"How to prune a cactus" sounds like a lead-in to a bad joke, the answer to which would be: "Carefully!" But caution is indeed the watchword when working with desert cacti (Cactaceae), since their protective spines can be very painful if they find their way into your skin. Fortunately, this is a very infrequent task for your to-do list. Unless seriously diseased or damaged, cacti can typically go for years without being trimmed.
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The main purpose for occasional trimming of healthy cactus plants is to minimize their susceptibility to disease and optimize their appearance. Because of this, most cacti will not require any kind of ongoing pruning care. If your plant needs to be pruned, it can be done in just a few simple steps.
How to Prune a Cactus
Step 1: Put On Safety Gear
Take steps to make sure you are safe while pruning. Cover yourself completely with protective clothing when pruning large cactus plants outdoors. Wear long sleeves and pants and put on heavy leather gloves and shoes or boots. For added protection, cover your face with a bandana, wear a hat and don plastic safety goggles to protect your eyes. When cutting small, indoor plants, such extensive measures generally are not necessary; gardening gloves are usually sufficient, just make sure they are thick enough to protect your hands from the sharp spines.
Step 2: Prepare the Ground
Cover the ground around the cactus you are pruning with an old sheet, carpet scraps or newspapers. This collects the spines and smaller pieces of the plant as you prune it and keeps them contained for easier disposal. Place a large trash can and shovel nearby to dispose of larger pieces of the cactus.
Step 3: Sterilize Your Tools
Sanitize the blades of your pruning tool by wiping them with isopropyl alcohol. Continue to clean your tool as you move from one plant to another to reduce the risk of spreading disease.
Step 4: Trim a Prickly Pear Cactus
Cut back to a new pad when trimming prickly pear cactus (Opuntia spp., USDA zones 4-9). Use a hand saw or loppers to cut off sections of the pads you want to remove, until you have removed them. You may want to use long-handled tongs to grasp cut pads and bring them down in a controlled manner. Finally, according to Joe Marcus of the Lady Bird Johnson Wildflower Center, make a clean cut a few inches above the stem where you will make your final cut. This prevents the stem from splitting.
Step 5: Prune a Columnar Cactus
Prune columnar cacti, such as saguaro (Carnegiea gigantea, zones 9-11) or organpipe (Stenocereus thurberi, zones 9-11), with either the hand saw or garden lopper. Cut off knobs or offshoot columns to prune back weak or discolored sections of the cactus. To thin out an overgrown plant, cut off one or more entire columns at the base of the cactus.
Step 6: Clean Up the Area Carefully
Discard the pruned pieces of the cactus carefully. Use the shovel to scoop up larger pieces and carry them to the trash can for disposal. Carefully gather up the corners or edges of the ground cover containing the smaller pieces and carry the entire bundle to the trash can.