It protects against under watering as well as over watering. The WaterStik is a break through in soil moisture measuring technology that makes it possible to accurately monitor the water content in any indoor potted plant's soil. I have been testing one for about three months and have over watered three plants, under watered three plants and properly watered three more (or so I thought). The WaterStick worked as advertised in all areas and even help correct my watering patterns.
Here is how it works: Simply place the WaterStik into the your plants soil. The top will Automatically blink when plant needs water or has recived too much water. The color of light indicates the plant's status . It Uses full spectrum LED technology and depth gauge technology allows to different plant types and watering habits.