Cactus can take a long time to grow by seed, but there are faster alternatives to multiplying your cacti collection, such as rooting. You can easily propagate cactus by taking a cutting from one cactus, rooting it, and thereby making another plant. Although different types of cacti will root in different ways, there are a few general guidelines you can follow to propagate cacti. What you will need is: cacti, a knife, rooting hormone, cactus potting mix, and a pot. Spring is the ideal time of year to propagate, as the cactus is coming out of dormancy and initiating growth.
Cacti that are columnar, pad-forming, and segmented are generally able to root by taking cuttings. Before beginning, make sure your blade is sterilized and sharp. Each kind of cactus will be cut differently. Columnar cactus will be cut into large segments. You will want the mother plant to be at least 18 inches long. Make two cuts, dividing the plant into thirds. You will use the top two sections to root. Make sure you remember which part is down on the middle section. Allow the pieces to sit out in a dark, dry area for 1 to 14 days. A callous will form over the cut; this will prevent the segment from rotting when you go to plant it.
Once the callous has formed on your cutting, dip the callous end (this is where the roots will form) into rooting hormone. Then, plant the section about three inches deep into a pot filled with well-draining cactus potting mix. Give it bright light (but not direct sun) and water, allowing it to dry entirely between waterings. It can take months for the roots to form.
To propagate pad-forming or branching cacti, cut at the joint. To propagate prickly pear, for instance, by cutting at the joint, you will want a single pad as a section to root. For segmented cacti, like Christmas Cactus, you can use multiple stem segments from the same plant. Whatever kind of cactus you are trying to root, make sure you take cuttings from a healthy plant, and never over water!