iStation is Plants personal water reservoir featured with a valve, so you can control the flow of water going to your plant even to drops, completely shut off the water.
Volume: can hold up to 2 cups of water (500ml, or 16oz)
Height: 30cm, or 12in (from top to bottom)
It is made with PLA plastic. 
"...small plants used 1 tablespoon [of water] per day, while large plants used slightly less than 2 tablespoons per day." (www.greenhousemag.com); So simply fill iStation up with water, place it in the soil inside the flower pot, adjust the flow of water to intermittent single drops.