Cacti are succulent plants that store water in their stems and often in their roots. They protect themselves with a tough wax-covered cuticle to lessen water loss as well as a spine covering to shade the skin and discourage herbivores. However, if injuries expose moist internal tissues, naturally occurring pathogens such as bacteria and fungi can feed on the stored food and moisture. If the wound is kept dry, some cacti cope by secreting a slimy polysaccharide that coats the wound and dries into a waterproof layer. Others form a tough corky layer over the wound. It can take weeks or months to complete the process. If the wound is near or in the soil, or humid weather or rain occurs, the chance of infection goes up. If infection occurs, the wound begins to rot, with pathogens eventually spreading through the entire plant.
Shallow Surface Wounds
Shallow wounds resulting from the nick of a hoe, a string trimmer slice, or a scrape should be treated immediately. Treat and disinfect the wound with hydrogen peroxide or dust it with horticultural sulfur or scouring powder containing bleach. Don't water the wound area and protect it from rain. Shallow wounds usually heal in a week or less in summer, longer in winter. Inspect your cacti occasionally to monitor chewing damage from wildlife, commoner in drought years, protecting the plants with fencing if necessary and treating the wounds.
Sometimes wildlife can make deeper wounds, such as pocket gophers digging up from the bottom of the plant, or rabbits chewing the sides. Clean and disinfect the wound, removing any dirt or beginning pockets of rot. All the tissue exposed should look white and clean. It will take longer for deep or extensive wounds to heal, often weeks or months in cool weather. For a deep puncture wound, excise any damaged tissue around the puncture and to expose the extent of the wound so air can reach the bottom. Disinfect and check for healing, which will be slower because less surface area is exposed to free air movement. When you take a stem cutting of a columnar cactus for rooting, make a slanting cut so water doesn't sit on the cut surface of the mother plant. A large diameter cut can take several months to heal.
Wounds From Transplanting or Propagating
Root wounds also need to be healed. When you dig up a cactus for relocation, use a sharp shovel and dig around the plant for about a foot, cleanly severing the roots you encounter. Then dig under the plant, levering it up so you can cut or cleanly break deeper roots. Shake off the soil and move the plant into the shade, where you can cleanly recut all broken roots and dust them with horticultural sulfur or rooting hormone containing fungicide. Allow root ends to heal before replanting the cactus, which should take a few days to several weeks in warm weather, depending on the diameter of the wounds. It takes longer to heal wounds in cool weather or if the plant is dormant.