Calystegia japonica Choisy is an invasive, perennial, rhizomatous weed of forage crops in Japan. Although the species is currently spreading in poorly drained upland areas that were formerly under paddy culture, the dynamics of the rhizome system in hydric soils have not been investigated. The current study was conducted to determine the efficiency of water stress as a means of controlling this species. Clonal plants were established in pots in April 1997 and April 1998, using stored rhizomes collected from a single plant in the autumn of 1996 at Itinohe town in Iwate. Pots were set in tanks and plants were watered with an auto-watering apparatus to maintain adequate growth conditions for the experimental periods. Moderate and severe water stresses were later applied by flooding tanks with water to depths of 20cm below the soil surface or 5cm above the soil surface. Within each treatment, flooded conditions were maintained for one or two months. To study rhizome dynamics in the post-emergence stage, water stresses were imposed on 15 May using plants grown in 1997. To study rhizome dynamics in the pre-emergence stage, water stresses were imposed on 15 April using plants grown in 1998. Plant dry weight, rhizome dry weight, the number of buds on rhizomes, and rhizome total non-structural carbohydrate (TNC) concentration were measured. Moderate water stress had no effect on plant growth during pre- and post-emergence stages (Tables 1 and 4). However, severe water stress applied during pre-emergence strongly inhibited plant growth (Table 1). The reduction in growth was most pronounced after two months of water stress. In this treatment, we observed early stages of rhizome senescence when water levels were lowered to control levels on 15 June 1998, and complete senescence by 15 August 1998 (Tables 1 and 3). In contrast, we found no evidence of senescence when severe water stress was applied for two months during post-emergence. Although growth was negligible during the imposition of water stress, shoots appeared soon after release from water stress and growth continued until 15 August 1997 (Tables 4 and 5). Table 2 summarizes statistical analysis by ANOVA for rhizome dry weight and the number of buds on rhizome in the pre-emergence stage. Both response variables were significantly affected by water depth as a main factor (p<0.05), with rhizome dry weight being affected by the duration of water stress as a subfactor (p<0.05). No significant interaction was observed between water depth and duration of stress for either item. The results suggest that submerging the afflicted field with water for two months starting before the emergence of shoots in early spring would be an effective measure of controlling the spread of Calystegia japonica Choisy. At the same time, we propose that forage crops that can tolerate relatively wet soils, such as forage rice, Japanese millet, Job's tears or fall panicum, should be grown instead of the more traditional forage corn. In the following year, forage corn could then be grown upon restoration of the soil to less hydric conditions suitable for cultivating such crops. In conclusion, we consider that implementing these measures should provide a very practical, cost-effective and environmentally friendly alternative to the current practice of using herbicides to control Calystegia japonica Choisy.

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Barbarea vulgaris R. Br. (Cruciferae) is a naturalized weed that originated from Europe. Our questionnaire revealed that the species was found in all six prefectures of the Tohoku area in 1993 and 1996. From the distribution survey in 2001, we found that B. vulgaris had adapted to the area and had established itself as a weed throughout the Tohoku area. The weed abounded especially in the Aomori, Yokote, Kitakami, Sizukuishi and Tohno regions. B. vulgaris has spread in the Yokote region during the past eight years. Most of the populations were found along the roadside and levees near irrigation canals for paddy fields, and they also occurred on riverbanks. Furthermore, they were found in levees where the bottom soil of irrigation canals had been deposited. B. vulgaris seeds submerged for 731 days still germinated, indicating that they could survive in water for two years. These results suggest that B. vulgaris is spread by the seed dispersed by water from irrigation canals and rivers.

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