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Roots of cacti are relatively shallow, with mean depths of 7 to 11 cm for various species native to the Sonoran Desert and 15 cm for cultivated opuntioids; the cultivated vine cactus Hylocereus undatus has even shallower roots. Such shallowness facilitates water uptake after light rainfalls but exposes the roots to extreme temperatures occurring near the soil surface. The uptake of the vital stain neutral red into root cortical cells was decreased by extreme temperatures, with 50% inhibition (LT50) occurring at an average of -7 degrees C for low temperatures and 57 degrees C for high temperatures for Nopalea cochenillifera, Opuntia ficus-indica, and O. robusta and -2 degrees C and 52 degrees C, respectively, for H. undatus growing at moderate day/night air temperatures of 25/20 degrees C. The opuntioid LT50's decreased 1.2 degrees C as the day/night air temperatures were lowered by 20 degrees C and increased 4.4 degrees C as they were raised by 20 degrees C, indicating the possibility of seasonal acclimation to changing ambient temperatures. An equation is presented to predict soil temperature as a function of depth in the soil and time, which can be used to assess root growth, respiration, and layers with lethal temperatures. In this regard, the sensitivity of roots to low temperatures should not be a limitation for present and future cultivation of most cacti. Increases in air and soil temperatures accompanying global climate change could cause fewer roots of cacti to be in the uppermost soil layers, which can affect plant water uptake, but much greater limitations should occur for other (non-CAM) perennials.