There’s not a hard-and-fast rule for how often to water
Mexican fence post cactus. The best way to determine this is to check the soil and only water when it’s bone dry. You can either stick your finger in the pot or use a moisture meter to check the soil below the surface. When you plant it in a deep pot, you can do this with a stick or chopstick. If it feels even a little bit moist, wait a few days and check it again.
Most people will need to water
Mexican fence post cactus about every two weeks in summer and once a month in winter, but there are several factors that can change the frequency. The section below lists some considerations that can help you to determine how often to water.