... should be removed at a given mowing," reads the Lawn Management Through the Seasons guide from the Penn State Center for Turfgrass Science. "Thus, if the turf is cut at two inches, it should be mowed when it reaches a height no greater than three inches."
Run a Google search on "one third rule mowing" and you will get pages and pages on this "rule." From Cornell University, this explanation:
After raising your cutting height, the next most important thing you can do is to observe the “One-Third Rule” when mowing: never remove more than one-third of the grass blade. That means if your mowing height is 3 inches, you need to mow when the grass is about 4.5 inches tall.
This is pretty standard advice, but where does it come from, and how strictly does one need to follow this rule? This is based, as far as I can tell, on a really interesting paper by Franklin Crider published in 1955: Root-growth Stoppage Resulting from Defoliation of Grass.
I read this in its entirety last weekend, and was struck by the way the percentage removal was done. It was not by length of leaf. The percentage of cutting was determined as a percent of the verdure volume, not leaf length or plant height. I really liked the paper, and was interested to learn about the root blacking technique and the absolute cessation of root growth that occurred with certain defoliation treatments.
Most interesting to me were the data and discussion on just how much the root system is decreased by mowing. There is also a nice photo and discussion of root system size when grass plants are mown at all, versus left "unmolested" for a growing season:
The roots of the clipped plants [clipped 3 times over 247 days for cool-season species; 2 to 4 times per 146 days for warm-season species] weighed only one-eighth as much as the roots of the unclipped ones. This striking difference in root production by clipped and unclipped plants was manifest as well in the development of the plants as a whole. Compared with [sic] unmolested plants, the mature, clipped specimens were greatly lacking in size and vigor."
I like the thought of trying not to cut grass too short, and trying not to remove too much of the leaf at one mowing. But if the grass must be cut a different way at times, then go for it. Doug Brede's Turfgrass Maintenance Reduction Handbook has a great section on the one third rule in which he explains just how absurd it is, calling "an absolute like the one third rule ... strangely out of place" in a discipline like turf management that usually "deals in shades of gray."
And Brede has a fine replacement for the one third rule too.
So what can we use in place of the one-third rule? What general guideline can be employed to govern mowing frequency?
How about the plugged-up mower rule: "If your mower plugs up when you're mowing, you let it grow too tall." This guideline makes more sense for the turf caretaker who's battling practical limitations of budget, equipment, labor, and weather. This guideline also allows added flexibility for managing low maintenance turf.
The Crider paper on root growth and defoliation is interesting but one can read it and realize a few things:
- It is about forage grass more than turf.
- It does not measure turf or surface performance, rather it looks at root growth.
- It was not based on mowing height and cutting a percentage of leaf length; it was based on grass allowed to grow for two months and then cut to different percentages of verdure volume.
For more about mowing, original research, dogma, and the one third rule, see: