In Caesar setting there is a temperature setting that should be the installation temperature, usually 21C. Does that setting accept a negative temperature as -35C? The plant I am studying during winter has that temperature so I would like to study the the worst scenario, I stalkation temp -35C

Yes you can input a negative value here.

Exercise caution.

If your pipe is cut in a warehouse at 21C, but "installed" at -35C, you should still be using 21C.