Growing a cactus from a cutting is not as hard as you might think! One of the most common mistakes people make is planting the cutting before it has callused. Follow these instructions for rooting a cactus cutting and you will surely be successful.
CUT: Make a straight, clean cut with a sharp knife. Use a saw for larger plants with woody skeletons. The application of powdered sulfur on the cut surfaces helps prevent infection. Cactus cuttings root most easily when taken during their natural growth season (usually in warm weather).
DRY: Place the cutting in a cool, dry area out of direct sunlight until the wound is fully callused over. This will take a few days for a small cut surface, or a week for a large cut surface. Erect stems should be kept erect during this period, or else turned frequently, to prevent the formation of roots along the side that is on the bottom, and to prevent possible curvature of the stem.
PLANT: Plant the cutting in pure sand or a well-drained soil mixture just deep enough to hold it upright. The soil mixture should contain enough gravel, coarse sand, perlite, or pumice to ensure good drainage. Test the drainage by running water through the pot or rooting bed to be sure it drains quickly. During cool or humid weather, cactus cuttings should be rooted in an especially well-drained mixture of half coarse sand and half soil or pure sand if you have it. Rooting is best accomplished with some shade to prevent sunburn of the plant. Cuttings in full sun will require more water and sunburn very easily. For cacti which crawl along the ground or that have long thin stems, place the cutting on top of the soil mix, sand or directly on top of the loose soil. For prickly pears or chollas, or any other cactus that branches freely, place the pad or stem in the soil or on its side, so that new growth will be clean and upright.
WATER: Water immediately after planting and thereafter every time the planting mix becomes totally dry. Never allow the planting mix to remain totally dry for more than a few days. Depending on pot size, soil texture, and weather, drying may take as short a time as 24 hours or as long as three to four months. The main reason for rooting failure is rot. This is caused by too much water, especially in cool or humid months when soil does not dry out quickly. In cool weather, it may not be necessary to water the plant after the initial watering until the weather begins to turn warm.
CHECK: 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 shriveled.