The Desert cacti and most of the succulents are very tolerant of high temperatures and bright full sunshine as long as there is plenty of fresh air around them.
In a closed greenhouse or sun room/conservatory in the height of Summer, temperatures inside can easily reach well over the hundred Fahrenheit mark, 38 Celsius, and your plants can ‘cook’ and although these plants love the heat and the sun, air ventilation is an absolute must so avoid temperatures indoors reaching above 90 Fahrenheit, (32 Celsius) which is not hard for me to do here in Ireland haha!
The Epiphytic cacti (Rainforest cacti) prefer to be kept in a more shadier spot away from intense heat and sunshine. These plants are from tropical rainforests that are shadier and cooler and more humid than the dry and intense heat of the arid regions where the desert cacti are from, so avoid exposing the Epiphytic cacti plants to very high temperatures during the high heat of Summer.
So just like intense heat during the Summer months will have an effect on the plants, during the Winter months, the colder temperatures will also affect on the plants and precautions should also be taken,
When it comes to overwintering these plants, the Desert Cacti and most of the other Succulents are happier to be kept at a minimum temperature of 5- 7c around 45f as long as the soil medium is completely dry, and the plants are overwintered in a dry atmosphere away from excess damp and humidity. There are some plants however both cacti and succulents that do require some watering over Winter you can find out more information on this by clicking here
Some of the Mammillaria’s, Rebutia’s and Agave’s, can take very low temperatures during winter, but as I have no personal experience of keeping them below 4c for any long periods I can not talk about this from my own experience but the general consensus is that they will be fine at lower temperature.
Melocacti and other warmth loving cacti such as Discocactus, and Uebelmannia need to be kept at a minimum temperature of 12-15c around 60f. The Epiphytic cacti also prefer to be kept at a minimum temperature no lower than 8-10c around 50f, lower than this and their stems are prone to scarring and spotting.
Check out my video I made on How to Overwinter your Cacti and Succulents, where I talk in more detail about the ideal temperatures to overwinter them.
Seedlings need to be kept warmer during their first Winter and I would not recommend the temperature to drop any lower than 10c 50f as they are more sensitive to damp and rot than mature plants.
Check out a video I made on How to Overwinter your Cacti and Succulent seedlings where I talk in more detail about the ideal temperatures in which to keep your seedlings in to get them through their first Winter.