Start Cactus from Cutting
I have a 30 year old cactus that has grown to be 5 feet tall. It was knocked over and the top foot of the plant broke off. I was hoping to start a new one from that piece but it keeps rotting in the soil.
Re-cut the bottom of the cactus you are trying to root. Let it sit overnight so the cut edge begins to seal. Then try rooting it in vermiculite or sand. Once rooted you can move it to a well-drained potting or cactus mix.