A hypothesis concerning the most important time for sunlight to fall on turfgrass

Adam Garr asked if there is a great resource on morning light for turf:

Can anyone point me to a great resource explaining the importance of morning sun on greens that I can share with members?

I have a hypothesis about this. I wrote it on Twitter in response to Adam’s question, but I got it slightly wrong, in that I wrote max DLI when I meant max PPFD — or something like that. Here’s my hypothesis.

The most important time for full sunlight to reach a turfgrass surface corresponds to the optimum temperatures for photosynthesis for that species. More specifically, the longer the duration of time at which the PPFD on the turf surface is near the maximum the grass can use, while at the same time the air temperature is close to an optimum for photosynthesis, will optimize turf performance.

Turf grows best in full sun. But full sun is a concept—the reality is there are clouds, trees, buildings, mountains, and whatever other causes of shade that exist. My hypothesis would predict that morning sun, when temperatures are cool, would be better for cool-season grass in the heat of the summer, when afternoon temperatures are better for photorespiration than for photosynthesis. My hypothesis also predicts that morning sun is less important for warm-season grass in hot weather, because it is the afternoon sun combined with high temperatures that produces the maximum carbohydrates for those species.

I’ve got lots more to write and say about this. For a couple of things you can look at now, if you are interested in this topic:

  1. It is, I hope, more general than the title suggests. How much do clouds affect photosynthetic irradiance? Measures of light at Thailand, Hong Kong, Vietnam, and Japan.

  2. Screencast of my presentation on Eternal Sunshine of the Spotless Green about light, shade, and turf.