In Japan, the uncontrolled growth of morning glory (Ipomoea spp.) in soybean fields has become a severe issue, necessitating an effective control method for buried seeds. We investigated the effects of using a polyethylene mulch sheet cover to reduce buried seeds by heating the soil. Field experiments were conducted in a soybean field in the Hiroshima Prefecture between 2017 and 2019. The mean soil temperature was noted to be higher near the field surface. The average soil temperature (April–October) at 15 cm below the soil surface was the highest on plots covered with clear mulching sheets (24.8–26.2°C), while the plots covered with black mulching sheets were slightly cooler (23.6–23.9°C), followed by uncovered plots (21.4–21.9°C). The relative humidity of the soil surface in the plots covered with mulch sheets was maintained at approximately 30% higher than that of the uncovered plots. Although the cumulative number of seedlings of I. coccinea during the soybean cultivation period was lower in the plots covered with mulch sheets before soybean sowing than in the uncovered plots (72–77% of the uncovered plots), the effect of reducing buried seeds was limited. The heating effect of the mulch sheets was higher in the plots from spring to fall, and the percentage of buried seeds after one soybean cultivation period ranged from 26 to 44%, lower than that of the uncovered plots. Therefore, covering the ground with a mulch sheet effectively reduced the number of buried seeds of I. coccinea and was considered a highly effective treatment, even in a single year.

In Japan, the spread of morning glory (Ipomoea spp.) in soybean fields has become a serious problem, necessitating establishment of effective methods for controlling buried seeds. Therefore, in this study, we investigated interspecific differences in germination responses of four morning glory species, namely, I. coccinea (red morning glory), I. lacunosa (pitted morning glory), I. triloba (three-lobed morning glory), and I. hederacea var. integriuscula (entireleaf morning glory) related to temperature and also the effect of polyethylene sheet cover-induced soil heating on reduction of viability of buried seeds. In both laboratory and field experiments, interspecific differences were observed in temperature sensitivity related to seed survival and breaking of dormancy. I. coccinea, I. lacunosa, and I. triloba exhibited seed decay when heated to 35–40°C or higher under moist conditions (more than three days), with a decline in seed survival rates after prolonged heating. The higher the temperature and longer the period of heating, the lower the seed viability. In contrast, the effect of heating on I. hederacea var. integriuscula was lower than that on the other species. In the field, the viability of hard seeds of I. coccinea, I. lacunosa, and I. triloba was considerably reduced by mulching sheet cover-induced soil heating, indicating that the mulching cover was effective in reducing the number of seeds buried up to 5 cm below the soil surface (rate of decrease: 42–99%). Therefore, covering crops with mulching sheets in summer can effectively reduce the viability of buried seeds of I. coccinea, I. lacunosa, and I. triloba.