40, 30, & 20

Midday photosynthetic irradiance is reduced by more than 50% by clouds at El Cortijo on Gran Canaria, Spain

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.

When I wrote this in 2015, I didn’t have an easy way to get DLI values. I’ve subsequently made this Global DLI app which can generate a summary chart of DLI for any location.

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.

Monthly sunshine hours can be estimated in DLI units too. One hundred 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.

With DLI data, or by estimating DLI from sunshine hours, 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.