Effects of Use of Frost Protective Fan on Frost Injury to Tea Plants in March 2010 at Makinohara, Shizuoka

Serious frost damage to tea plants was observed in Shizuoka prefecture in March 2010. In this report, we present the results of a survey conducted on a tea field in which tea plants were injured by frost on March 30, 2010. A survey was conducted at a 0.1 ha tea field located in Makinohara, Shizuoka. Two frost protective fans (1.98 kW, 0.9 m diameter triple-blade) were mounted on 7-m-high steel towers installed at the tea field. The fan installed in the southern part of the field was operated continuously, whereas the fan installed in the northern part was virtually inactive because of energy saver control. Temperatures were monitored at 15 cm and 160 cm above the canopy of the tea plants and 7m above the ground. Wind speed and direction were measured at 1.9m above the canopy of the tea plants. Two weeks after the occurrence of frost injury, the canopy shoots were visually inspected in order to evaluate the damage. The observations and results of the survey can be summarized as follows: 1)Between 2:00 am and 6:00 am on March 30, the nighttime air temperatures at both 15 cm above the canopy of the tea plants and 7 m above the ground dropped below freezing temperature. 2)The frost injuries to the tea plants under the continuous operation of a frost protection fan in an area of 570 m2 were classified into four types: 0 (no injury), 1 (frost injury without necrosis), 2 (frost injury with necrosis), and 3 (necrosis). Type 0 accounted for 0% of the tea field area; type 1, 24%; type 2, 37%; and type 3, 39% respectively. 3)The continuous wind generated by the frost protection fan reduced the inversion strength between 15 cm and 160 cm above the canopy of the tea plants. We observed that the temperature inversion was weaker when the fan was operated continuously than when the fan was inactive. However, the area protected by the fans was 260m2 that was determined by the first plucking area of tea shoots. 4)The temperature reduction on March 30 was occurred by a combination of both advective and radiative frost conditions, so that the frost protective fan could not protect tea plants completely against frost injury. However, the using of the fan is effective to avoid delay plucking and to be harvested tea shoots under the severe frost condition.