Question
I only water my big (12" diameter x 14" tall) cactus once a month, but it's
looking much lighter yellowy-green at the bottom, and it seems to be
rotting--almost melting--on one side (at the belly, not the bottom).

I bought it from a plant store where it had been in the window for 10 years in
the same pot, and that would make me think it's not a soil drainage problem.
It hasn't gotten a huge amount of light in the past 6 months (east exposure,
cold winter), so I wonder if that's it? Do you think it would be OK in the sun
for the summer on my Brooklyn roof deck? Thank you!

Answer
Justine,

Sorry for the delay, I have had no internet service for almost a week. It is very possible the lack of light may have contributed to the problem. If it is rot, stop watering it. It may or may not survive, but continuing to water a cactus that is rotting will only make it much worse.

Putting it outside might be good as long as it does not get too much light. If it has not been in direct sunlight, it may also get sunburned, also bad. Make sure to give it afternoon shade if you put it outside and keep the rain off and it should hopefully recover. Good luck.

Sincerely,

Greg