When soil is plowed in the spring, pigweed seeds that have been buried in the soil all winter are churned up to the surface and redeposited just under the surface  The brief exposure of the seeds to sunlight stimulates receptors, which have become highly sensitive to sunlight during the months the seeds were buried in the soil, and the stimulated receptors trigger germination
Without the prolonged darkness, followed by exposure to sunlight, the seeds do not germinate
Fewer pigweed plants will grow in the field if it is plowed at all than if it is plowed only at night.