This nitrogen fixing plant is shown below in mid summer. As you can see it is in the Mimosa family (Mimosaceae). This native perennial plant is 2-5' tall, which occasionally branches out, and has a ferny appearance grows in the prairie region of North America. This plant has a central taproot, and disperses by reseeding itself.
This picture was taken at a PermacultureNursery.com experimental nursery plot located at Montview Neighborhood Farm in Northampton MA. The seed for this cultivar was acquired from the Land Institute which they are growing to select for non-shattering traits for potential mechanical harvesting. The blue green leafy vegetable growing with it is sea kale (Crambe maritima) - there should probably be more space between plants.