After this post about when it is too hot to grow bentgrass, I had e-mail conversations with a few people and decided to get some more data and have another look at this. The three main things I wanted to do in looking at this again:

1. Get more precise data, from better locations. The script I used to get the original data was reading data from airport locations, which in some cases (Osaka Itami and New York JFK in particular) seemed to be giving unusually high temperatures. Also, those data from Weather Underground were in integer increments of temperature, but data in my new analysis are from the NOAA, the JMA, and the BOM, with precision to a tenth of a degree C.

2. To include a desert location, where the daytime highs can be well above 40°C (104°F), yet high quality bentgrass is still produced. This is to illustrate the importance of nighttime lows (and average temperatures) rather than daytime highs. I include data from Carefree, Arizona, in this analysis, where there were 199 days with a high temperature of 40°C or above from 2010 to 2014. The highest temperature was 45.6°C (114°F). This has some personal interest for me. If you've heard my story of why and when I decided to be a turfgrass scientist, it happened one summer when I was 19 years old working on the grow-in of the Apache Course at Desert Mountain. I was amazed by a couple things -- that creeping bentgrass could grow so well in such temperatures, and that bermudagrass could grow in the rocky desert soil. Understanding how that was possible, why it was possible, is something I wanted to study.

3. To include data from Sydney, Australia. Why? It's a great place, it gets hot there, and there is lots of bentgrass in Sydney.

I've put the scripts to get the data, some of the data, links to download data where I could not scrape it, and code to make these charts, in this GitHub repository.

Scott Griffith pointed out something really important, that it is the nighttime lows that really matter when it comes to heat stress on bentgrass that leads to a decline in turf quality. He mentioned 65°F (18°C) and 70°F (21°C) as nighttime low threshholds at which there is a noticeable effect on the bentgrass.

First, I looked at these cities (I've tried to stay away from large urban airports, and to get data from government weather stations close to concentrations of golf courses) and the number of days with a nightime low above 18° in each of the past 5 years. I would think of this as something like summer length. Bentgrass is not going to fail at 18°C, but if the temperature remains above 18°C for a long time, that is a long summer, and the temperatures will be higher than optimum for bentgrass every day.

Notice that at the 18°C level of nighttime lows, Carefree in Arizona, where there were 199 days in this timespan with highs of 40°C or above, is right in the middle of the pack for days with lows above 18°C. Although the summer might be intense there, it is not exceptionally long.

Ramping up the heat stress, I looked at the 70°F (21°C) threshhold level that is more indicative of when we really start to see bentgrass struggle.

Here we have something more like the length of intense summer. Jacksonville Beach is a place where we don't really see creeping bentgrass. Other locations, like Augusta, Carefree, Knoxville, or in Japan, we could see bentgrass or warm-season species. In Japan it would be almost all bentgrass, actually, in the Kanto and Kansai regions from which data are shown. And in New York and Sydney it would be typical to have only cool-season turf on greens.

Augusta has had 75 to 100 days of this intense summer each year; in Carefree it is a bit less; at Koshigaya, near Tokyo, there is a duration of more than 60 days. This shows the mean number of days with lows above 21°C in each of the past 5 years.

It would seem that the critical number of days with lows above 21°C would be somewhere between Augusta and Jacksonville. That is, we know that bentgrass doesn't work in Jacksonville, when there are more than 140 days with lows above 21°C, but it is possible in Augusta and Carefree and Koshigaya.

When Bingru Huang has looked at this problem in temperature-controlled environments, she has found that 23°C is a critical soil temperature above which bentgrass fails in a lot of ways. Her paper on this with Pote and Xu is one everyone should read. By modifying both soil and air temperatures, they were able to determine that it is the soil temperature that is most important. From Huang et al.:

Our results in this study suggest that rootzone temperature of 23°C [73°F] was the critical level for root activities, Pn [net photosynthesis], and overall turf growth. When soil temperature was higher than 23°C, all growth and physiological parameters were negatively affected, even if shoots were kept at optimum temperature (20°C).

During summer, the soil temperature is always going to be higher than the nighttime low temperature. The lower the temperature at night, the lower the soil temperature can get. So looking at the nighttime lows gives some indication of how much negative affect there will be on root activity, photosynthesis, and growth. Look at Carefree, Arizona, where the temperature was above 45°C on 3 and 5 July, 2011, and on 29 June 2013. The lows on those days were 23.3, 21.7, and 26.7°C. Or at Sydney, where it was a blistering 45.8°C on 18 January 2013! The low was 21.7°C. And then on the 19^th, the morning after that scorcher, the low was 21.5°C.

Bentgrass can survive the really hot days, as long as the night temperatures get relatively low, because that allows the soil temperatures to drop.

It is really hot when the nighttime low is higher than 23°C, and one can expect soil temperatures to be even higher, probably not dropping below 24°C.

Here's a look at these really hot (>23°C) temperatures for these same cities.

And here are the average counts over 5 years.

I think at this temperature it is easier to see when bentgrass can work, and when it can't. I'd pick a 60 day threshhold of low temperatures above 23°C and say that is the number. Bentgrass is a challenge when it is hot in places like Hyogo and Augusta and Saitama and Carefree, but it is possible, and there are less than 60 days in an average year when the low temperature is higher than 23°C. In a bad summer, bad for bentgrass anyway, the number of days with lows above 23°C may spike higher than 60, and that is when there can be widespread failure of bentgrass.

On a year by year variation, there can be quite a difference in number of days with these high nighttime lows.

Look at Augusta, for example. when the 2010 summer had more than 60 days with lows above 23°C, and 2012 and 2013 had less than 25 days.

Based on a suggestion from Albert Bancroft, I've also tried to separate those days when it is really hot, and those days when it is hot but the nightime low drops below 23 or 21°C.

First, the days when it is really hot, which I say is when the high reaches 30°C and the low remains above 23°C. Next, the days when it is hot (30°C or above) but there is a little relief at night because the low is less than 23°C. And then the days when there is a real cool night for the bentgrass, when it is above 30°C for a high, but the nighttime low is less than 21°C.

Those days when the low drops below 21 and the high is above 30 are pretty good growing weather for bentgrass. Although there are some really hot days in Carefree, Augusta, and Knoxville, the relatively high number of days in these locations with a cool night temperature and a high day temperature give bentgrass a chance to recover.

This is even more clear when the days with low of 18 or below and a high of 30 and above are shown.