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January 2015

"No more than one third of the the total leaf surface ...

... 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."

Breaking the one-third rule on bermudagrass in Vietnam

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:

  1. It is about forage grass more than turf.
  2. It does not measure turf or surface performance, rather it looks at root growth.
  3. 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:

Estimating turfgrass nutrient use

In a presentation at the Ontario Golf Superintendents Association conference, I explained how I answer the two important questions of turfgrass nutrition. These are 1) do I need to apply this element as fertilizer, and 2) how much do I need to apply?

These slides, and associated handout, describe the process.

I explained that one can answer the two questions by obtaining three quantities, which I called a, b, and c. These are the amount of an element that the grass uses (a), the amount that must be kept as reserve in the soil (b), and the amount actually present in the soil (c).

This simplifies to the amount we need (a + b) and the amount we have (c). The need to supply the element, and the quantity required, is then the difference between the amount we need and the amount we have (a + b - c).

I was well-titivated for my time on stage, although I wasn't nearly as eloquent as Pat Jones, who spoke after lunch about the turf and landscape industry, and who has a way with words on Twitter too.

More to come about my second presentation at this excellent conference, and on some of the other information shared at this outstanding OGSA event.


And isn't Niagara Falls a great venue for a conference? What a view from the conference hotel!


Does stressed turf require more nutrient supply, or less?

In a recent conversation, Carmen Magro and I showed a fundamental difference in thinking about turf nutrition.

My approach, which I explain in Everything you need to know about turfgrass nutrition in 1 lecture, is summarised by this statement:

If the grass is supplied with enough of an element, adding more of that element will provide no benefit.

Which is why I was so happy to answer when Magro asked "Why are some still saying NPK only for fertility in turf?" In fact, if there is enough P and K in the soil, I'll be happy to say N is the only element required!

As the discussion continued, it seems our point of difference is in whether the degree of stress on turf influences the nutrient requirement. The definition of plant stress that I like to use is Grime's. Stress is:

the external constraints which limit the rate of dry matter production of all or part of the vegetation.

One is always trying to produce good turf, grass that is as healthy as possible. That grass when healthy is going to have a certain concentration of mineral nutrients in it. Creeping bentgrass for example is going to have about 4% N, 2% K, 0.5% P, 0.5% Ca, 0.2% Mg, and so on. As the degree of outside stress increases, let's say from temperature, or drought, or close mowing, the rate of dry matter production will be reduced. There is a consequent decrease in the plant demand for nutrients. So if the quantity of nutrients supplied were sufficient at a lesser degree of outside stress, how can it be that the nutrient demand increases when stress increases?

If the stress is not caused by a nutrient deficiency, and is rather caused by something else, where is the connection between that outside stress and a grass requirement for increased supply of a particular element?

For more about this, see:

More turfgrass tweet activity

Rather than number of followers, or number of tweets, I wanted to look at the activity or response to a tweet, in terms of favorites and retweets. For the accounts that I followed, or that followed me, as of 24 January, I downloaded the last 500 tweets using this script.

I looked at only those accounts with more than 500 tweets and I left in most golf and sports related accounts for perspective. I removed accounts that were unrelated to turf or sports that are played on turf, although this manual removal may not be perfect.

This is the top 50 accounts, looking at simply the sum of mean favorites and mean retweets for the original tweets out of the last 500 tweets.

For those wondering how to get these data, this gist shows how one can use the twitteR package in R to query the Twitter REST API. As shown in this gist, the tweet activity data can be calculated for X tweets from a specified user's timeline. Or, also shown here, one can get the followers and or friends from a given user's account, and then get the tweet activity data for each of those accounts in a loop.

Zoysia and growth potential in Beijing and Seoul

This chart shows the temperature-based growth potential for cool (C3) and warm (C4) season grasses at Beijing and Seoul.

It is too cold in winter for any type of grass to grow, and during the spring, summer, and autumn, C3 species will grow more than C4 species. Of course, C4 species use less water than C3 species, tend to be more salt tolerant, and in this type of climate, will require less mowing due to the shorter growing season.

