Mark Hunt's Weatherblog had an interesting statement this week – "why I think the Growth Potential [Optimum] Temperature is wrong for Poa annua." Regarding recent temperatures in Southeast England, he wrote:

For me I think Poa has been under stress for the last two weeks and so the GP model should be showing this with a lowered daily figure, but it’s not and therefore I think the optimum temperature is set too high at 20ºC, it should be more like 18ºC, so that’s what I’m going to change to in 2015.

I think this is a good idea. The objective of the growth potential (GP) model is to generate a number between 0 and 1 that can be an index of potential to grow, or high temperature stress, or low temperature dormancy, and this can be adjusted to fit the conditions and grass species at a particular site.

Jason Haines wrote about his adjustment of the GP to 18ºC for Poa annua greens in Canada. When I was in Iceland last year, I wrote that the 20ºC optimum temperature in the standard GP equation would likely need to be adjusted to fit the observed growth under Icelandic conditions. One of the great things about the GP equation is the ease with which one can adjust it. 

Growth potential (GP) curves shift left or right if one changes the optimum temperature in the growth potential equation

Now I don't advise changing this too much if you don't have to. The standard optimum temperatures of 20ºC for cool-season (C₃) grass and 31ºC for warm-season (C₄) grass work well for most turfgrass species. And it is the simplicity and consistency of the GP concept that makes it so useful.

With C₃ grasses, I would consider adjusting the optimum temperature down if looking at Poa annua (as Mark wrote) or fine fescue (pictured above in Iceland). With C₄ grasses, the only one I would adjust is kikuyugrass (Pennisetum clandestinum), which will have an optimum temperature somewhere in the mid-20s. Kikuyugrass thrives in temperatures warmer than optimum for C₃ grass, but at much cooler temperatures than optimum for other C₄ grasses. 

For example, kikuyugrass is the dominant grass in the mild climate of Kodaikanal (above) in the Palani Hills of Tamil Nadu, just as it is at Real Club de Golf de Las Palmas (below). One would need to adjust the optimum temperature in the growth potential equation for this species in particular.