The One-Third Rule is Bunk (and why we should still preach it like gospel) - The Edge from the National Association of Landscape Professionals

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The One-Third Rule is Bunk (and why we should still preach it like gospel)

As a brand new lawn care specialist, one of the first things you learned was to tell your customers to follow the one-third rule of mowing.  What if I told you that the one-third rule was flat-out wrong, and then recommended that you keep right on preaching it?

Wait, what?  But I was told it was true!

When it comes to mowing, lawn care professionals preach the one-third rule, that is only remove one-third of the height of the lawn at any one mowing.  For instance, if the lawn is to be cut at two inches, mow it when it gets to be three inches tall.

We preach this because infrequent mowing can lead to shock; that’s when the lawn yellows after cutting. We also want to minimize clumping or excessive clippings that lead to the smothering of the underlying turfgrass, which will then lead to reductions in density and uniformity.  In short, a well-tended lawn is cut on a regular and predictable basis.

But where does this nugget of wisdom come from? Virtually every recommendation on mowing frequency contains this reference in some way or another.  Is there a single research study at its genesis?

Sometimes, we accept things without knowing exactly where they came from.

It turns out that there is a single research study behind the one-third rule and it has nothing to do with turfgrass management or for that matter height of cut.  I was reading the Asian Turfgrass Center blog (one of my favorites and hopefully it’ll become one of yours, too) and Micah Woods tackled this topic which you can read here.

The upshot of this can be summarized as follows:

“The 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.”

So we find out that this advice is aimed at cattle ranchers and not turfgrass managers?  Wow…  The good doctor goes on to say that he finds the 1/3d rule to be absurd and offers a different recommendation:

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.

Don’t worry, your entire world isn’t collapsing around you.

That’s fine advice if you’re running a golf course or a landscape maintenance company and the turfgrass you are managing is right in front of you and you are making the decision as to when to mow and how tall.  That’s not the case with professional lawn care where the very best lawn care application ever can be quickly ruined by a single bungled mowing.

As lawn care professionals, our challenge is not to write research papers, run golf courses, or make Fenway Park look great.  Our challenge is to convince our customers to care for their lawns in such a way that will enhance the service we provide, not to detract from it.  Our customers need something simple and easily remembered.

The One-Third Rule Is Still The Best Recommendation We Can Make.  Why?

In the professional lawn care industry, the one-third rule works because it’s an idea that people can easily grasp and put right into practice.  Agronomically, it’s sound because we know from an avalanche of other research that lawns that are mowed regularly are denser and more uniform in appearance than ones that are not mowed in that way.

As it turns out, if we recommend that our customers cut at somewhere between two and three inches, the rate of leaf growth throughout most of the season is such that, if you stick to the rule, the lawn will require mowing once a week.  If we can get customers to mow once a week at two to three inches with a sharp blade, that alone will be a big step towards a great looking lawn.

So in spite of it’s humble beginnings in describing the proper way to grow forage grasses, following the one-third rule still makes a lot of sense for maintaining home lawns.

Find more technical tips in our Technical Resource Library in the NALP Member Center.