This research was conducted to evaluate the light requirement for redroot and smooth pigweed germination in soil, and how this requirement is affected by germination temperature and seasonal periodicity in seed dormancy. Seed enclosed in nylon mesh bags was buried in the field in December 1993 and 1994 and was recovered throughout the spring and summer of the following year, respectively. Germination was highest with red light or at 30 C. The requirement for red light was more pronounced at 20 vs. 30 C. The saturating fluence of red light was as low as 3 μmol m−2 in buried seed and 1,000 μmol m−2 in unburied control seed, depending on germination temperature. The effect of light and germination temperature pigweed germination also changed throughout the growing season. Our results indicate that light may be a requirement for germination in only the most dormant weed seed in the soil seedbank.
