The Mexican Fence Post Cactus is highly drought-resistant, requiring very little water in order to grow. In the drier months, plan on watering this cactus once or twice per month, allowing for the soil to dry between watering sessions. In winter and spring, you can reduce the watering frequency further, as your cactus will need even less moisture. Avoid overwatering, which is a common problem when tending to indoor cacti.
To check the water levels of your plant’s soil, place a water meter into the soil, where it will give you an accurate indication. If you do not own a water meter, stick one finger knuckle-deep into the soil. At this depth, you will be able to accurately account for the soil’s level of moistness.