Monthly Turfgrass Roundup: June 2015
Turfgrass Mystery: what's wrong with this picture?

Photosynthetically active radiation: this is what cool season grass may "see"

I've shared a series of charts (links at the bottom of this post) that show the photosynthetically active radiation (PAR) by time of day at Corvallis, Oregon. These charts are based on measurements made on 5 minute increments during 2014.

Those charts show the photosynthetic photo flux density (PPFD) on the y-axis and the time of day on the x-axis. At night, the PPFD is 0, and during the day, the PPFD is > 0, reaching a maximum of about 2000 μmol m-2 s-1 at midday during summer when there are no clouds blocking the sun.

However, cool-season (C3) grasses are considered to have a light saturation point of about 1000 μmol m-2 s-1. That is, the PAR from a PPFD up to about 1000 μmol m-2 s-1 can be used, and at that point the grass can't use any more light. So the PPFD that can be used by the grass may look something like this.

PPFD every 5 minutes in 2014 capped at 1000

When the measurements made every 5 minutes are filtered to include only those time increments in which the temperatures are reasonably suitable for photosynthesis -- that is, not too hot, and not too cold -- and then morning or afternoon shade are imposed, the charts look like this (morning shade, afternoon shade).

PPFD capped at 1000 with morning shade

PPFD capped at 1000 with afternoon shade

These posts show additional charts and calculations of total PAR over the course of a year:


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