Playing golf on a Zoysia japonica fairway near Seoul, March

Zoysia japonica is in common use as a fairway and rough turf around Seoul, more so than it is in Beijing.

Bentgrass greens and Zoysia japonica through the green near Seoul, March

Based on the similar temperatures between Seoul and Beijing, Zoysia japonica would certainly perform well in the summer in Beijing. But with colder winters in Beijing than in Seoul, one would need to be more concerned about potential winterkill. It would seem that Beijing winters would be almost too cold for Zoysia japonica, based on the temperatures at which this species was killed in this experiment by Patton and Reicher.

March golf near Seoul

Nonsense, facts that aren't facts, and turf in 3 dimensions

I received this inquiry about nutrient application and soil nutrient concentration:

I have in mind that somebody wrote that one rise any element by 6.7 ppm when it is applied by 1g/m². Is that true or is it different for any element so that i have to change the factor in my calculation?

This 6.7 ppm factor is correct when thinking of the turf system in 3 dimensions. How does this work out? It is like this. Assuming a rootzone depth of 10 cm, and a soil bulk density of 1.5 g/cm3, then the mass of the rootzone in 1 m2 is 150 kg. An application of 1 g/m2 of an element to the surface, assuming distribution throughout the rootzone, is going to cause an increase of 6.7 ppm (1000 mg/150 kg). Likewise, nutrient harvest can be estimated in the same way.

If you like to use other units, or a different rootzone depth, or have a soil with a different bulk density, the calculations are elementary.

I've written before about the ease of such calculations using metric units. The calculations can be made in other units, but I find it especially easy to visualize these in 2-dimensional and 3-dimensional space using metric units.

Being familiar with 2-dimensional and 3-dimensional thinking can help to pick out nonsense pretty quickly. Here are two examples.

I saw this tweet about a cubic mile of fog being made up of less than a gallon of water. That just didn't sound right at all. This is a 3-dimensional space we are thinking of, and I remembered that in non-foggy conditions, I'd read one can get up to half a liter of dew in 1 m2 with the amount of  water in a column of air something like 10 to 50 meters high. I'd read this in Nobel's Physicochemical and Environmental Plant Physiology, and looked it up again, and it turns out that at an air temperature of 10°C, the column of air would need to be 53 m high to provide that much water. My memory wasn't exactly right, but now I had the basic information I needed.

Now obviously, 53 m3 is a hell of a lot less than a cubic mile. One cubic mile is about 4,165,509,529 m3. Roughly estimating that a half liter of water is slightly more than an eighth of a gallon, one can calculate that there are 78,594,519 units of 53 m3 in a cubic mile, each with potentially an eighth of a gallon of water, so in a cubic mile of fog there may be something like 9,824,315 gallons. This is easier if one just keeps it all in metric units, but the point is, being aware of what the volume is, one can think in 3-dimensional terms and get a rough estimate of just how outrageous such a statement of less than one gallon of water in a cubic mile of fog is.

One can find other estimates, such as 56,000 gallons of condensed water in a cubic mile of fog. I prefer to consider the total amount of water, and that obviously is way way more than a gallon.

There was also the estimate of how the length of roots in a lawn was supposedly extending to a distance equivalent to 15 round trips between the sun and the earth.


That just sounds outrageous from the start, for anyone who has seen turfgrass roots, an average lawn, and has some idea of how far the sun is from the earth. Even with generous estimates of root length, and including root hairs, the cumulative root distance falls well short of even a one way trip to the sun. Thinking in 2-dimensional and 3-dimensional terms makes these estimates easy.

So whether one wants to know a simple conversion between nutrient uptake and depletion from the soil, fertilizer addition and increase in the soil, water application and increase in soil moisture, plant water use and decrease in soil moisture content, or expand this to consider whether nonsense "facts" can possibly be true, thinking about turf in 2 and 3 dimensions can be useful.

Turfgrass on Twitter: how accounts share information

I downloaded the last 200 tweets of the accounts I follow and of the accounts that follow me. I cleaned the data a bit, removing obviously non-turfgrass accounts (although I'll have missed a few), and omitting inactive accounts and those that haven't sent many tweets. Then I looked at which accounts share by retweeting, and which accounts don't retweet so much.

