Row Spacing and Seeding Rate
Altering crop row spacing and seeding rates can create a thicker, denser crop canopy, which in turn can help suppress weeds. Each weed species requires a certain amount of light to germinate and grow, and a thick crop canopy can greatly reduce the amount of sunlight that understory weeds and weed seeds have access to. A dense, healthy crop may also outcompete weeds for other resources like water and nutrients.
Horseweed is a good example of a weed that does not do well under a thick crop canopy. It requires high amounts of light exposure to grow. Research at Penn State is showing that when the field is covered by a thick cover crop in the early spring, horseweed growth is greatly suppressed due to the shading that the cover crop achieves.