a hydroponic planter includes an outer container and an inner liner . the liner includes a central inverted cup - shaped reservoir for a liquid nutrient solution . the outer wall of the liner includes slots extending substantially above the height of the reservoir , to provide aeration of the roots of plants growing in the planter . a combined light and liquid level indicator determines the minimum amount of light in which the planter can be placed , shows when to add nutrient solution to the planter , and indicates when too little , sufficient or too much nutrient solution has been added . the light / liquid level monitor includes three light - transmitting rods of different lengths extending between the outer container and the liner ; one extends to the bottom of the liner , another to below the top of the reservoir , and the third to near the top of the reservoir . the rods have flat upper indicating surfaces and conical ends so that the indicating surface becomes dark in appearance when the conical end is immersed in liquid .