Question
We have three sets of Mexican Fence Post Cactus planted in a new landscape design.  Our lot is at about 2800 feet elevation near Tucson. Some people have suggested the use of Styrofoam cups on each "post" to prevent frost damage during the winter.  My question is, should the cups stay on day and night until the risk of frost is past, or should the cups be removed during the day or between frost threats?  Thank you!

Larry

Answer
I believe you have Pachycereus marginatus which are a tad more sensitive to frost cold, tho from what I read can tolerate temps to 25 degrees.  Are they new?  Frost can call scarring. The best thing for you to do is find a cactus expert in Tucson as I understand it depends on where you are located.  

I would, however,take the cups off if there is no frost predicted both day and night.  If you leave these cups on for awhile remember the cacti will have to get used to the sun again, tho I doubt you'd have to protect them for that to be a worry.

You cold also contact the Tuscan Cactus and Succulent Society for more info.