What is often the most critical limiting factor for a weed when the crop outgrows it?

When a crop outgrows a weed, light becomes the most critical limiting factor for the weed's survival and growth. Weeds often rely on light for photosynthesis, which is essential for their energy production and overall health. As the crop develops and its canopy closes, it shades the weeds beneath, significantly reducing their access to sunlight. This decrease in light availability impairs the weeds' ability to photosynthesize and grow effectively, leading to reduced vigor and potential death.

In contrast, while nutrients, water, and pest pressure are important aspects of plant health, they may still be adequate or available to the weeds even when overshadowed by the crop. A well-managed crop can utilize available resources efficiently, outcompeting weeds for light before these other factors become limiting. Thus, in scenarios where a crop has outgrown weeds, the immediate impact of light limitation on the weeds' survival is the most significant factor.