This is a question for those who've studied lithops in their habitat.
I've killed many lithops from accidentally leaving them under direct sun for too long, even in open air. They basically popped like hot blister.
from what I've read lithops grow in open field often devoid of shades and flush with the surrounding soil. These ground often heats up considerably under the unrelenting South African sun,
Even if they are mostly buried, how did the tissue touching the heated surroundings not get toasted (that's how my cultivated lithops usually get killed)? Forgive my lack of understanding as I've never observed a specimen in Little Karoo.
attached is a photo of a L. dinteri v. brevis I accidentally overheated, it showed moments after the initial damage. It later died
You are probably living above 50 latitude, as we are in the London area. Consequently winter sunshine (when it occurs) is very weak and short-lived; the plants loose their resistance to insolation during the winter months. Come spring and summer, we have a very big contrast in light levels. In SA and Namibia, plant are growing around 30 latitude with less cloud cover and they never experience low light levels as we do. Some lithops do grow among dark rocks but many are in fields of white quartz which reflect the sun's radiation. Touch the quartz on a hot SA day and it feels cool. I saw the widespread L. lesliei on one occasion north of Kimberly and it was actually growing in a grassy area where the grass when in growth during the summer rainy season would provide some shade.
Thank you Terry, that’s very insightful. I do live below 50 latitude at 44 in Toronto, but winter is cold so the Lithops goes behind a dingy windowsill. Goes to show the general “full sun with good air flow” advice might be too simplistic.
Full sun and good air flow might be too simplistic, but I've never scorched any Lithops when I've had the fan on them. That is probably part of the solution, but also just the simple "the ground is cool". Cooler than a small pot anyway. Also, air temperatures even in my greenhouse are not as extreme as many places or even many other UK greenhouses, so the air movement is very effective at cooling.
It can be informative to measure the temperature of a Lithops body in the sun. These regularly exceed 50C without damage, although I try to keep them below that if I can. Lithops are unfortunately very efficient at warming themselves up, most likely precisely because they are sitting up to their necks in the cool ground. They do take a while to get really hot, but the process is inexorable and in the greenhouse the plants really have no way to cool down without a fan on them.
Thank you Ian, it's a hurdle for me to overcome as 99% of the ones I killed died by such manner. I'm amused that you actually took the trouble to measure your plants' temperatures, presumably with IR thermometers?
