Golf Greens Mowing The 1/3rd rule and Clip Rate 
An important consideration in the mowing of any turfgrass surface is that of the ‘1/3rd rule’, which is really more of a useful guide than a strict figure to be adhered to throughout. However, for most turfgrasses removing no more than 33-40% of leaf at any one time helps to reduce harm and stress to the turfgrass plant. Removing less than this will be more beneficial, but there are trade-offs to be made with how often a surface can be mown with available resources, and how cost-effective increased mowing would be.
By harm and stress in this context it means reducing the available area from which a leaf can photosynthesise and produce food for use and storage, and also the exposed area of the cut leaf which is then open for disease infection and discolouration from drying winds, dry conditions and just die off of damaged cells.
For longer maintained turf there is more leeway available in mowing frequency and trying to adhere to the 1/3rd rule, but for fine turf this becomes more limiting due to the shortness of the grass which is being maintained. The daily rate of growth, even when expressed as millimetres per day, can be relatively high for a golf green, especially when a fertiliser has recently been applied, available nutrients are in optimum or greater supply, the soil is well supplied with available water, temperatures are optimum for growth, as are light levels.
To maintain a fine surface a useful guide to how often the surface should be mown, as a minimum, to adhere to the 1/3rd rule can be estimated from what is called the clip rate, or frequency of clip. This is distance travelled forward which a cylinder mower makes between each cut a blade makes with the bottom blade. This rate is influenced by the speed of the mower, how many blades are present on the cylinder and how fast the cylinder is turning. Too fast, too few blades and a slower revolving of the cylinder will produce a poor uneven cut. The aim is to have the clip rate at no greater than the desired height of cut.
The clip rate can be represented by a simple formula:
Clip rate (mm)=(Speed (mm/min))/(no.of blades (typically 9-15) x turning speed (rpm))
Example 1:
• Mower speed: 4 km/hr (2.5 mph) = 66,667mm per minute
• No. of blades: 10
• Turning Speed: 2,000rpm
Clip rate (mm)=66,667/(11 x 2000)= 3.03mm
The above type of mower, along with operating practices would be suitable for a sward being cut at 3mm (which is very low indeed), typically at least daily if the 1/3rd rule is to be adhered to, which might be challenging at many times of the year.
Example 2:
• Mower speed: 5 km/hr (3.1 mph) = 83,333mm per minute
• No. of blades: 9
• Turning Speed: 2,000rpm
Clip rate (mm)=83,333/(11 x 2000)= 3.79mm
The above type of mower, along with operating practices would be suitable for a sward being cut at 4mm (which is still low), typically at least 5-6 cuts per week, but realistically daily, if this increased speed etc. was to be maintained and if the 1/3rd rule is to be adhered to.
The following tables show the finer details of the relationship height of cut and the maximum growth height if no more than 1/3rd of the leaf is to be removed.
The first table shows the different mowing requirements as intervals of days that the grass would need to be cut depending on the grass growth rate.
The second table shows the mowing frequency per 7-day week, that the grass would need to be cut depending on the grass growth rate.
The traditional and typical heights of cut for the main growing season and out of growing season have been boldened, i.e. 3/16" (4.76mm, typically in practice being 5mm) and 5/16” (7.93mm, but in practice this would be 8mm) respectively.
Interpreting the tables
If a height of cut setting of 3/16" is desired, then the tables show that (and assuming the 1/3rd rule is not to be exceeded) the height of the grass should not be any greater than 7mm (or 7.14mm if precise measure being taken) before being cut. If the grass was growing at a rate of 1mm per day then this would also mean that intervals between cuts should be no more than every 2.4 days, which would also mean that mowing frequency would be on 3 occasions per week (2.9 is the figure, so in practice this would be 29 cuts per 70 days, as you can’t have 2.9 cuts – it’s either 2 or 3). Daily mowing would be required where growth is at the high of 2.5mm per day.
During the periods over the autumn and winter when small amounts of growth are taking place, for example 0.25mm to 0.5mm per day, and with a height of cut at 5/16", this would equate to a minimum frequency of cut of weekly to fortnightly, basically a topping of the sward.
What the tables do show is that the lower the height of cut and the greater the growth rate, the more frequent the sward must be cut. In practice the lower heights of cut will not be able to adhere to the desired 1/3rd rule because growth during the subsequent 24-hour period will exceed the maximum ‘allowed’ before the following cut which would take off more than 1/3rd of a leaf height.
Very short maintained swards will require greater inputs of nutrition, water, synthetic pesticides and top-dressings, as well as mechanical activities, if they are to maintain swards of a high quality. Greens may be maintained and sustained on this basis by some clubs, however, this does not equate to them being sustainably managed, which they are not. The two terms are frequently used to mean the same thing, i.e. sustain equals sustainable, but this is not what sustainability means.
Adapting maintenance practices, not just mowing ones, to better suit the needs of the prevailing weather and ground conditions is important if a healthy sward is to be continuously encouraged.