It is impossible to see details, like leaves or flowers, so just only a wild guess: cover crop of Buckwheat.
From my observation Buckwheat can bloom at any time during the season, about one month after being sown, whenever it is. In my garden Buckwheat that I sowed on August 10 is just getting ready to bloom. In a day or two it should be in full bloom.
Thank you. I found the same information here. They mention Mattawa and bees, so I am sure now it is buckwheat What about buckwheat? :: Spring 2014 :: Washington State Magazine