Unmanaged dense hinoki (Chamaecyparis obtusa) plantations in Japan tend to suffer from severe surface soil erosion. Thinning plays an important role in soil conservation through development of understory vegetation. To examine the possibility that the weeding conducted during thinning enhances surface soil erosion, we investigated splash detachment, pedestal height, forest floor cover and throughfall for four different experimental treatments, testing thinning and weeding and the combination of thinning and weeding in a hinoki plantation. High splash detachment was observed for weeding treatments. Generalized linear mixed modeling results suggest that weeding increases splash detachment. This increase in splash detachment is caused by clearing shrubs that have a low enough height to decrease the impact of rainfall. For the same treatment type, low forest floor cover was also observed in the first year after weeding due to the decline of litter cover. Broadleaf litter fall was decreased by weeding, although forest floor vegetation cover was similarly low for all treatments in the first year. These results suggest that weeding conducted during thinning promotes splash erosion in hinoki plantations.