Question
I have three cactus that my sons planted 15 years ago.  I have repotted them several times in cactus potting mix.  They live on my glassed in porch facing the southeast.  I believe one of them is a barrel cactus.  It is only about three inches tall and it has black around the roots.  The tallest one is almost a foot tall and appears to be healthy. no problems.  The third is kind of four sided with lots of spines and it appears to be turning gray and shriveling up.  I have not overwatered although I was alarmed when I notice one was shriveled and leaning a bit.  They are all three in the same pot.  The did have what appeared to be a whitish mold type of round areas which I washed off with a q-tip and water.  They didn't come back.  There are no bugs in the soil.  Since
they are so old and my sons planted them from seed so long ago I want to save them if I can.  I believe the barrel is beyone help because it has black root.  Is there a way I can air graft it or start a new plant from the top part that is still healthy?  When I was in high school we learned how to air graft plants with stems so there must me a way to do it with a cactus as well.  Please help me if you can, I want to save these cactus desperately since they are so old and my sons planted them when they were little boys and are grown and married now.

Answer
Dear Lorrie,

Unfortunately, I'm not an expert at grafting cacti, however this site may help in that regard:

http://www.cactus-and-succulents.com/text/grafting-cactus-and-succulents.htm

Just keep in mind for future reference, not all cacti come from deserts. Some cacti in Chile survive off of fog that accumulates on them and records show that rain has not fallen in hundreds of years. Other cacti from southern Mexico might get over 25 inches of rain per year, which is well above what is considered a desert.

What I am getting at is that planting different cacti in the same pot can be a problem as far as watering goes. It sounds like based on your description that one cactus is OK, one is rotting (from too much water) and one is shriveling up (from a lack of water).

Let me know if you need any additional information. Thanks.

Sincerely,

Greg