Watch this video and hear the claim “you cannot over water”. The official EarthBox website says the same thing.
“With the overflow hole you can never over water, so always keep the reservoir filled.”
The true facts based on science:
“Self-watering” is a marketing hype term used only in the consumer market. It is used to describe a planter that provides water to plants using sub-irrigation based on capillary action.
It is a misleading and inaccurate term that has led to the widespread misuse of sub-irrigation planters.
If you constantly top off the water reservoir, you will most definitely over water the plant(s) in the "self-watering" planter.
Unlike animals, plants have no intelligence to start and stop drinking.
EarthBox (and many others) tell a “white lie” when they say you cannot over water. The unsaid caveat is this is likely only when the planter is located outside in full sunlight.
Under 10,000 foot-candles (100,000 lux) the plant’s (vegetables in particular) rate of photosynthesis is so high that it will use all the water it can get. The plant will use all the water in a matter of days depending on the temperature and weather conditions.
Move the EarthBox (or any other “self-watering” planter) into your garage, under a porch or into any low light area and watch the plant die from over watering when you use the same constant refill method.
The rate of photosynthesis and water usage will slow down in low light and the plant will drown from a lack of oxygen.
Sub-irrigation is clearly the best way to water plants in containers but it's important to understand some simple plant biology and how capillary action works.
You are the self in self-watering. The best way to manage a sub-irrigation planter is for you to monitor the soil moisture.
Use a soil probe for plants in larger opaque containers. Maintain (grow) smaller plants in clear plastic containers and you can see the level of soil moisture along with the root system status. Recycled pop/soda bottles make superb sub-irrigation planters.
Only add water to the reservoir when the soil moisture indicates that it’s time to do it rather than the water level status in the reservoir.