Saguaro arm dropped of cactus. Is it possible to replant the arm? What steps would be necessary to do so? This arm is quite large measures four feet has been detached for and on the ground for about one month?
The broken end should have been cut and dipped or rooting hormone applied before allowing the end to scar over.
This will help from rot setting in.
You could still try applying rooting hormone to the end before placing it in a cactus mix and wait and see.
If successful it will take some time.