What is the aim of rolling and what does it achieve, compared to not rolling?
When rolling a cricket pitch we are aiming to achieve a number of outcomes:
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1. To compact the soil – this makes the soil harder, meaning it deforms less when impacted by the ball. Without getting too technical, we want both the pitch and the ball to deform as elastically as possible (any deformation during impact is recovered – helping the ball bounce) but not to deform plastically (the deformation is permanent, absorbing energy from the impact and reducing pace and bounce). Hard pitches will offer good bounce, soft pitches will dent and the ball will then start to bounce variably (go up and down) depending on where it lands. The roller can make a pitch harder to a certain extent, but it is the combination of rolling and drying that causes pitches to get match hard – and this relationship between roller and soil moisture content is the basis of the research that Peter Shipton and I did at Cranfield University to develop the ECB Rolling Guidelines.
2. To make the pitch smooth – to remove any variations in level over the pitch that will cause variable bounce. Variations in level will come from a number of factors including shrinking and swelling of the soil, frost heave (in some winters), deformation by players (and groundstaff when its too wet), earthworms and the weather. The aim of rolling is to iron these variations out and make the pitch more consistent.
3. To modify the colour and condition of the grass plant. From work I have been doing with Lee Fortis, Head Groundsman at the Oval, we can see that grass coverage and colour, and in particular the consistency of grass colour, has on pitch pace and consistency of pace. We can manage the pace of a pitch by carefully controlling the amount of moisture in a pitch, the rolling of a pitch, the amount of grass removal and the cut height. A denser sward cover with a greater proportion of green leaf will offer more pace because of lower surface-ball friction, but excessive green grass can cause more seam movement, so content needs to be managed carefully. It can also lead to more variable pace. Managing the colour and density of the grass sward is the most critical challenge for the groundsman in determining the performance of pitches once the pitches are hard and smooth – particularly those used for 4-5 day games as the change over time is greater.
What do rollers do?
When used optimally, rollers will predominantly push the soil downwards forcing the particles of soil closer together making them more compact and therefore harder. The roller also pushes the soil forwards slightly which helps to smooth it out at the surface and make it even harder, but we do not want too much forward movement, or we could make a pitch uneven, and in the worst case – corrugated. The grass is critical in this process -providing a flexible reinforcement of the soil that helps to keep the soil moving downwards and not excessively forwards. This can be seen in the videos I produced at Cranfield University back in 2009 and still available on YouTube https://www.youtube.com/watch?v=EtalCo06ae8
The effect of the roller is in part a function of roller design (in particular the mass of the roller, the diameter of the roller and the width of the roller), which all affect roller contact stress (force per unit area) and the direction of stress (larger diameter rollers have a greater component of vertical soil movement than smaller diameter rollers). Most commercially available cricket rollers are only effective to about 75 mm depth (depending on these roller factors and the moisture content).
Even more critical than roller design is the moisture content of the pitch – too wet and the pitch can be damaged by the roller causing horizontal movement (and very little vertical movement), but more commonly a lack of compaction because the water content in the soil cannot be compressed.
One of the key findings of the rolling trials at Cranfield University is that rolling is most effective at the optimum moisture content and that rolling method should be to carry out a few passes (say 4-6) and then let the soil dry before rolling again. A lot of time is wasted rolling wet soils that will not compact. A lot of time is wasted rolling dry soils that cannot be compacted by the weight of the roller. It is all about getting the rolling right for your pitch in relation to its moisture content and your roller. Early rolling when the soils are more moist allows improved smoothness and compaction, but pitches only really get harder as they begin to dry out and the soil shrinks. As rolling progresses through the pitch preparation process, the roller becomes less effective at smoothing and compacting, and more about preparing the grass plant. You can optimise rolling relatively easily – have a look at the rolling guidelines, experiment (safely – within small changes of moisture content and timing)) and draw on your experience as you prepare what works on your pitches.
