[dddaver]

I read a long time ago it was 87F. My cab is running at 87F now. So I Googled it .Most returns say high temps should never exceed 85F. But some also said it was dependent on airflow and if CO2 is being added. And even some that said their temperatures in their greenhouses often exceed mid 90s and plants still flourish (relating to that airflow increase I imagine).

I can only say my plants still grow well at 87F. I have a small 4" Vortex (had that sucker for 12 years) sucking in fresh air from outside, but not sure of cfm. Enough it seems though.

So, I think it really depends. But does anyone know for sure of the definitive max temp where cannabis sativa L. growth stops?

[starke]

Lighting choice makes a difference also. My girls are much happier under full spectrum LEDs at about 84-86 degrees. Several others on here have mentioned running higher temps with LEDs compared to MH/CMH etc.

[Phaeton]

All things are relative. The maximum temperature for growth is probably quite a bit higher than expected, well into the 100's F.
Efficiency of growth is also a benchmark.
86 F gives the combination of high efficiency energy conversion and rapid growth. This falls off as the temperature rises.
As the temperature go down the fall off is slower, with growth not stopping until less than 60 F.

Anecdotes: During summer a warm spell had daytime temps running 105 F and night temps 89 F.
This went on for almost two weeks. All the plants lived and produced on time. Yields were a tad bit light, but not a major loss.
Winter day temperatures run 80 F and nights get down to 49 F. This goes on for almost six weeks. All the plants live and harvests are on time. Buds are smaller but not a lot smaller.

The target is the 86 F that has tested out best in laboratory conditions. The temp sensor for that is 2" above the top bud directly in the beam of the grow light.
Sensors for ambient air and container temps are also used, but canopy temps are the important numbers.

Too cool is better than too hot, the harvests between 80 and 86 are pretty much identical. The harvests between 86 and 92 are more significantly different.