Most people water their plants from the top, even though plants actually absorb water from the bottom-up. Self-watering planters, on the other hand, usually have water reservoirs that sit at the bottom of a pot to be drawn from as needed through a process called capillary action. Essentially, a plant’s root system attracts water from the reservoir and transports it upward due to the water’s adhesion, cohesion, and surface tension (thank you, physics!). Once it has reached the plant’s leaves, the water can be used for photosynthesis and other essential plant processes.
When a houseplant receives too much water, that water sits at the bottom of the pot, oversaturates the roots, and makes capillary action impossible, which is why overwatering is a leading cause of root rot and plant death. But since self-watering planters keep the water supply separate from your actual plant, they don’t drown roots.
When a houseplant doesn’t get enough water, the water it does get tends to stay on top of the soil, drying out the roots below. As long as your self-watering planter is refilled with water periodically, you don’t have to worry about this either.