The balance is to achieve no more rolling than is necessary to get good quality pitches. There is a point where you are wasting your time because the roller is having little or no effect, but under cook it and you will have slower pitches with variable bounce. With my own pitches, I want to be sure that I have done enough rolling before I worry about doing too much rolling – but as we are all volunteers with day jobs on our groundstaff, we do not have the time to do excessive rolling, and I know that even if I could spend all day on the roller is not improving our pitches because the increase in hardness with each pass is getting smaller and smaller – it is definitely the law of diminishing returns. The first-class grounds I work with all do their rolling slightly differently, but I can reassure you that none of them are spending all day on the roller.
Pre-season rolling GuidelinesIt’s that time of year isn’t it? The two questions I get asked at this time of year are (in this order) are:
1. When can I start pre-season rolling?
2. Is it worth it?
The questions need to be answered in the reverse order. Pre-season rolling in the right conditions is definitely worth it. Over winter, as soils get wet they swell and the surface can lift (usually unevenly), also the tolerances that can be achieved during end-of season renovations are not as tight as those needed for pitch preparation, and finally in very cold years, frost heave can be experienced too. Pre-season rolling should be done in four directions (across play, on each diagonal and then finishing in the direction of play – often referred to as the ‘Union Jack Pattern’). This allows optimum smoothness to be achieved and can remove small differences in compaction around the roller edge. Pre-season is often the only chance to roll across the square because later in the season once the season starts it can be difficult to roll in any other direction than in the direction of play because of the different preparation states of different pitches.
It is also harder later in the season to get the whole square to the optimum moisture content for rolling whilst – because it can be really hard to evenly wet a square with irrigation. As long as we have good surface levels – nature will get our squares evenly wet. In winter rainfall is higher, pitches are uncovered and evapotranspiration rates (the rate at which water is removed from the soil by the sun and the grass plant) are at their lowest. In summary the square needs to be compacted, it needs to be smoothed and conditions across the whole width of the square are at their optimum as long as the square is not too wet.
Above, I said ‘Pre-season rolling in the right conditions is definitely worth it’. In the wrong conditions – it will do more harm than good. Which brings me to the answer to the first question. There is no single day, week or month in which you should start pre-season rolling – I don’t think you can buy a grounds management calendar that has a pre-printed ‘Start of Pre-season Rolling Season’ day on it, and that is a good thing. The process of starting pre-season rolling in February because it is February is a risky approach – particularly in February 2020 because it has been so wet. The reason for this is that it all depends on ground conditions – which depend on weather conditions.
A rule of thumb is that if you can’t cut it with a cylinder mower without leaving a crease – it is too wet for the roller. It is better to be too late with pre-season rolling than it is to be too early and although above I said that later in the season it is hard to work across the square – early in the season, conditions are more uniform and you can defer your pre-season rolling until you are in a position to prepare pitches if your squares are too wet. You only need to prepare one pitch for the first game of the season and if necessary, this can be combined with in-season pre-season rolling. I know that sounds daft but if the square is too wet you will cause damage that cannot be rolled out – this is worse than being slightly less effective with your pre-season rolling. DO not misinterpret this as ‘pre-season rolling is not worth it’ or that it is not necessary – it’s just you can catch up on it later.
The traditional approach of building up the weight and size of the roller as you move through pre-season rolling (using ever increasing mower sizes then moving on to un-ballasted and then ballasted rollers) is built on the experience of every grounds manager that has gone before us -they probably all went too heavy too early at one point in their career. Again, the precautionary principle applies - start with light rollers /cylinder mowers and build up the weight as you become confident that the square can take it. You will crease the ground a bit and you will need to work these creases out but if it is too wet you will cause undulations in the square, smear the soil or create deeper creases that you cannot roll out. This is all made worse by worm casting which disrupts levels and can be easily smeared losing grass cover. The real art of cricket ground management is to move up the weights as quickly as you can without causing damage and that art will come from experience of your square. This will mean that you will have the most effective pre-season rolling. Consult the rolling guidelines for more information.
