has been re-conditioned. If it proves impossible, therefore, to get an appropriate grooming, verti-cutting and sand topdressing programme implemented throughout the growing season, make do with what proves possible and continue to make good with remedial scarification and hollow-tining treatments each autumn. The players and the greens staff must accept one or other of these alternatives if they are ever to achieve the putting surfaces required.
If only particular greens are affected, examine the configuration of the surface to see if localized wetness could be resulting from surface water being shed into enclosed hollows. If so, consider how these hollows can be taken out. Slit draining through the hollow is an option, but the efficiency of the drainage through the surface to the drain will have to be frequently renewed by spiking and/or sand-filling hollow-tine core holes.
Where surface configuration does not appear to account for a particular problem of persistent wetness, then examine the soil in depth, looking for a well-defined layer, suspended within the soil profile, that could be impeding vertical drainage. If present, decide if it could be treated satisfactorily by core extraction and sand in-filling. Aim to remove cores of the problem layer so as to link through to a free-draining material below, and then secure the bypass route by sand in-filling. The machine required will depend on depth.
Any general problem of soil compaction that continues in depth will originate in an inappropriate soil texture. To treat, this will require either deep disruption by verti-draining tines, sand-filling and provision for lateral discharge, or complete reconstruction with appropriate materials.
The ideal for any green is a good initial construction with appropriate materials, and a true, shedding surface, maintained throughout the growing season by routinely grooming, verti-cutting and sand topdressing. Scarification and hollow-tine coring then can be held in reserve for remedial treatment in those seasons when the normal maintenance programme has not been fulfilled, or when surface disruption is welcome because there is to be a major effort at overseeding.