Question
I have had a castle cactus for over five years now. It is over a foot high.  We built another home, so the cactus was kept in a low light environment for two months. Since I brought it back, one by one, the stems are slowly getting weaker and fall. I have to cut them and the process is starting again for another stem. It is the third stem now, and I can see the others weakening. I don't think I water it too much, say every two weeks, it is in a very lighted area with full sun, when it is available (I live in Canada# from the sunroom, and an average of 15 to 25 C #55 to 70 F).

I tried not to get it to freeze, when I brought it back, but it's been over a month now.

What can I do to help it survive?

Answer
Dear Pierre,

Look at the roots. It souldns lke the plant is rotting from the roots up.  If they are brown, flat, unhealthy looking then i don't think you can do anything to save your plant. If the roots look healthy then put the plant in a smaller pot so it's a bit crowded and don't water it till you see new growth.  If some roots are heathy and some not, cut off the dead roots with a sterile knife or scissors with each cut and brush what's left with fungicide.  Then leave it out of the soil for about a week to let the cut dry. If that is the case your chance of saving the plant decreases dramatically.

It does sound like root rot tho.  Castle cactus aren't the easiest to grow.  If kept in low light AND watered for a couple of months it might have been enough to kill it.

Do check first to see if the roots are heathy and if they are, when you remove a limb use a sterile knife and brush the cut with fungicide. Only water when the soil is completely dry and then drench it. Be careful of full sun till it is established.