Growth Ratio
Growth ratio is a new-to-me metric for analyzing putting surface clipping volume and turning it into decisions about how much nitrogen to apply.
I'm a big believer in the importance of context; the more context you have on a subject, the more you will comprehend what you’re reading. With that, I recommend the following reading to prepare for what I’m going to write.
Jason Haines came up with the growth ratio, or as he calls it–The Turfgrass Speedo.1 Micah Woods also writes about the growth ratio as a useful way to assess the growth of your grass at any time of year.
One might ask; ‘why pay attention to growth at this level?’ Or one might say: ‘I can tell by looking in the baskets.’ You might be able to tell by looking in the baskets and that may very well be all you need. If this works for you; keep it up, I’ve got no beef with that, but I think you can get to another level by paying attention with much more detail than just looking in the baskets.
I’ve written a lot about clipping volume, so I’m not going to go into that here. I’m going to assume that if you’re still reading, you monitor your clipping volume, or at least understand the concept.
Growth ratio (GR) is a way to use your clipping volume data to tell you exactly how much your grass is growing, compared to what would be expected, given the temperature. If you’ve been tracking clipping volume, this is the next metric you should try. This is the first year I’ve tracked the GR on our greens and it has proven to be exactly what Jason and Micah have said it is–a very useful tool.
The GR is an equation and Dr. Woods explains it like this in the blog post linked above.
Clipping volume/(20)GP=GR
where…
GR is the turfgrass growth ratio
Clipping volume is the daily clipping volume, expressed in units of mL/m2. In my case it is the daily average clipping volume across all putting surfaces on the course.
20 is the standard amount of clippings, set at 20 mL/m2. (Footnote: 20 mL/m2 is considered to be the ideal amount of clippings when the growth potential is at its maximum of 1. This is explained better in Jason and Micah’s links above.
GP is the temperature-based turfgrass growth potential developed by PACE Turf.2
For example: if I collected an average of 10 mL/m2 across all greens on the course, and the GP on that day was 1, the GR would be 0.5.
Jason explains in his post, that when managing Poa greens, felt the GR producing the best surfaces was something close to 1. As he tracked GR, he found reducing the GR below 1 for an extended period of time would favor the creeping bentgrass in his stand, by exposing his Poa to low growth issues like anthracnose and excessive wear and tear.
Our greens are pure A-4 bentgrass, with only small spots of Poa on a handful of greens. Over a season of tracking this data on our putting surfaces, I feel confident a GR of 0.2 to 0.3 is pretty ideal for bentgrass. What do I see that tells me this is the right GR for our surfaces?
At that level of growth, we are able to meet or exceed our desired playing conditions…
…and the grass grows enough to be able to heal ballmarks, but not so much that it creates a need to disrupt the surfaces with grooming, verti-cutting, or in-season topdressing.
Below you can see what a season of GR looks like on our putting surfaces. This chart has been truncated at 1.00. Early in the spring, the GP is low, but clipping volume tends to be high due to winter growth and sand pick up. The GR during this time is off the chart, but I don’t worry about it, because as regular growth and weather ensue, the numbers settle down.
The solid line is the 14-day trailing average GR. There was a spike in the 14-day average in June in which the 14-day average was up near 1.00 for a week or so. I wasn’t happy with the surfaces at that time. No matter what we did, we couldn’t seem to get the green speed I would have expected. The surface was also a little bit scuffy, which is what I call it when the day’s activity leaves a lot of leaf tissue scuffed up around the holes. Typically a turf manager might react to such conditions, by vertical mowing, or topdressing. Rather than disrupt, my decision was to withhold nitrogen for a short period of time and get the GR back down below 0.5. You can see how this looked on the chart above. Once we dropped the GR, I became much happier with what I was seeing in the green speed data and visually, I no longer saw the scuffing around holes.
Since the flare up of GR in June, we’ve kept the 14-day average between 0.25 and 0.4. The result of having the GR in this range has been consistently excellent putting conditions.
In my next post, I’m going to write about how we use the GR to inform our decisions on how much nitrogen to apply. I’ll also touch on how PGRs factor into our program.
Short for speedometer, not an ode to the skimpy men’s swimwear
GP can be expressed both as a number in which the maximum is 1, or as a percentage in which the maximum is 100. For the purpose of this formula, 1 is used as the maximum GP.
Great read. This is our first year of fully using GR so guide inputs. Our course is predominantly Pencross Bent and I've found a GR of 0.4 to be ideal for our needs. It is enough to keep up with ball marks and can withstand the heavy traffic on our course (upwards of 400 golfers on our busiest days) with minimal need for verticutting, grooming, or Topdressing. So far this season we've applied 0.6#N/1000sqft, trinexipac @ 250GDD application interval, verticut once, light topdress twice to a depth of less than 1mm, and 1/2" hollow tine to a 3.5" depth with topdress to fill holes and 1mm on the surface and greens are better than ever.