Is there any truth to the ‘only-one-third’ rule?

Many lawn maintenance professionals say to only cut one-third of your leaf-height when mowing. Sharp mower blades are important with mowing, but the amount you take off can affect its recovery. That fresh lawn smell – soothing as it may be for us humans – is actually a distress signal for grass as it goes into repair mode.

The ‘One-Third-Rule’ has been drummed into many turfgrass professionals and the wider lawn-loving public over many years.

Is there any substance to this reckoning and where did it originate?

It relates to the length of leaf that you should limit yourself to taking off at each mow. The reasoning behind it is to lessen the stresses on the grass during the destructive mowing process. By removing only one-third of the leaf during a mowing event, you will limit the amount of photosynthesizing material removed and allow the plant to maintain reasonable growth by minimising this stress among others.

It is also based on the correlation between the percentage of top-growth removed and the subsequent effect this removal has on root-growth.

The research behind the theory

It is little-known that root-growth can be severely impeded, and actually cease for a period of time, depending on the amount of top-growth removed. The research that first introduced this theory was carried out by Franklin Crider in 1955 whilst he was an employee with the US Department of Agriculture. He undertook the research on pasture and forage-grasses. Three separate studies were conducted within his experiment. The amount and frequency of cutting of the grass leaves (forage removal) were compared to root-growth. His first experiment evaluated cutting leaf tissue at different intervals on cool-season turf species and Florida paspalum and Bermuda grass among his warm-season grass experiments. The second experiment examined the effects of cutting on root-growth in the field. The last experiment examined different percentages of top-growth removed on different grasses for multiple cuttings.

What happens when more is removed

Crider showed when more than half of the forage is removed from a plant, root growth stops within the first day or two afterward. It stays stopped anywhere for a further six to 18 days, with an average of 11 days, depending on the grass-type. For real-world lawn maintenance, this means if you were to cut more than half of the top-growth of your lawn grass, at an interval of less than 11 days, the roots would never fully recover. This could lead to greater stresses and eventual decline.

Crider also showed in his experiments the effect that a single removal of top growth, in 10-percent increments, has on root growth. When 40-percent or less of the forage is removed, 0-percent of the roots stop growing. However, when 50-percent-or-more of the forage is removed, an increasing percentage of the roots stop growing; and when 90 percent of the forage is removed, 100 percent of the roots stop growing. In other words, leaving more than a third of the leaf at any time the grass is mown during the growing season allows the roots to continue to grow. If the roots keep growing, the plant will remain healthy and perform better than under the stresses placed on it by harder mowing and slower root-growth. Although Crider’s experiments were not examined in turfgrass settings, the results are still referred to by today’s turf managers and cited in textbooks on the subject.

There are many parts of grass-physiology that extend to a majority of species, whether they be forage, pasture, or home and sports-field grasses. His research ultimately proved what was theorised; in that cutting too much leaf tissue off results in the stoppage of root-growth and placing unnecessary burden on the turf.

The long and short of it

The One-Third Rule has been around for decades and is known to minimise plant stress and should be adhered to wherever practical for your own lawn’s best performance. Consistent mowing height and frequency will always put your lawn in a better position to perform well and overcome the various other stresses that inevitably occur with weather and wear and tear. Lawn care doesn’t need to be difficult and simple hints from Turfco can help with products to keep your lawn in top condition this summer.

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Read more on sharpening your mower blades

For more information on lawn care visit our Lawn Care page.

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