The Story of the 1/3 Rule

October 1, 2009

Recently, I was skimming through a textbook about maintenance reduction practices for turfgrass settings and I stumbled upon a section that discussed the one-third rule of mowing. I’m sure most everyone has heard about this recommendation as it appears in almost every textbook and extension publication. This common rule states that no more than one-third of the plant should ever be removed in a single mowing. However, this rule is commonly ignored. You probably remember many other “rules” from your introductory turfgrass classes that are also commonly violated: irrigate deeply and infrequently, never mow when the grass is wet, avoid fertilization during the summer months, etc… So are there any absolute truths when it comes to turfgrass management practices? The truth of the matter is that turfgrass management often deals in shades of gray. There seems to be exceptions to almost every rule.

Most recommendations stem from research, which is usually a good thing. But sometimes the inference space of the research is extended beyond the original scope of the project. For instance, a study investigating the growth of bentgrass cultivars will only provide information about the growth of bentgrass cultivars. The results of the study should not be applied to other species of turfgrass such as bluegrasses or ryegrasses. The recommendation of the one-third rule is a classic example of a research project being extended beyond its inference space.

The 1/3 Rule is Born
The one-third rule originated from a greenhouse study done by U.S. Department of Agriculture scientists about 60 years ago on forage type Kentucky bluegrass. The goal of the study was to investigate the effect of cattle defoliation on the growth of the roots. Results of the study showed the grass (or roots) didn’t die when more than one-third was removed. The growth of the roots simply paused for a short time before resuming normal growth. The findings from this study were eventually morphed into the creation of the one-third rule.

More Recent Research
A similar defoliation study was recreated in 1986 by a group of turfgrass scientists at North Carolina State University (Shepard et al, 1989). This study was conducted in the field with a “high” and “low” maintenance tall fescue maintained at 3 and 6 inches, respectively. Each grass was allowed to grow 30, 50, 100 and 300% taller than the maintained height before being mowed. The 50% defoliation treatment signified the maximum guideline of the one-third rule – 30% was within the recommended guideline, and the 100 and 300% treatments violated the one-third rule. The researchers found that the tall fescue could grow to double the original height before being defoliated without negative consequences (tall fescue maintained at 3 inches could grow to 6 inches before being cut without serious consequences). The 300% treatment resulted in senescence to a portion of the tall fescue stand. The results of the NC State study indicated that the one-third rule has little merit, yet the recommendation lives on. One reason could be the detrimental effect of excess grass clippings that shade the turf underneath. Either way, the one-third rule is by far an unconditional truth.

So what is the moral of this story? Recommendations should not always be taken at face value. Ask questions and spend some time investigating how they came about before taking action. You may just be surprised when you learn the rest of the story.

NC State University Citation:
Shepard, D.P., J.M. Dipaola, and W.M. Lewis. 1989. Effects of clipping regime on turf quality and mowing requirement. Agron Abstr. p.165

Marcus Jones

Graduate Research Assistant