Just a word of warning, particularly those working with volunteers – early pre-season rolling is not the time to train up new groundstaff on the roller, or the time for you to accept an offer from another well-meaning volunteer of little or no experience ‘to do that boring task while you get on with something more complicated’ – the person rolling needs to be competent at managing the roller in forwards and reverse and to be able to spot the signs of the square being too soft.
If we use a heavier roller will we get better wickets?
The answer is ‘it depends’ (of course!) There are many things to consider when choosing a roller for your club but it is usually size and cost that are the over-riding concerns. Heavier rollers tend to be bigger rollers and so storage in the grounds shed is often a primary concern. Anything with a cab or roll-over protection is likely to need good clearance and room in the maintenance shed. Bigger rollers also tend to be more expensive but it's not simply a case of buying the biggest roller you can afford. The discussions above have shown that by matching roller specification to the club's pitches and other resources, it is possible to buy an effective roller and save money or to improve pitch preparation by purchasing the right kit.
Heavier rollers are more effective at drier moisture contents – this is a double-edged sword. It means that they will compact drier pitches more, but it also means pitches have to be drier and without covers getting pitches dry enough for heavy rollers can be a challenge.
The other variable is the impact of the roller on the grass plant itself. Very heavy rollers can adversely affect the grass plant drawing moisture out of the leaf. They can also increase evaporation near the surface and dry out the surface of a pitch. This can be worked to the advantage of the grounds manager when producing one-day pitches but increases the challenge when producing 3-5 day pitches or club pitches that have to be used over a number of games/days in the weekend.
The optimum weight of roller will depend on all these factors. Typical cricket rollers are in the range of 1.5 -2.2 tonnes; many clubs will be using 3 tonne road rollers and 3 tonne rollers are used at first class grounds in combination with 2 t rollers and various ballast methods. The use of heavier rollers tends to be in the early stages of prep using 2 tonne rollers as the preparation progresses towards match day. A lot of community clubs will find it hard to get dry enough conditions to use rollers bigger than 2 tonnes and certainly bigger than 3 tonnes. A 2 tonne roller used in the right conditions is more of an allrounder. The key thing is to remember to use it effectively by controlling moisture through the prep – using available moisture and allowing drying between spells of rolling.
Do you have any other key rolling advice?
1. Get on top of your thatch. Work hard, particularly at the end of the season to remove and control thatch. There is no weight of roller than can effectively compact thatchy pitches – the surface will be like a spring, rising behind the roller just after it is pushed down. You need to deal with the problem at source and remove the thatch.
2. Look after your grass plants – maintaining a good grass density is important to good pitches. The grass plant is the main mechanism for drying pitches during the season – the deeper the roots the deeper the drying. Over rolling tends to desiccate the grass too much and whilst it might leave a shiny white pitch – this might not be give good pace and bounce.
3. Too many people underestimate the danger of rolling because the machines are moving slowly and the work can be repetitive. All operators should be trained, competent and risk assessed. Roller maintenance is critical and it is important to keep rollers in good, safe working order. Rollers can injure and even kill. Always use a safe stop, kill the engine and engage the break (ideally off the square of course) before moving in front or behind a roller. Always conduct a risk assessment – whether as a professional or a volunteer.
About the author
I’m Dr Iain James and I am currently, Technical Director at TGMS and specialising in cricket – working at grounds from all levels of the game from my own at Olney Town CC in Buckinghamshire, where I work on the ground as a volunteer, to first class and international grounds such as the Kia Oval, Emirates Old Trafford and Lord’s. In the coming weeks I shall be moving to take on a new role at the ECB helping to improve facilities right across the game – I am looking forward to this new and exciting challenge.
Before becoming a consultant, I was Senior Lecturer in Sports Surface Engineering at Cranfield University and led both the rolling and aeration in cricket projects there. This blog is based on my experience as a volunteer groundsman, as a consultant and the findings of research that my colleague Dr Peter Shipton and I carried out at Cranfield University and that was funded by the ECB and the UK Engineering and Physical Sciences Research Council.