According to the companion planting bible, Carrots Love Tomatoes, chamomile increases wheat yields (plant 1 part chamomile to every 100 parts wheat). Bachelor Buttons (a flower) aids rye production when planted in a 1:100 ratio.
It should be noted that grains grow 3+ feet tall, and often require little water, so don't expect your companion plants to perform their best given this shaded, dry environment.
No. Grains tend to close out other plants. (As such, they are not very good companion plants for your vegetables!) However, if thistle, burdock, or another invasive plant appears it's best to get it while you can.