By assumption, a cactus can withstand very high temperatures given that they live in the desert. In general temperatures above 32ºC (90ºF) will result in the cacti losing sufficient moisture and cause the cacti to “burn” or “scorch.” When a cactus burns, the plant begins to turn yellowish, and the body of the plant withers or shrivels up. If you have your plants in a greenhouse, make sure that it's well ventilated during the summer. Cactus too close to a glass window can burn. If the temperature drops below 30 degrees, move your plants inside or cover them. Never cover plants with plastic. Burlap works better and allows the plants to breathe.