We have found that using buckwheat as a mid to late summer cover crop works well for us. It grows well in hot weather, provides late season forage for the bees, and if the flowers go to seed, we fence in the chickens to eat the seeds, fertilizing the garden for the next growing season.
You may notice the chard is still hanging in there quite well. Once it makes it through the heat of summer it revitalizes and is the last thing to die back in the garden.