I'm tolerant of non rhizomatic grass but anything with rhizomes is the devil incarnate!
Studies in corn fields have reported 30-40% loss of crop when weeds were present at sprouting time.
John Polk wrote:
Fruit trees should have no grass under them until they are well established (+/- 3 years) in their spot. After that, grass could help them. Orchard grass is still commonly planted in orchards.
Some plants are not too badly harmed by grass, and for others, it creates a serious problem. Many plants (and corn is a prime example) need lots of sun, and have a "shade avoidance mode" built into them. Upon sprouting, if they are getting green spectrum light bouncing onto the undersides of their leaves they send a signal to the brain that says "I gotta get real tall QUICK". All energy that would have gone into root development is now diverted into stalk development. They will be the tallest stalks in the field. but they will never have the root structure needed to produce a decent crop. Studies in corn fields have reported 30-40% loss of crop when weeds were present at sprouting time.