How do you propagate a 25 year old columnar cactus?
To begin, protect your fingers from cactus spines by wearing sturdy gloves or by wrapping them in tape. Using a serrated knife, make a clean cut on the cactus. Dust the cutting with sulphur or a rooting compound. Allow the cut area to air dry until it is callused over which will protect it from most soil born diseases. Depending on the size of your specimen, this may take a few days to a week or more.
Once dry, use a propagation mix of half organic matter and half inorganic material such as 50% pumice or perlite and 50% peat or compost. Cutting should be placed deeply enough in the container so that it is stable. Place in a bright area and water immediately, but then do not water again until the rooting medium is dry.
Depending on the specific type of columnar cactus, the growing tip may not show signs of new growth for some time. The cutting should feel firmly anchored in the soil per this note from the Desert Botanical Garden in Phoenix:
‘Check for roots every two weeks by gently moving the plant in the soil, using tongs or wearing gloves. If there is strong resistance, the cutting is rooted. New growth is evidence that rooting has occurred, but sudden swelling (turgor) of the stem is better proof that water-absorbing roots are present. As long as the cutting still contains moisture, and is not diseased, it still has the potential to eventually make roots, even if it is somewhat shrivelled.’