Polk is a ubiquitous weed here in the near South (Maryland, USA). Polk is only picked when it is quite young in the Spring. The leafy shoots are edible then, but it ain't no big taste treat. It probably has some nutritional value.
The plant can grow nearly five feet tall with a stem an inch or more in diameter. As a mature plant, I believe the leaves are poisonous. The plant also produces clusters of purple berries upon which birds feed. Laundry on the line, convertible tops, and even siding can develop purple spots from the droppings of birds who have eaten the Polk berries.
Roger in Baltimore