Question
I have a mexican fence post cactus that is 3 feet tall in a 12" container.    The sticker on the pot says it's latin name is "lemaireocereus marginata".  It's actually two cacti side by side.  One is slightly shorter and leaning a little towards the taller cactus.  It reminded me of a man and woman, with the woman leaning towards the man.  I bought it a year ago as an anniversary present for my husband and myself.

I have a brown thumb and cacti are some of the only things I can (usually) keep alive.  My gardening mantra is,  "If the deer don't eat it and I don't kill it it stays!" I'm not so sure what to do with my mexican fence post "couple".   They had been in my screened porch since last November or so, when it was raining a lot.  I just moved the pot outdoors in the past two weeks.  It is sitting in front of my house, full sun, eastern exposure.  So it gets sun most of the day.

Over the time it was indoors, it has developed tan spots, especially near the base.  And now I notice the top is turning black.  The top is getting worse every day.  I hate to see this plant die - it would really sadden me.  I want to plant it in the ground but I'm not sure if that's a good idea.  It would be near where it is now - eastern exposure, sunny, but it rains a lot in the winter.  I live in California in the Santa Cruz mountains.

Can you advise me what to do?  Why is the top turning black?  Why does it have tan splotches?  Should I transplant it in the ground?  How do I protect it in the rainy season?  I hope you can help before it's too late.

Sincerely,

Gina

Answer
Dear Gina,

Brown spots are typically caused by sucking insects but also may develop depending on the age of your plant. The black tip is not good and usually indicates damage of some kind.

To save the plant, you may want to cut off the black part, starting the cut about an inch below where the black ends. Be very careful not to cut yourself or damage the cactus. If after you remove the top of the plant you see a black spot, continue to cut off an inch more at a time until you no longer see a black spot.

Hopefully this helps.

Sincerely,

Greg