*Medium size is best for 4.1 - 7.1″ diameter pots. Sustee is unlike any other moisture sensor - you can leave it in the soil and it works for 6 to 9 months without the accuracy rapidly degrading. The inner core will tell you when to water the plant. This multi-award-winning design is maintenance free and works with all types of plants. No batteries required. Product Details ・Color of the inner core changes based on pF values. White means the plant needs watering; blue means it has enough water. ・The inner core needs to be replaced after 6-9 months in order to work optimally. (Sold separately) ・The inner core is made from biodegradable fabric. ・Collaborated with Tokyo University of Agriculture to improve the design and function of each stick. ・Made in the Japan, ships from USA Size Options: S (0.4 x 0.3 x 4.7″) - best for 2.4 - 3.5″ diameter pots M (0.4 x 0.3 x 7.1″) - best for 4.1 - 7.1″ diameter pots L (0.4 x 0.3 x 10″) - best for 7.1 - 10.2″ diameter pots