Euphorbia graminea Jacq. (Euphorbiaceae), which is native to Mexico, was first recorded at Okinawa-honto Island, Japan in 2004. The authors recognized E. graminea growing in a wide range of Miyako-jima Island and invading sugarcane fields. To confirm the E. graminea distribution in the Miyako-jima region and to elucidate its expansion process, we conducted a herbarium survey, re-examination of earlier works related to weeds in sugarcane fields, and field surveys of 2017–2018 and 2019. The first record of the species in the Miyako-jima region was in 2007. Subsequently, within about five years, it expanded its distribution quickly and invaded sugarcane fields. Within about five years, it spread to almost all areas, except for a few islands. It was found in 20–30% of all the sugarcane fields in 2019. Our results suggest that E. graminea has become a major weed in sugarcane fields of the Miyako-jima region. Considering the rapid expansion of its distribution in the Miyako-jima area, it is expected to be necessary to monitor the alien species carefully in sugarcane fields of other regions of Okinawa Prefecture.

Winter cereal crops have been severely infested with wild oat (Avena fatua L.) in Ibaraki, Japan, especially in the western part of the prefecture. For effective use of soil-applied herbicides, seedling emergence of wild oat should be considered. In this study, using wild oat seeds collected from 10 populations in Ibaraki, we evaluated variation in seed dormancy using a germination test and a census of seedling emergence. We found four different types of seed dormancy among these populations: shallow (one population), medium (two populations), medium-deep (two populations), and deep dormancy (five populations). Most seedlings in the five populations falling under shallow, medium, and medium-deep dormancy types emerged by the end of December. In the five populations falling under deep dormancy, seedlings emerged throughout the growing season, from November to April. Following census of seedling emergence, it was found that medium-deep or deep dormancy populations had more viable seeds than shallow or medium dormancy populations, and that the highest seed viability was 57.0%. Ungerminated seeds could emerge the following year. In the production of winter cereals, it is considered that wild oat populations with deep seed dormancy are difficult to control using herbicides. In that case, multiple methods such as cultural control should be included.

In Japan, the spread of morning glory (Ipomoea spp.) in soybean fields has become a serious problem, thus necessitating the establishment of an effective control method. Therefore, in this study, we sought to clarify the optimum application system of imazamox ammonium salt (IAS) with bentazone to control the spread of morning glory and to investigate the associated degree of phytotoxicity in soybean. Field experiments were conducted from 2017 to 2019, using I. hederacea (L.) Jacq. var. integriuscula A. Gray as the target control species. Dry matter weight of I. hederacea var. integriuscula tended to decrease in autumn, causing a delay in the timing of IAS application. The application at the second trifoliate leaf stage (TLS) of soybean was the most effective. The control rate was higher when what is combined with bentazone; systematic application of IAS and bentazone at the first and third–fourth TLS of soybean was the most effective control strategy. We detected no statistically significant reduction in soybean yield as a consequence of IAS application. However, we noted marked differences in the annual yield; it decreased by approximately 27% in 2019. This reduction in soybean yield is attributable to a delayed flowering period and the observed reduction in the total numbers of nodes, pods, and grains due to the damage caused by IAS. Furthermore, we speculate that a low temperature and sunshine duration after IAS application contributed to phytotoxicity in soybean.