I was listening to Frank Rossi talking with Bert McCarty on the TurfNet Radio Network. It was an interesting conversation, about lots of things including ultradwarf bermuda, pigments, and heat stress on bentgrass.

One thing struck me when they were talking about ultradwarf bermudagrass, and Rossi mentioned that "light levels change, maybe temperatures change, but certainly light levels is a driving force" in the slowdown in growth at the end of summer. McCarty confirmed that "light starts it off" as the "grass starts to slow up" even though it is still hot.

I wondered if that was right. It seemed to me that temperatures would go down to slow growth before light would. Well, I looked up some data, and they were absolutely right.

Here's the average monthly temperature for the last 10 years at McClellanville, South Carolina. Temperatures for 2015 are included to October 4. Looks like July and August are the hottest months, no surprise, and I see a drop going to September.

Then I looked up the global solar radiation, converted to photosynthetically active radiation expressed as a daily light integral, and calculated the monthly averages. Again, 2015 data are just up to October 4. The DLI peaks in June and then looks like a steady decrease to December.

Ok, I had the temperature and the light, but how to see which one is dropping off at a different time? To do that, I looked at the change from month to month. I calculated something called the log percent (L%), which is $100(log_{e}(y/x))$. In this case I let $y$ be the value for the month, and $x$ be the value for the previous month. This calculation gives a symmetric, additive, and normed measure of the relative change in light, and of the relative change in temperature.

I didn't compare December to January, so there is no L% for January. That's alright, because I was most interested in what happened at the end of summer, specifically in August and September. The values on this chart for February represent the L% change from January, March shows the change from February, and so on.

It is really clear. Dr. McCarty was exactly right.

In July the temperature is still increasing from the previous month, and light is decreasing from its peak in June.

Then in August there is a big drop in light, and on average no change in temperature. And even in September, the relative drop in light is higher than is the relative drop in temperature. It is not until October and November that the temperature decrease is more than the light decrease.

It's good to know this!