First, a histogram of the retweet fraction (0 is no retweets out of the last 200 tweets, 1 is 100% of the last 200 tweets were retweets. The mean fraction for retweets of all these turfgrass accounts is 0.329. That is, for the average account, about 33 out of 100 tweets were retweets.

Hist200retweetfractionThen I plotted the 50 accounts that have the highest fraction of retweets, and the 50 accounts with the lowest fraction of retweets.

 For more about Turfgrass on Twitter, see:

4 versions of the same topic

Over the course of a week last October, I spoke about turfgrass nutrient requirements in Hawaii, Washington, Idaho, and Oregon. Each time, I discussed this topic in a slightly different way.

With a look at these presentation slides, which all essentially cover the same topic, but are different in their title and in their approach and in the localized estimates of nutrient use, one can get a good overview of how I go about answering the two most important questions of turfgrass nutrition. These are 1) Do we need to add this element as fertilizer? and 2) If so, how much of the element do we need to add?

As you will see from these slides, I suggest nutrients be supplied to ensure that there is enough present to meet all the grass requirements. That is, make sure there is enough of each element so that the grass use of that element can be met, while still keeping a reserve amount of that element present in the soil. And although I didn't write it on the slides, I would have said it during the presentations. It is silly to add more of an element than the grass can use or than the soil can hold.

First, from Hawaii, where we talked about The secret to preventing nutrient deficiencies: why K fertilizer is almost always required but Ca is not, and the controlling role of N, among other examples to illustrate the point.

Then, in Washington, I talked about Leaves of Grass: a practical understanding of turfgrass nutrient use.

Next was in Idaho, where the title was A Modern Method for Estimating Turfgrass Nutrient Requirements.

And then was at Oregon State University, where I had a chance to teach a session of the Turfgrass Management class in the Horticulture Departiment. Getting right to the point, since I only had one chance at it, I taught Everything you need to know about turfgrass nutrition in 1 lecture.

Turfgrass on Twitter: an updated impact index

In yesterday's plot, I showed the average number of retweets, added to the average number of favorites, for the original tweets out of the last 200 sent by that account. That index gives some idea of how much interest a tweet from that account generates, as measured by retweets and favorites.

Today's chart shows something that incorporates both the number of tweets and the interest generated by the average tweet. I multiplied the total number of tweets from the account by the sum of 1.5 times the mean retweets per original tweet plus 0.5 times the mean favorites per original tweet. The chart is here. Scroll to the bottom to read more about the data.

These data were downloaded on January 14 and 15. I took the accounts that follow me, or that I follow, using those as being a general selection of turfgrass industry accounts. Then I downloaded the last 200 tweets from those accounts and calculated these indices. I manually removed accounts that were not turfgrass related, but I have been inconsistent in leaving in or removing golf course architects and golf journalists and club managers. These are the top 100 out of about 2000 accounts on this impact index.

I think this is a better index than only the average retweets and favorites per tweet, because this accounts for both the quantity and the interestingingness of the content being shared.

By the way, today is the last day for nominations for the 2015 Super Social Media Awards.

Analyzing tweet activity

With the GCI and Aquatrols Super Social Media Awards coming up at #GCITweetUp15, I wondered if one could take a quantitative approach to identifying the most influential people/accounts in the turfgrass industry.

I downloaded the most recent 200 tweets for all the accounts I follow on Twitter and for all the accounts that follow me. I figure that with a few exceptions, this will be a reasonable collection of turfgrass industry accounts.

Then I excluded those (I may have missed some) that are not in the turf industry, and I also excluded those relatively inactive accounts that have not sent a tweet or a retweet in the past month. Then, from those 200 most recent tweets, I counted the average number or retweets and favorites for each original (not a retweet) tweet, and I plot all those with an average retweet + favorite count above 3, for those tweets among their last 200 that are not retweets.

What do you think? Are these some of the most influential accounts?