Which factor is a direct cause of winterkill in crops?

Winterkill in crops typically occurs due to a combination of environmental stresses that can undermine plant health during the cold months. Heaving, which occurs when soil freezes and thaws repeatedly, can directly disrupt the root systems of crops. This process can displace plant roots from their ideal growing conditions, exposing them to extreme cold temperatures or desiccation, leading to significant damage or death of the plant.

In contrast, while inadequate irrigation, excessive summer rain, and pest infestations can negatively impact crop health, they do not directly cause winterkill. Inadequate irrigation primarily affects the plants during the growing season, while excessive rainfall in the summer can lead to waterlogged soils, which may not have immediate effects during winter. Pest infestations typically affect plant health through feeding damage and disease spread but are not directly associated with the winter kill process.