The number of leaves on an onion plant, which is an important element of the yield, is related to the bulb diameter and weight. In order to estimate the number of leaves of an onion plant based on the accumulated temperature, we compared several calcu lation methods. The fitness of the estimation formula for a specific calculation method was verified by examining different variables: sowing date, cropping type, and method of phosphoric acid fertilization (broadcast or locally applied). As a result, the accuracy of the estimation method using the daily average temperature was lower than the triangle method and the average temperature method with a base temperature of 3.5°C or 5.0°C. In addition, when the base temperature to calculate the effective temperature was set for the accumulated temperature method or triangle method, a base temperature of 5.0°C was more accurate than that of 3.5°C. Therefore, our results indicate that the formula of the average temperature method with the base temperature set to 5.0°C is highly correlated with the effective temperature and number of leaves, and the prediction estimation error is small. In different cropping types, the average temperature method with a base temperature of 5.0°C was the most accurate. Based on our results, the estimation formula with the highest estimation accuracy is the average temperature method with a base tempera ture of 5.0°C. However, further examination is required involving other varieties and regions, but the coefficient of the estimated formula suggests that onion plants develop 1.5 leaves per accumulated temperature above 5.0°C of 100°C・day.
