Question
hi maureen, i have been growing cacti for years, im not particularly botanical about my endeavors, more trial and error! from my collection i have two tall Mexican style(sorry i don't know there names) cacti that i keep outside, i am in the UK and we have just had two large snow drops, during the first the cacti survived but this time round the tops of both have become mushy and floppy, i have put them close to the house, under shelter and wrapped them in fleece.  is this enough or can i do more to save them??? i have read various blogs on the subject which has left me confused. there are schools on chopping the rotten part off, others on bringing the cacti inside removing the soil and allowing it to dry until spring, others suggesting if i cut the rotten part off it will leave the wound vulnerable to cold again. is there a definitive rescue strategy? please help!!! These two cacti are my biggest prize specimens i do not want to loose them. please please please help them live!!!!  

Answer
Dear Emma,
This is what I'd do. Because the tops have rot it will spread if you don't do something about it, sorry to say. I would bring them inside.  Being left out all winter generally stresses true non-cold hardy cacti and the added stress of snow and increased moisture probably tipped them over the top.Do you have any idea what kind of cacti you have?
The rot that develops is a fungus and if not removed will spread.  First unpot the plants and shake of as much of the soil as possible without disturbing the roots too much. Then with a sharp knife cut the rot back to healthy tissue. Clean the knife after each cut with alcohol or passing it through a flame.  When you think you've cut it all off check the vascular bundles for orange-brown marks that indicate that the fungus has spread even farther so you have to cut back more. Destroy the infected material and the soil (don't compost it). Leave the plant out awhile to dry. I also dust the wound with a powder fungicide but if you don't have that dust with rooting powder or charcoal powder.  I don't know many people that do this but i get paranoid about the open wound being exposed to some other creepy invader and if nothing else it makes me feel better.
Then either clean and disinfect the pots the plants were in or start over with new pots.  Make sure the soil drains well. I use sand (I use sifted builders sand that is coarse),and perlite. You can use other ingredients to. I don't use soil. If you do, sterilize it. So for me, it's 1/3-1/2 peat, then equal proportions sand and perlite. I throw in a bit of dried blood and bone meal and some sm pieces of charcoal and dry fungicide, but that's just what i like. The important thing is that it drains well while retaining some moisture. Wash the dust out of the sand and perlite. Just as long as it crumbles and doesn't stick when you squeeze it. There are tons of cactus mix recipes. Don't water for a week after potting, stake if wobbly, then water infrequently and mist once a week or so. Keep cool and sunny indoors till reestablished. Cacti are dormant in winter. From Oct.to March I don't water at all, just occasionally mist and keep the cacti in a bedroom that is unheated. With your cacti i wouldn't be so harsh.  When it's warmer put in a sunny place remember not to scorch. If you send a picture maybe I can ID the plants. Also, if the plants have offshoots cut a couple off to root just in case.  Rooting cactus is a whole other story.

Best of luck.  don't hesitate to contact me again if needed.  I love the UK, by the way.