Question
I have a old cactus that I have mistakenly over watered this winter and it appears to be rotting from the top down.  I've let the soil dry out and the rest of the plant seems healthy.  Should I remove the top rotting part or will it come back?  If I remove it do I remove the entire arm of the cactus or just the rotting section?  And finally is there any thing I should do to the cut to help it seal up?

Answer
Dear Tom,

For some reason I have had a bunch of questions this month about this same problem. Generally there are two reasons why a plant dies from the top down. Either the roots died a while ago (for whatever reason) or the plant was frozen. I can't tell without pictures what the base of the plant looks like, but if you know it was frost that did the damage, the plant may heal and branch from the base near the soil. Only then trim off the dead parts.

There is only one exception, if your cactus is actually a Euphorbia, you should cut off the affected area. The reason being is that a Euphorbia has an internal fluid called latex that will spread the rot very quickly throughout the plant. The way you tell the difference is that generally a Euphorbia will have only 2 spines in pairs along the edge of the cactus, whereas a cactus will have many spines.

If the plant was not exposed to freezing temperatures, then I am afraid there's not much you can do. Hopefully this helps.

Sincerely,

Greg