A cactus that has been growing upright but is now leaning like the Tower of Pisa should be a concern for the gardener. The plant may be perfectly healthy, or it may be sick. Careful observations can help determine why the plant is leaning. If a problem is detected early, solutions are available that will save the cactus.
Even if your cactus is planted in full sun, it may lean. Plants naturally grow toward the sunlight. The barrel cactus grows southward toward the sun so much that it practically lies over on one side. If the cactus is otherwise healthy, this is not a danger. However, if a cactus planted in partial shade is leaning, it may need more sunlight. A cactus in this condition will look sickly and have weak, flimsy stems that are yellow-green. The plant should gradually be exposed to stronger sunlight to prevent sunburn.
If the cactus is leaning from the ground, it may be loosely planted in the soil. Improper watering may be the reason. After watering a cactus, the soil must be allowed to completely dry out before watering again. Watering too frequently prevents the soil from hardening around the base of the cactus and causes the soil to loosen. Provided there is no root rot, either transplanting the cactus in dry soil or packing more dirt around it is a solution. Also, adding a layer of small rocks or gravel on the surface can stabilize the cactus.
Rot is caused from fungi and bacteria growing on the plant. The most common reason for rot is watering the cactus too often. The infected spot will have a watery, slimy, black appearance. Rot begins at the base of the cactus and works its way to the top. Rotting plants start to lean, then collapse and die. If rot is detected early, dig up the cactus and replant it in dry soil. Otherwise, remove the cactus from the soil, and cut the healthy top away from the diseased part. Allow the top to dry, and dust the cut surface with a fungicide before replanting it in sandy soil.
Insects eating the stem of the cactus can weaken the plant and cause it to lean. A plant infested with mealy bugs will stop growing and take on a sickly, deformed appearance. Mealy bugs should be removed with tweezers or a cotton swab dipped in alcohol. Ants nesting at the base of the cactus eat away at the stem causing it to lean and eventually fall over. Wash away the ants with a strong, lukewarm stream of water. A cactus weakened by insects may develop fungi and rot and will die if not treated.