One-Third Rule

A rule of thumb used in mowing whereby the length of grass removed per cut is no more than one-third of the height of the grass.

For example, if the grass height is measured as 30mm, then no more than 10mm will be removed during a mowing session: The height of cut would therefore be no less than 20mm.

Removing excessive amounts of grass per cut will weaken a sward, thinning it and reducing root depth.

Research from 1955 identified the effects of removing different percentages of leaf height from a sward, which resulted in the cessation of root growth following each mowing session, where 50% or greater of leaf height was removed.

Where 40% was removed this still maintained positive root growth, although to a lesser extent than 30% in the experiment. The cut-off point between root growth still occurring and ceasing to grow would appear to lie between 40% and 50%.

Whilst the research was on heights of grass from 1″ to 3″, it laid the foundation for turfgrass maintenance and the ‘not to remove more than 1/3rd of the grass leaf per cut mowing rule’.

The less the amount of grass height removed per cut resulted in greater root growth occurring, presumably due to reduced stress caused by the action of mowing and also with more green leaves still being available to produce carbohydrates from photosynthesis.

From a practical perspective the removal of 40%-45% of leaf might be considered as part of a mowing regime for lower quality grass swards, possibly mixed in with the one-third rule, without having a noticeably negative impact on the end product because of the lower quality requirements and expectations from that type of turf.
Crider, F.J. (1955) ‘Root growth stoppage resulting from defoliation of grass’; Technical Bulletin No. 1102, USDA,