Some months ago, we discussed morning sun and its importance for cool season grass.
I mentioned that I have a hypothesis about timing of light, one that goes something like this:
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.
For cool season (C3) grass, I expect morning sun to be relatively more important than afternoon sun, because the temperatures in the morning will tend to be more suitable for photosynthesis than afternoon temperatures when it may be too hot. Note that this will vary based on where one is located. If the high temperatures in the afternoon are just right for C3 growth, then for that location I predict afternoon sun would be more important than morning sun.
To test this hypothesis, I downloaded the temperatures and sunshine hours for Mobara, Japan, in 10 minute increments from 1 January 2014 to 31 December 2014. Then I calculated the temperature-based growth potential (GP) for each 10 minute increment of the year, and I used the Ångström equation to calculate the PPFD. Then I selected only those increments during which the GP was greater than or equal to (>=) 0.5 and the PPFD was >= 0.5 of the the maximum potential PPFD for that time. Those times are shown in this chart, representing 62,870 total minutes during 2014.
You'll notice that there are not many times in the winter with low PPFD on this chart. that is because the temperatures at Mobara on winter mornings would tend to be too cold for GP to be above 0.5, so the times with warm enough temperatures to have GP above 0.5 will correspond with times of the day when PPFD is relatively high.
You can also notice that in July and August, there are not many times shown with a high PPFD. That is because it is so hot at Mobara in those months that the afternoon times, with a high PPFD, will have a GP below 0.5, and thus those 10 minute increments are not shown.
To simulate morning shade, I then reduced the PPFD by 80% for all times before 10 a.m. That reduces the amount of time with both GP and PPFD above 0.5 of their potential by 33%, from 62,870 minutes to 42,220 minutes. The simulated morning shade before 10 a.m. is shown in this chart.
One loses all the summer morning points, because by the time the shade is gone at 10 a.m. in Mobara, the temperatures are so hot that the C3 GP is less than 0.5.
Afternoon shade is not desirable either, but it is better than morning shade if there is a climate like Mobara. With simulated afternoon shade reducing PPFD by 80% for all times after 14:00, there is a 19% reduction in hours meeting the GP & PPFD cutoffs of 0.5 or more of potential. Compared to no artificial shade, the morning shade before 10:00 reduced the time for optimum growth by 33%, and the afternoon shade reduced the time for optimum growth bv 19%. This chart shows the afternoon shade.