In both the 2006 and 2007 growing seasons, early season plant heights without plastic mulch were lower than those with mulch (Fig. 3). This could be due to the early emergence with mulch conditions. In 2006 treatment M60, with the earliest mulch removal, had among the greatest plant heights. For four times, plant height of treatment M60 was significantly greater than other mulched treatments (F-test, P < 0.05). Other mulched treatments had very similar plant heights and the difference among these treatments was not significant according to F-test (P > 0.05). This suggested that later mulch-removal had little effect on plant height. In 2007, however, it was found that mulching duration had significantly (F-test, P < 0.01) affected plant height during the early growing period (from June 6 to June 26): M0 < M40 < M60 < M90 ≈ Mw. This height difference can be explained by the higher soil temperature under mulch, which would enhance stem elongation according to Marinus and Bodlaender (1975). During the later growing period, removing mulch had little effect on plant height: treatment M90 had quite similar height to Mw. This was probably due to the small soil temperature difference between mulched and non-mulched soil during that period when crop canopy was very large and nearly shaded the entire soil surface.