I spoke about light -- photosynthetically active radiation, to be specific -- in this presentation at the Japan Turf Show.

I was asked what daily light integral (DLI) is required for different grasses. My answer was, for warm-season grasses on putting greens, I'd look at the moving average of DLI, and I think good numbers are 40 for bermudagrass, 30 for seashore paspalum, and 20 for korai (Zoysia matrella).

If the DLI is above 40, bermuda won't have any light problems. If the DLI is less than 40, it will be a challenge. For seashore paspalum, I'd estimate that value of no problem above, and challenge below, to be 30. For korai, I'd put the number at 20. And for cool-season grasses, I guess the number is about 20 also. I base my guesses on observations of turfgrass performance in locations with varying DLI. Fortunately there is some ongoing research in this area that should give more accurate values than my guesses.

DLI values aren't always available; sunshine hours data are around -- at least the average sunshine hours data are available for a lot of places around the world. And to make a rough estimate of DLI from sunshine hours, one can estimate each hour of sunshine will give 5 moles of photons per square meter. Thus, on a day with 5 sunshine hours, one could estimate the DLI to be 25.

The climate charts at this website have normal sunshine hours data for a lot of places. To look at numbers in tabular format, the climate information on the Hong Kong Observatory website has sunshine data in an easy to view format.

On the charts I've made, I sometimes showed the average daily hours of sunshine, and sometimes the average monthly total. If daily, multiply by 5 to get an estimate of DLI. If monthly, 100 hours of sunshine gives a DLI of about 16 moles of photons per day; 200 hours is a DLI of about 33 per day; and in a month with 300 sunshine hours the average DLI would be about 49 each day.

One could, for example, look at locations such as Atlanta and Ishigaki and plot the sunshine hours for an entire year. I like to look at the combination of temperature and sunshine for each day, to see the area encompassed on the chart.

One could also display the sunshine hours on their own, with time on the x-axis.

Either way, one can see that more than half the year the sunshine hours in Atlanta are more than 200 per month, and for more than half the year in Ishigaki the sunshine hours are less than 200 per month. Looking even more carefully, it seems like Atlanta has about half the year at 230 or above, and Ishigaki has about half the year at 150 or above. From a chart like this, and a conversion of those sunshine hours to estimate DLI, one can get an idea if there is enough photosynthetically active radiation to easily manage a certain species, or if such management will be a challenge.