In 2007, treatment M60 had the greatest weight of the largest tubers (W ≥ 200), which was similar to the results of 2006 (Table 6). There was a clear pattern: the yield of the largest tubers was less with mulching duration longer than 60 days, although the difference was not significant according to F-test (P > 0.05). Treatment M60 had the greatest number of the largest tubers, and the difference between treatments M60 and M40 was significant (P < 0.05). The difference in the number of marketable tubers (W ≥ 50 g) among treatments was insignificant. In addition, total tuber number was not significantly affected by mulch or mulching duration, which was different from 2006. This could be explained by the slightly lower air temperature and more cloudy days in 2007 and is consistent with results showing that tuber number could be reduced by plastic mulch during hot weather (Wang et al., 2009).