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Effects of time and weeding method on the regeneration and growth of Japanese yam (Dioscorea japonica) vines and tubers in tea gardens

For the weed control of Japanese yams, farmers pull out the vines by hand because the chemical control of Japanese yams in tea plantations is difficult as the vines grow from the area below the tea trees, where the herbicides have limited reach. This study compared the regeneration and growth of vines after controlling weeds using the following methods: 1) cutting the vine at 30 cm height, 2) cutting the vine at ground level, and 3) pulling out the vine from the base (at the node of the tuber and vine). All three treatments were performed during three weeding seasons to determine the most effective method and time to control Japanese yams in tea gardens. In addition, we investigated the deterioration of old tubers and the formation of new tubers by studying their weight changes because each old tuber disappears and forms a new tuber each year. However, our results indicated that new tubers were formed before the disappearance of old tubers, and there were no border time periods where both old and new tubers were absent. Furthermore, the new tubers grew in a short period from July to August. Therefore, we recommend repeatedly pulling out Japanese yam vines during July for effective weed control. Moreover, when vines of Japanese yam were not pulled out completely or were cut in May or July, branches grew rapidly and reached the canopy of tea trees after 2–4 weeks. Furthermore, when vines were pulled out at the ground level, 40 % of vines reached the canopy after one month. Therefore, it is difficult to control Japanese yam completely by pulling out vines alone.