We investigated the effects of root-zone heating in early morning to improve the growth of celery and reduce the cost of electricity for root-zone heating during winter in a hilly and mountainous area of Japan. Celery plants were transplanted to a soilless culture system in a sloped greenhouse. Polyethylene bags (30 x 90 cm) filled with bark composts were used as a substrate in the system. We used a heating wire and a thermostat for the root-zone heating. Four heating treatments were set after transplanting: 24h constant heating; CH, early morning (03:00-09:00) heating; MH, daytime (09:00-15:00) heating; DH, and non-heating; NH. The treatments were continued after transplanting through harvesting. The thermostat for root-zone heating was set at 15°C during the heating time. In CH and MH, the fresh weights of plants and the marketable part weights were larger than NH significantly. There were no significant difference in marketable part weights in CH and MH. We thought that MH was more effective in celery growth than other treatments, because the ratio of the difference in marketable part weights (dW) from NH to the diifference in root-zone temperature (dT) from NH was significantly larger than CH and DH. We also thought that maintaining favourable root-zone temperature from the morning to the evening was effective, and that at night was a little effective, in celery growth. The electricity consumption for the root-zone heating in DH was the lowest. However, the electricity cost was the lowest in MH, thanks to the dual fee system for electricity. We consequently concluded that root-zone heating in early-morning was more effective to improve celery growth in winter than constant heating the whole day.

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