Sustee Medium size is best for 4.1 - 7.1″ diameter pots. Sustee Aquameter (Moisture Sensor) conveniently changes color to notify that your plants need to be watered. This multi-award-winning design is maintenance-free and works with all types of plants. No batteries required. After about 6-9 months, Sustee insert will need to be replaced in order to work optimally.