Question
Hi...I live in Austin Texas, I have a Peruvian Apple cactus that I planted about 2 years ago, and it have reached a height of about 12', plus it had grown several arms.  It was doing Fantastic, until the latest Siberian Arctic air mass that came last week, temps dropped into the teen's, and now it looks awful!  The main stalk looks shriveled about half way up and some of the arms have turned yellow and look like they are dead, one has even fallen over.  What do you recommend I do with it to help it?

Answer
Get out your saw and sterilize it. Sterilize it for every new cut as well.  The yellow you are seeing indicates rot and that needs to be cut back to where the vascular system doesn't show any yellow, orange, black, etc.  You also need to cut off the main stalk that has shriveled because if it doesn't have rot now, it will soon.  Microorganisms and fungus will love that compromised tissue. Dust all the areas that you've cut. You may have to cut further back than you originally thought because of the  damage inside.Dust each wound with a fungicide if you have it or rooting powder or even charcoal dust. They will callous over. Winter isn't he best time to be cutting but you really don't have much choice.  The cactus is dormant so keep it dry.  Don't water it till you start to see some new growth.  The problem with cacti is that you often won't know the full damage sometimes for months, but hopefully if you treat and protect this plant it will do fine. It may not look the same or have flowers or fruit this year, but hopefully you'll learn to love the new look. If you do have a healthy arm you might want to cut it off, let it callous and pot it up indoors, just in case.

Good luck.  I know how hard it is when a plant gets sick or damaged. It seems your part of the country has had a really hard winter.